Integrate optional terrain transforms

- Add versioned terrain providers that run before content placement.
- Resolve transformed terrain heights, occupancy, and cave metadata through bounded caches.
- Initialize native structures after binding the modded generation runtime.
- Cover transform behavior, provider resolution, and structure binding with regression tests.

adapters/modded-common/src/main/java/art/arcane/iris/modded/IrisModdedChunkGenerator.java

@@ -242,6 +242,7 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
         this.announced.set(false);
         resetRuntimeCaches();
         this.engineBinding.complete(replacement);
+        this.nativeGenerator.initializeStructureState(level);
         // Bind time: a feature-order cycle in the new pack is reported here, once, and degrades to features-off.
         // Never waits on a build owned by another thread: this method owns the generator monitor and the build
         // path can need it.
@@ -442,12 +443,14 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
         }
         Engine current = engineIfBound();
         if (NativeDimensionRuntime.sameWorld(boundLevel, level) && current != null && current.getComplex() != null) {
+            nativeGenerator.initializeStructureState(level);
             return;
         }
         requireCompletedShutdown(engine);
         unloading = false;
         Engine bound = bindEngine(level);
         nativeGenerator.installVolumeIndex(level, bound);
+        nativeGenerator.initializeStructureState(level);
         // Bind time: a feature-order cycle is reported here, once, and degrades to features-off. Non-waiting for
         // the same reason as repointAndBind: this method owns the generator monitor.
         importedFeatures.prepareWithoutWaiting(bound);
@@ -1097,6 +1100,12 @@ public final class IrisModdedChunkGenerator implements NativeGeneratorOwner, Nat
         return current.getDimensionStackContext() != null;
     }
 
+    @Override
+    public boolean terrainTransformed() {
+        Engine current = engine;
+        return current != null && current.getComplex().getTerrainTransform() != null;
+    }
+
     @Override
     public int runtimeId(Engine current) {
         return current.getCacheID();

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedMixinAuditTest.java

@@ -42,6 +42,7 @@ public class ModdedMixinAuditTest {
             "\\[\\[mixins]]\\s*config\\s*=\\s*\"([^\"]+)\"");
     // One applied mixin per config proves the config was registered, which is all the audit detects.
     private static final Set<String> UNAUDITED = Set.of(
+            "volmlib.entity.mixins.json/ServerLevelStructureBootstrapMixin",
             "volmlib.entity.mixins.json/WorldGenerationWriteGuardMixin",
             "volmlib.entity.mixins.json/ChunkMapTerrainReceiptAccess",
             "volmlib.entity.mixins.json/ChunkTerrainReceiptMixin",

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedStructureBindingOrderTest.java

@@ -0,0 +1,145 @@
+package art.arcane.iris.modded;
+
+import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.EnginePlatformHooks;
+import art.arcane.iris.generation.runtime.IrisComplex;
+import art.arcane.iris.pack.loading.IrisData;
+import art.arcane.volmlib.nativelib.minecraft26_2.modded.NativeModdedChunkGenerator;
+import art.arcane.volmlib.nativelib.minecraft26_2.modded.NativeModdedServer;
+import art.arcane.volmlib.nativelib.terrain.NativeWorld;
+import net.minecraft.SharedConstants;
+import net.minecraft.server.Bootstrap;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.mockito.MockedStatic;
+
+import java.io.File;
+import java.lang.reflect.Field;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.junit.Assert.assertSame;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyLong;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class ModdedStructureBindingOrderTest {
+    @BeforeClass
+    public static void bootstrap() {
+        SharedConstants.tryDetectVersion();
+        Bootstrap.bootStrap();
+    }
+
+    @Test
+    public void initialBindingPublishesReadyEngineBeforeStartingStructureSearches() throws Exception {
+        NativeModdedChunkGenerator<?, ?, ?> nativeGenerator = mock(NativeModdedChunkGenerator.class);
+        ModdedEngineBinding<Engine> binding = new ModdedEngineBinding<>(1L, TimeUnit.MILLISECONDS);
+        IrisModdedChunkGenerator generator = generator(nativeGenerator, binding);
+        NativeWorld world = mock(NativeWorld.class);
+        NativeModdedServer server = mock(NativeModdedServer.class);
+        Engine engine = readyEngine();
+        when(nativeGenerator.represents(world)).thenReturn(true);
+        when(server.generateStructures()).thenReturn(true);
+        doAnswer(invocation -> {
+            assertSame(engine, generator.awaitStructureEngine());
+            assertSame(engine, binding.await("overworld"));
+            return null;
+        }).when(nativeGenerator).initializeStructureState(world);
+
+        try (MockedStatic<NativeModdedServer> servers = mockStatic(NativeModdedServer.class);
+             MockedStatic<ModdedWorldEngines> engines = mockStatic(ModdedWorldEngines.class);
+             MockedStatic<ModdedIrisLog> log = mockStatic(ModdedIrisLog.class)) {
+            servers.when(() -> NativeModdedServer.forWorld(world)).thenReturn(server);
+            engines.when(() -> ModdedWorldEngines.get(eq(world), any(), any(), anyLong(), any())).thenReturn(engine);
+
+            generator.bindLevel(world);
+
+            verify(nativeGenerator).initializeStructureState(world);
+        }
+    }
+
+    @Test
+    public void levelBindingStartsDeferredStructuresAfterAnEarlierDataQueryBoundTheEngine() throws Exception {
+        NativeModdedChunkGenerator<?, ?, ?> nativeGenerator = mock(NativeModdedChunkGenerator.class);
+        ModdedEngineBinding<Engine> binding = new ModdedEngineBinding<>(1L, TimeUnit.MILLISECONDS);
+        IrisModdedChunkGenerator generator = generator(nativeGenerator, binding);
+        NativeWorld world = mock(NativeWorld.class);
+        Engine engine = readyEngine();
+        when(nativeGenerator.represents(world)).thenReturn(true);
+        when(world.nativeHandle()).thenReturn(new Object());
+        field("engine").set(generator, engine);
+        field("boundLevel").set(generator, world);
+        binding.complete(engine);
+        doAnswer(invocation -> {
+            assertSame(engine, generator.awaitStructureEngine());
+            assertSame(engine, binding.await("overworld"));
+            return null;
+        }).when(nativeGenerator).initializeStructureState(world);
+
+        generator.bindLevel(world);
+
+        verify(nativeGenerator).initializeStructureState(world);
+    }
+
+    @Test
+    public void repointPublishesReplacementBindingBeforeStartingStructureSearches() throws Exception {
+        NativeModdedChunkGenerator<?, ?, ?> nativeGenerator = mock(NativeModdedChunkGenerator.class);
+        ModdedEngineBinding<Engine> binding = new ModdedEngineBinding<>(1L, TimeUnit.MILLISECONDS);
+        IrisModdedChunkGenerator generator = generator(nativeGenerator, binding);
+        NativeWorld world = mock(NativeWorld.class);
+        NativeModdedServer server = mock(NativeModdedServer.class);
+        Engine replacement = readyEngine();
+        when(nativeGenerator.represents(world)).thenReturn(true);
+        when(server.generateStructures()).thenReturn(true);
+        doAnswer(invocation -> {
+            assertSame(replacement, generator.awaitStructureEngine());
+            assertSame(replacement, binding.await("overworld"));
+            return null;
+        }).when(nativeGenerator).initializeStructureState(world);
+
+        try (MockedStatic<NativeModdedServer> servers = mockStatic(NativeModdedServer.class);
+             MockedStatic<ModdedWorldEngines> engines = mockStatic(ModdedWorldEngines.class)) {
+            servers.when(() -> NativeModdedServer.forWorld(world)).thenReturn(server);
+            engines.when(() -> ModdedWorldEngines.prepareReplacement(eq(world), eq("overworld"),
+                    eq("overworld"), eq(42L), any())).thenReturn(replacement);
+
+            generator.repointAndBind(world, "overworld", "overworld", 42L);
+
+            verify(nativeGenerator).initializeStructureState(world);
+        }
+    }
+
+    private static IrisModdedChunkGenerator generator(NativeModdedChunkGenerator<?, ?, ?> nativeGenerator,
+                                                       ModdedEngineBinding<Engine> binding) throws Exception {
+        IrisModdedChunkGenerator generator = mock(IrisModdedChunkGenerator.class, CALLS_REAL_METHODS);
+        field("nativeGenerator").set(generator, nativeGenerator);
+        field("engineBinding").set(generator, binding);
+        field("dimensionKey").set(generator, "overworld");
+        field("generationMode").set(generator, ModdedGenerationMode.PERSISTENT_RESTORE);
+        field("announced").set(generator, new AtomicBoolean());
+        field("importedFeatures").set(generator, mock(ModdedImportedFeatureStage.class));
+        return generator;
+    }
+
+    private static Engine readyEngine() {
+        Engine engine = mock(Engine.class);
+        IrisData data = mock(IrisData.class);
+        when(engine.getComplex()).thenReturn(mock(IrisComplex.class));
+        when(engine.getData()).thenReturn(data);
+        when(data.getDataFolder()).thenReturn(new File("build/structure-binding-test/pack"));
+        when(engine.getPlatformHooks()).thenReturn(mock(EnginePlatformHooks.class));
+        return engine;
+    }
+
+    private static Field field(String name) throws NoSuchFieldException {
+        Field field = IrisModdedChunkGenerator.class.getDeclaredField(name);
+        field.setAccessible(true);
+        return field;
+    }
+}

adapters/modded-common/src/test/java/art/arcane/volmlib/nativelib/minecraft26_2/modded/mixin/NativeStructureBootstrapTest.java

@@ -0,0 +1,70 @@
+package art.arcane.volmlib.nativelib.minecraft26_2.modded.mixin;
+
+import art.arcane.volmlib.nativelib.minecraft26_2.modded.NativeModdedChunkGenerator;
+import art.arcane.volmlib.nativelib.terrain.NativeWorld;
+import net.minecraft.SharedConstants;
+import net.minecraft.server.Bootstrap;
+import net.minecraft.server.level.ServerChunkCache;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.level.chunk.ChunkGenerator;
+import net.minecraft.world.level.chunk.ChunkGeneratorStructureState;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class NativeStructureBootstrapTest {
+    @BeforeClass
+    public static void bootstrap() {
+        SharedConstants.tryDetectVersion();
+        Bootstrap.bootStrap();
+    }
+
+    @Test
+    public void levelConstructionDefersNativeStructureInitialization() {
+        ServerLevelStructureBootstrapMixin mixin = mock(ServerLevelStructureBootstrapMixin.class, CALLS_REAL_METHODS);
+        ServerChunkCache chunks = mock(ServerChunkCache.class);
+        NativeModdedChunkGenerator<?, ?, ?> generator = mock(NativeModdedChunkGenerator.class);
+        ChunkGeneratorStructureState state = mock(ChunkGeneratorStructureState.class);
+        when(mixin.getChunkSource()).thenReturn(chunks);
+        when(chunks.getGenerator()).thenReturn(generator);
+
+        mixin.iris$initializeStructureState(state);
+
+        verify(state, never()).ensureStructuresGenerated();
+    }
+
+    @Test
+    public void levelConstructionStillInitializesVanillaStructures() {
+        ServerLevelStructureBootstrapMixin mixin = mock(ServerLevelStructureBootstrapMixin.class, CALLS_REAL_METHODS);
+        ServerChunkCache chunks = mock(ServerChunkCache.class);
+        ChunkGenerator generator = mock(ChunkGenerator.class);
+        ChunkGeneratorStructureState state = mock(ChunkGeneratorStructureState.class);
+        when(mixin.getChunkSource()).thenReturn(chunks);
+        when(chunks.getGenerator()).thenReturn(generator);
+
+        mixin.iris$initializeStructureState(state);
+
+        verify(state).ensureStructuresGenerated();
+    }
+
+    @Test
+    public void boundNativeWorldInitializesItsDeferredStructures() {
+        NativeModdedChunkGenerator<?, ?, ?> generator = mock(NativeModdedChunkGenerator.class, CALLS_REAL_METHODS);
+        NativeWorld world = mock(NativeWorld.class);
+        ServerLevel level = mock(ServerLevel.class);
+        ServerChunkCache chunks = mock(ServerChunkCache.class);
+        ChunkGeneratorStructureState state = mock(ChunkGeneratorStructureState.class);
+        when(world.nativeHandle()).thenReturn(level);
+        when(level.getChunkSource()).thenReturn(chunks);
+        when(chunks.getGeneratorState()).thenReturn(state);
+
+        generator.initializeStructureState(world);
+
+        verify(state).ensureStructuresGenerated();
+    }
+}

core/src/main/java/art/arcane/iris/generation/mantle/CaveTerrainSnapshot.java

@@ -29,6 +29,7 @@ public final class CaveTerrainSnapshot {
     private final MatterCavern[][] caverns;
     private final HydrologyCaveCell[][] hydrology;
     private boolean hydrologyPresent;
+    private volatile long retainedBytes;
 
     private CaveTerrainSnapshot(int chunkX, int chunkZ, int sections) {
         this.chunkX = chunkX;
@@ -63,6 +64,36 @@ public final class CaveTerrainSnapshot {
         return hydrologyPresent;
     }
 
+    public long estimatedRetainedBytes() {
+        long cached = retainedBytes;
+        if (cached != 0L) {
+            return cached;
+        }
+        long bytes = 80L + ((long) caverns.length + hydrology.length) * 8L;
+        for (MatterCavern[] section : caverns) {
+            if (section != null) {
+                bytes += 16L + (long) section.length * 8L;
+                for (MatterCavern cell : section) {
+                    if (cell != null) {
+                        bytes += 96L + (long) cell.getCustomBiome().length() * 2L;
+                    }
+                }
+            }
+        }
+        for (HydrologyCaveCell[] section : hydrology) {
+            if (section != null) {
+                bytes += 16L + (long) section.length * 8L;
+                for (HydrologyCaveCell cell : section) {
+                    if (cell != null) {
+                        bytes += 208L + ((long) cell.fluidProfileKey().length() + cell.floodedBiomeKey().length()) * 2L;
+                    }
+                }
+            }
+        }
+        retainedBytes = bytes;
+        return bytes;
+    }
+
     public MatterCavern cavern(int x, int y, int z) {
         if (y < 0) {
             return null;

core/src/main/java/art/arcane/iris/generation/mantle/EngineMantle.java

@@ -28,6 +28,7 @@ import art.arcane.iris.generation.runtime.Engine;
 import art.arcane.iris.generation.runtime.EngineTarget;
 import art.arcane.iris.generation.decoration.tree.TreeBlockMaterial;
 import art.arcane.iris.generation.terrain.IrisDimension;
+import art.arcane.iris.generation.terrain.Terrain3DColumn;
 import art.arcane.iris.generation.hydrology.cave.HydrologyCaveCell;
 import art.arcane.iris.generation.hydrology.cave.HydrologyCaveStorage;
 import art.arcane.iris.pack.value.IrisPosition;
@@ -108,6 +109,12 @@ public interface EngineMantle extends MatterGenerator {
     }
 
     default int getHighest(int x, int z, IrisData data, boolean ignoreFluid) {
+        IrisComplex complex = getComplex();
+        int transformed = complex == null || complex.getTerrainTransform() == null ? Integer.MIN_VALUE
+                : complex.transformedHeight(x, z, ignoreFluid);
+        if (transformed != Integer.MIN_VALUE) {
+            return transformed;
+        }
         return ignoreFluid ? trueHeight(x, z) : Math.max(trueHeight(x, z), getFluidHeight(x, z));
     }
 
@@ -116,6 +123,10 @@ public interface EngineMantle extends MatterGenerator {
     }
 
     default boolean isCarved(int x, int h, int z) {
+        Terrain3DColumn transformed = getComplex().transformedColumn(x, z);
+        if (transformed != null) {
+            return h >= 0 && h < transformed.topY() && !transformed.isSolid(h);
+        }
         HydrologyCaveCell hydrology = HydrologyCaveStorage.getIfPresent(getMantle(), x, h, z);
         if (hydrology != null) {
             return hydrology.carves();
@@ -142,6 +153,12 @@ public interface EngineMantle extends MatterGenerator {
     }
 
     default int getFluidHeight(int x, int z) {
+        IrisComplex complex = getComplex();
+        int transformed = complex == null || complex.getTerrainTransform() == null ? Integer.MIN_VALUE
+                : complex.transformedFluidHeight(x, z);
+        if (transformed != Integer.MIN_VALUE) {
+            return transformed;
+        }
         return (int) Math.round(getComplex().getRiverWaterSurfaceStream().get(x, z));
     }
 

core/src/main/java/art/arcane/iris/generation/mantle/MantleWriter.java

@@ -582,6 +582,10 @@ public class MantleWriter implements ObjectPassPlacer, AutoCloseable {
     }
 
     public boolean isPrerequisiteCarved(int x, int y, int z) {
+        Terrain3DColumn transformed = engineMantle.getComplex().transformedColumn(x, z);
+        if (transformed != null) {
+            return y >= 0 && y < transformed.topY() && !transformed.isSolid(y);
+        }
         HydrologyCaveCell hydrology = getPrerequisiteDataIfPresent(x, y, z, HydrologyCaveCell.class);
         if (hydrology != null) {
             return hydrology.carves();
@@ -592,6 +596,10 @@ public class MantleWriter implements ObjectPassPlacer, AutoCloseable {
 
     public byte[] getPrerequisiteCarvedColumn(int x, int z, int height) {
         int cappedHeight = Math.min(Math.max(height, 0), mantle.getWorldHeight());
+        Terrain3DColumn transformed = engineMantle.getComplex().transformedColumn(x, z);
+        if (transformed != null) {
+            return carvedColumn(transformed, cappedHeight);
+        }
         byte[] carvedColumn = new byte[cappedHeight];
         if (cappedHeight <= 0) {
             return carvedColumn;
@@ -961,6 +969,10 @@ public class MantleWriter implements ObjectPassPlacer, AutoCloseable {
 
     @Override
     public boolean isCarved(int x, int y, int z) {
+        Terrain3DColumn transformed = engineMantle.getComplex().transformedColumn(x, z);
+        if (transformed != null) {
+            return y >= 0 && y < transformed.topY() && !transformed.isSolid(y);
+        }
         HydrologyCaveCell hydrology = getDataIfPresent(x, y, z, HydrologyCaveCell.class);
         if (hydrology != null) {
             return hydrology.carves();
@@ -972,6 +984,10 @@ public class MantleWriter implements ObjectPassPlacer, AutoCloseable {
 
     public byte[] getCarvedColumn(int x, int z, int height) {
         int cappedHeight = Math.min(Math.max(height, 0), mantle.getWorldHeight());
+        Terrain3DColumn transformed = engineMantle.getComplex().transformedColumn(x, z);
+        if (transformed != null) {
+            return carvedColumn(transformed, cappedHeight);
+        }
         byte[] carvedColumn = new byte[cappedHeight];
         if (cappedHeight <= 0) {
             return carvedColumn;
@@ -1589,6 +1605,17 @@ public class MantleWriter implements ObjectPassPlacer, AutoCloseable {
         }
     }
 
+    private static byte[] carvedColumn(Terrain3DColumn terrain, int height) {
+        byte[] result = new byte[height];
+        int maximumY = Math.min(height, terrain.topY());
+        for (int y = 0; y < maximumY; y++) {
+            if (!terrain.isSolid(y)) {
+                result[y] = 1;
+            }
+        }
+        return result;
+    }
+
     public final class ObjectPlacementCapture implements AutoCloseable {
         private final ObjectContinuationBundle.PlacementKey key;
         private final Long2ObjectOpenHashMap<Matter> fragments = new Long2ObjectOpenHashMap<>();

core/src/main/java/art/arcane/iris/generation/mode/ModeOverworld.java

@@ -31,6 +31,7 @@ import art.arcane.iris.generation.stage.IrisDepositModifier;
 import art.arcane.iris.generation.stage.IrisFloatingChildBiomeModifier;
 import art.arcane.iris.generation.stage.IrisPerfectionModifier;
 import art.arcane.iris.generation.stage.IrisPostModifier;
+import art.arcane.iris.generation.context.ChunkContext;
 
 public class ModeOverworld extends IrisEngineMode implements EngineMode {
     public ModeOverworld(Engine engine) {
@@ -45,7 +46,7 @@ public class ModeOverworld extends IrisEngineMode implements EngineMode {
         IrisCustomModifier custom = new IrisCustomModifier(getEngine());
         IrisFloatingChildBiomeModifier floatingChildBiomes = new IrisFloatingChildBiomeModifier(getEngine());
         EngineStage sBiome = (x, z, k, p, m, c) -> biome.actuate(x, z, p, m, c);
-        EngineStage sGenMatter = (x, z, k, p, m, c) -> generateTerrainMatter(x >> 4, z >> 4, m || getEngine().isStudio(), c);
+        EngineStage sGenMatter = (x, z, k, p, m, c) -> generateTerrainMatterForChunk(x >> 4, z >> 4, m, c);
         EngineStage sTerrain = (x, z, k, p, m, c) -> terrain.actuate(x, z, k, m, c);
         EngineStage sDecorant = (x, z, k, p, m, c) -> decorant.actuate(x, z, k, m, c);
         EngineStage sCave = (x, z, k, p, m, c) -> cave.modify(x >> 4, z >> 4, k, m, c);
@@ -82,4 +83,10 @@ public class ModeOverworld extends IrisEngineMode implements EngineMode {
             registerStage(sCustom);
         }
     }
+
+    void generateTerrainMatterForChunk(int x, int z, boolean multicore, ChunkContext context) {
+        boolean studioParallel = getEngine().isStudio()
+                && !(context.isNaturalTerrain() && context.getComplex().getTerrainTransform() != null);
+        generateTerrainMatter(x, z, multicore || studioParallel, context);
+    }
 }

core/src/main/java/art/arcane/iris/generation/runtime/DimensionTerrainContext.java

@@ -125,8 +125,8 @@ public final class DimensionTerrainContext implements DataProvider {
                 engine.getDimension(),
                 engine.getData(),
                 engine.getHeight(),
-                natural ? complex.getNaturalHeightStream() : complex.getHeightStream(),
-                natural ? complex.getNaturalSlopeStream() : complex.getSlopeStream(),
+                natural ? complex.getNaturalHeightStream() : complex.getRawHeightStream(),
+                natural ? complex.getNaturalSlopeStream() : complex.getRawSlopeStream(),
                 fluidHeightStream,
                 natural ? complex.getNaturalTrueBiomeStream() : complex.getTrueBiomeStream(),
                 complex.getRegionStream(),
@@ -653,7 +653,7 @@ public final class DimensionTerrainContext implements DataProvider {
         if (selfReferencing) {
             return naturalSelf || naturalFallback
                     ? engine.getComplex().naturalTerrainColumn(x, z)
-                    : engine.getComplex().terrainColumn(x, z);
+                    : engine.getComplex().originalTerrainColumn(x, z);
         }
         if (terrain3D == null) {
             return null;

core/src/main/java/art/arcane/iris/generation/runtime/Engine.java

@@ -567,6 +567,10 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
     /** The terrain slope at a column, answered from natural terrain when the server thread may not wait. */
     @BlockCoordinates
     default double getSlope(int x, int z) {
+        IrisComplex complex = getComplex();
+        if (complex != null && complex.getTerrainTransform() != null) {
+            return complex.getSlopeStream().getDouble(x, z);
+        }
         if (answersFromNaturalTerrain(x, z)) {
             return getComplex().naturalSlope(x, z);
         }
@@ -580,6 +584,12 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     default int getHeight(int x, int z, boolean ignoreFluid) {
+        IrisComplex complex = getComplex();
+        int transformed = complex == null || complex.getTerrainTransform() == null ? Integer.MIN_VALUE
+                : complex.transformedHeight(x, z, ignoreFluid);
+        if (transformed != Integer.MIN_VALUE) {
+            return transformed;
+        }
         DimensionStackContext dimensionStackContext = getDimensionStackContext();
         if (dimensionStackContext != null) {
             return ignoreFluid
@@ -595,6 +605,12 @@ public interface Engine extends DataProvider, Fallible, BlockUpdater, Renderer,
 
     @BlockCoordinates
     static int hostHeight(Engine engine, int x, int z, boolean ignoreFluid) {
+        IrisComplex complex = engine.getComplex();
+        int transformed = complex == null || complex.getTerrainTransform() == null ? Integer.MIN_VALUE
+                : complex.transformedHeight(x, z, ignoreFluid);
+        if (transformed != Integer.MIN_VALUE) {
+            return transformed;
+        }
         EngineMantle mantle = engine.getMantle();
         if (mantle == null) {
             return engine.getHeight(x, z, ignoreFluid);

core/src/main/java/art/arcane/iris/generation/runtime/EngineMode.java

@@ -213,7 +213,11 @@ public interface EngineMode extends Staged {
 
         EngineStage[] stages = getStages().toArray(new EngineStage[0]);
         try (IrisContext.Scope chunkScope = IrisContext.open(getEngine(), generationSessionId, ctx)) {
+            if (getComplex().getTerrainTransform() == null) {
                 generateTerrain(x, z, blocks, biomes, multicore, ctx);
+            } else {
+                getComplex().getTerrainTransform().generate(x, z, blocks, biomes, multicore, ctx);
+            }
             recordNaturalTerrain(x, z, blocks, biomes, ctx);
             IrisDimensionStackActuator.clearHostNaturalMetadata(getEngine(), x, z, biomes.getHeight(), ctx);
             ctx.beginContent();

core/src/main/java/art/arcane/iris/generation/runtime/IrisComplex.java

@@ -54,6 +54,10 @@ import art.arcane.iris.generation.noise.IrisGenerator;
 import art.arcane.iris.generation.noise.IrisGeneratorStyle;
 import art.arcane.iris.generation.terrain.IrisDimension;
 import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRegistry;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRuntime;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformer;
 import art.arcane.iris.generation.noise.IrisInterpolator;
 import art.arcane.iris.generation.terrain.IrisMaterialPalette;
 import art.arcane.iris.generation.terrain.IrisRegion;
@@ -185,6 +189,20 @@ public class IrisComplex implements DataProvider {
     private final ProceduralTerrainHeightSampler proceduralTerrainHeight;
     @Getter(AccessLevel.NONE)
     private final transient Engine terrainEngine;
+    @Setter(AccessLevel.NONE)
+    private final transient TerrainTransformRuntime terrainTransform;
+    @Getter(AccessLevel.NONE)
+    @Setter(AccessLevel.NONE)
+    private final transient ProceduralStream<Double> transformedHeightStream;
+    @Getter(AccessLevel.NONE)
+    @Setter(AccessLevel.NONE)
+    private final transient ProceduralStream<Integer> transformedRoundedHeightStream;
+    @Getter(AccessLevel.NONE)
+    @Setter(AccessLevel.NONE)
+    private final transient ProceduralStream<Double> transformedTopStream;
+    @Getter(AccessLevel.NONE)
+    @Setter(AccessLevel.NONE)
+    private final transient ProceduralStream<Double> transformedSlopeStream;
     @Getter(AccessLevel.NONE)
     private transient Terrain3DRuntime terrain3D;
     private transient HydrologyBankTerrainRuntime hydrologyBanks3D;
@@ -494,6 +512,25 @@ public class IrisComplex implements DataProvider {
                             d.hashCode());
                 }), "", engine, cacheSize);
         //@done
+        TerrainTransformer transformer = engine.getDimension().getTerrainTransform() == null ? null
+                : TerrainTransformRegistry.discover().resolve(engine.getDimension().getTerrainTransform());
+        terrainTransform = transformer == null ? null
+                : new TerrainTransformRuntime(new TerrainTransformRuntime.Options(engine, this), transformer);
+        transformedHeightStream = transformer == null ? null : ProceduralStream.ofDouble((x, z) -> {
+            int transformed = transformedHeight((int) Math.floor(x), (int) Math.floor(z), true);
+            return transformed == Integer.MIN_VALUE ? heightStream.getDouble(x, z) : transformed;
+        });
+        transformedRoundedHeightStream = transformer == null ? null : ProceduralStream.ofDouble((x, z) -> {
+            int transformed = transformedHeight((int) Math.floor(x), (int) Math.floor(z), true);
+            return transformed == Integer.MIN_VALUE ? roundedHeighteightStream.get(x, z) : transformed;
+        }).round();
+        transformedTopStream = transformer == null ? null : ProceduralStream.ofDouble((x, z) -> {
+            int transformed = transformedHeight((int) Math.floor(x), (int) Math.floor(z), false);
+            return transformed == Integer.MIN_VALUE ? heightFluidStream.getDouble(x, z) : transformed;
+        });
+        ProceduralStream<Double> effectiveSlope = transformer == null ? null : transformedHeightStream.slope(3);
+        transformedSlopeStream = transformer == null ? null : ProceduralStream.ofDouble((x, z) ->
+                isNaturalTerrainContext() ? slopeStream.getDouble(x, z) : effectiveSlope.getDouble(x, z));
     }
 
     /**
@@ -689,6 +726,14 @@ public class IrisComplex implements DataProvider {
     }
 
     public Terrain3DColumn terrainColumn(int x, int z) {
+        Terrain3DColumn transformed = transformedColumn(x, z);
+        if (transformed != null) {
+            return transformed;
+        }
+        return originalTerrainColumn(x, z);
+    }
+
+    Terrain3DColumn originalTerrainColumn(int x, int z) {
         if (!hasTerrain3D()) {
             return null;
         }
@@ -1204,7 +1249,70 @@ public class IrisComplex implements DataProvider {
     }
 
     public ProceduralStream<Double> getHeightStream() {
-        return heightStream;
+        return terrainTransform == null ? heightStream : transformedHeightStream;
+    }
+
+    ProceduralStream<Double> getRawSlopeStream() {
+        return terrainEngine.getDimension().getTerrainTransform() == null ? slopeStream : heightStream.slope(3);
+    }
+
+    public ProceduralStream<Integer> getRoundedHeighteightStream() {
+        return terrainTransform == null ? roundedHeighteightStream : transformedRoundedHeightStream;
+    }
+
+    public ProceduralStream<Double> getHeightFluidStream() {
+        return terrainTransform == null ? heightFluidStream : transformedTopStream;
+    }
+
+    public ProceduralStream<Double> getSlopeStream() {
+        return terrainTransform == null ? slopeStream : transformedSlopeStream;
+    }
+
+    public int transformedHeight(int x, int z, boolean ignoreFluid) {
+        if (terrainTransform == null || isNaturalTerrainContext()) {
+            return Integer.MIN_VALUE;
+        }
+        return readsReadyTerrainOnly()
+                ? terrainTransform.readyHeight(x, z, ignoreFluid)
+                : terrainTransform.height(x, z, ignoreFluid);
+    }
+
+    public Terrain3DColumn transformedColumn(int x, int z) {
+        if (terrainTransform == null || isNaturalTerrainContext()) {
+            return null;
+        }
+        return readsReadyTerrainOnly()
+                ? terrainTransform.readyColumn(x, z) : terrainTransform.column(x, z);
+    }
+
+    public int transformedFluidHeight(int x, int z) {
+        if (terrainTransform == null || isNaturalTerrainContext()) {
+            return Integer.MIN_VALUE;
+        }
+        return readsReadyTerrainOnly()
+                ? terrainTransform.readyFluidHeight(x, z) : terrainTransform.fluidHeight(x, z);
+    }
+
+    private boolean readsReadyTerrainOnly() {
+        if (!terrainEngine.getPlatformHooks().isMainThread()) {
+            return false;
+        }
+        IrisContext context = IrisContext.get();
+        if (context == null || context.getEngine() != terrainEngine) {
+            return true;
+        }
+        ChunkContext chunk = context.getChunkContext();
+        if (chunk != null) {
+            return chunk.getComplex() != this || chunk.isNaturalTerrain();
+        }
+        if (context.getGenerationSessionId() == 0L || terrainEngine.getComplex() != this) {
+            return true;
+        }
+        if (terrainEngine instanceof IrisEngine irisEngine && irisEngine.threadState.assembly() != null
+                && irisEngine.threadState.assembly().mode == null) {
+            return true;
+        }
+        return terrainEngine.getMode() == null;
     }
 
     boolean isNaturalTerrainContext() {
@@ -2144,6 +2252,9 @@ public class IrisComplex implements DataProvider {
     }
 
     public void close() {
+        if (terrainTransform != null) {
+            terrainTransform.close();
+        }
         if (hydrologyBanks3D != null) {
             hydrologyBanks3D.clear();
         }

core/src/main/java/art/arcane/iris/generation/runtime/UpperDimensionContext.java

@@ -36,7 +36,7 @@ public class UpperDimensionContext implements DataProvider {
     private UpperDimensionContext(DimensionTerrainContext terrainContext, Engine engine) {
         this.terrainContext = terrainContext;
         IrisComplex complex = engine.getComplex();
-        lowerHeightStream = complex.getHeightStream();
+        lowerHeightStream = complex.getRawHeightStream();
         ceilingLayout = new CeilingLayout(engine.getHeight(), engine.getDimension().getUpperDimensionGap());
     }
 

core/src/main/java/art/arcane/iris/generation/stage/IrisCarveModifier.java

@@ -904,7 +904,7 @@ public class IrisCarveModifier extends EngineAssignedModifier<NativeBlockState>
                             zone.setFloor(floor);
                             zone.setCeiling(y - 1);
                             if (zone.isValid(resolver.height)
-                                    && !getComplex().isTerrain3DOpening(worldX, floor, worldZ)) {
+                                    && !isNaturalTerrainOpening(getComplex(), worldX, floor, worldZ)) {
                                 if (markerRoll(worldX, zone.ceiling, worldZ, 0x9E3779B97F4A7C15L)) {
                                     mantle.set(worldX, zone.ceiling, worldZ, MarkerMatter.CAVE_CEILING);
                                 }
@@ -992,7 +992,7 @@ public class IrisCarveModifier extends EngineAssignedModifier<NativeBlockState>
     }
 
     IrisBiome resolveCaveBoundaryBiome(MatterCavern cavern, int worldX, int y, int worldZ, CaveInputs resolverState, Long2ObjectOpenHashMap<IrisBiome> caveBiomeCache, Map<String, IrisBiome> customBiomeCache) {
-        if (resolverState.complex.isTerrain3DOpening(worldX, y, worldZ)) {
+        if (isNaturalTerrainOpening(resolverState.complex, worldX, y, worldZ)) {
             return resolverState.complex.getTrueBiomeStream().get(worldX, worldZ);
         }
         if (cavern != null && !cavern.getCustomBiome().isEmpty()) {
@@ -1001,6 +1001,18 @@ public class IrisCarveModifier extends EngineAssignedModifier<NativeBlockState>
         return resolveCaveBiome(caveBiomeCache, worldX, y, worldZ, resolverState);
     }
 
+    private boolean isNaturalTerrainOpening(IrisComplex complex, int x, int y, int z) {
+        if (complex.getTerrainTransform() == null) {
+            return complex.isTerrain3DOpening(x, y, z);
+        }
+        if (!complex.hasTerrain3D()) {
+            return false;
+        }
+        Terrain3DColumn column = complex.terrainColumn(x, z, complex.sampleHydrologyColumn(x, z));
+        return column != null && y >= column.minY() && y < column.topY() && !column.isSolid(y)
+                && !getEngine().isAdditionalTerrainOwned(x, y, z);
+    }
+
     /** The dimension's underground river bed palette, or null when nothing paints. */
     private IrisRiverMaterialConfig undergroundBedMaterial() {
         IrisHydrology hydrology = getDimension().getHydrology();

core/src/main/java/art/arcane/iris/generation/terrain/IrisDimension.java

@@ -39,6 +39,7 @@ import art.arcane.iris.generation.image.IrisImageMapBinding;
 import art.arcane.iris.generation.noise.IrisGeneratorStyle;
 import art.arcane.iris.generation.noise.IrisShapedGeneratorStyle;
 import art.arcane.iris.generation.noise.NoiseStyle;
+import art.arcane.iris.generation.terrain.transform.IrisTerrainTransform;
 import art.arcane.iris.pack.validation.CompatPools;
 import art.arcane.iris.pack.value.IrisRange;
 import art.arcane.iris.structure.nativegen.IrisImportedFeatureControl;
@@ -218,6 +219,8 @@ public class IrisDimension extends IrisRegistrant {
     private IrisHydrology hydrology = new IrisHydrology();
     @Description("Dimension-level river placement, routing, profile, biome, and geometry policy.")
     private IrisRiverPolicy riverPolicy = new IrisRiverPolicy();
+    @Description("Optional external terrain transformation with an exact provider version and pack-owned settings.")
+    private IrisTerrainTransform terrainTransform;
     @Description("Refuse to place surface objects and trees over carved surface openings.")
     private boolean requireObjectSurfaceSupport = true;
     @MinNumber(0)

core/src/main/java/art/arcane/iris/generation/terrain/Terrain3DColumn.java

@@ -1,6 +1,7 @@
 package art.arcane.iris.generation.terrain;
 
 import java.util.Arrays;
+import java.util.function.IntPredicate;
 
 public final class Terrain3DColumn {
     private final double baseHeight;
@@ -20,6 +21,29 @@ public final class Terrain3DColumn {
         return new Terrain3DColumn(baseHeight, top + 1, false, new int[]{0, top});
     }
 
+    public static Terrain3DColumn fromOccupancy(double baseHeight, int height, IntPredicate solid) {
+        if (height < 1) {
+            throw new IllegalArgumentException("Terrain column height must be positive");
+        }
+        int[] spans = new int[height + 1];
+        int count = 0;
+        boolean occupied = false;
+        for (int y = 0; y < height; y++) {
+            boolean next = solid.test(y);
+            if (next && !occupied) {
+                spans[count++] = y;
+            } else if (!next && occupied) {
+                spans[count++] = y - 1;
+            }
+            occupied = next;
+        }
+        if (occupied) {
+            spans[count++] = height - 1;
+        }
+        return new Terrain3DColumn(baseHeight, 0, true,
+                count == 0 ? new int[]{0, -1} : Arrays.copyOf(spans, count));
+    }
+
     public double baseHeight() {
         return baseHeight;
     }

core/src/main/java/art/arcane/iris/generation/terrain/transform/IrisTerrainTransform.java

@@ -0,0 +1,25 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import art.arcane.iris.pack.schema.annotation.Required;
+import art.arcane.volmlib.util.collection.KMap;
+import art.arcane.volmlib.util.documentation.Description;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import lombok.experimental.Accessors;
+
+@Accessors(chain = true)
+@NoArgsConstructor
+@Data
+@Description("An external terrain transformer with its exact implementation version and generation settings.")
+public class IrisTerrainTransform {
+    @Required
+    @Description("The installed terrain transform provider identifier.")
+    private String id;
+
+    @Required
+    @Description("The exact provider implementation version required to generate this dimension.")
+    private String version;
+
+    @Description("Provider settings saved with the generation pack. Values are strings interpreted by the provider.")
+    private KMap<String, String> settings = new KMap<>();
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainColumnTransformer.java

@@ -0,0 +1,6 @@
+package art.arcane.iris.generation.terrain.transform;
+
+@FunctionalInterface
+public interface TerrainColumnTransformer {
+    void transform(TerrainTransformContext context, int localX, int localZ);
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainTransformContext.java

@@ -0,0 +1,17 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
+
+public interface TerrainTransformContext {
+    int blockX();
+
+    int blockZ();
+
+    int minY();
+
+    int height();
+
+    NativeBlockState original(int worldX, int worldY, int worldZ);
+
+    void set(int localX, int localY, int localZ, NativeBlockState state);
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainTransformProvider.java

@@ -0,0 +1,11 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import java.util.Map;
+
+public interface TerrainTransformProvider {
+    String id();
+
+    String version();
+
+    TerrainTransformer create(Map<String, String> settings);
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainTransformRegistry.java

@@ -0,0 +1,62 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+import java.util.ServiceLoader;
+
+public final class TerrainTransformRegistry {
+    private final Map<ProviderIdentity, TerrainTransformProvider> providers;
+
+    public TerrainTransformRegistry(Iterable<TerrainTransformProvider> providers) {
+        Map<ProviderIdentity, TerrainTransformProvider> indexed = new HashMap<>();
+        for (TerrainTransformProvider provider : Objects.requireNonNull(providers, "terrain transform providers")) {
+            Objects.requireNonNull(provider, "terrain transform provider");
+            ProviderIdentity identity = new ProviderIdentity(provider.id(), provider.version());
+            if (indexed.putIfAbsent(identity, provider) != null) {
+                throw new IllegalArgumentException("Duplicate terrain transform provider " + identity + ".");
+            }
+        }
+        this.providers = Map.copyOf(indexed);
+    }
+
+    public static TerrainTransformRegistry discover() {
+        return new TerrainTransformRegistry(ServiceLoader.load(
+                TerrainTransformProvider.class, TerrainTransformProvider.class.getClassLoader()));
+    }
+
+    public TerrainTransformer resolve(IrisTerrainTransform descriptor) {
+        if (descriptor == null) {
+            return null;
+        }
+        ProviderIdentity identity = new ProviderIdentity(descriptor.getId(), descriptor.getVersion());
+        TerrainTransformProvider provider = providers.get(identity);
+        if (provider == null) {
+            throw new IllegalStateException("Required terrain transform provider " + identity + " is unavailable.");
+        }
+        Map<String, String> settings = Map.copyOf(Objects.requireNonNull(
+                descriptor.getSettings(), "terrain transform settings"));
+        TerrainTransformer transformer = Objects.requireNonNull(
+                provider.create(settings), "terrain transformer returned by " + identity);
+        int radius = transformer.radius();
+        if (radius < 0 || radius > 16) {
+            throw new IllegalArgumentException("Terrain transform provider " + identity
+                    + " requires radius " + radius + "; the supported range is 0 through 16 blocks.");
+        }
+        return transformer;
+    }
+
+    private static String requireIdentity(String value, String label) {
+        if (value == null || value.isBlank()) {
+            throw new IllegalArgumentException(label + " must not be blank.");
+        }
+        return value;
+    }
+
+    private record ProviderIdentity(String id, String version) {
+        private ProviderIdentity {
+            id = requireIdentity(id, "Terrain transform provider ID");
+            version = requireIdentity(version, "Terrain transform provider version");
+        }
+    }
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainTransformRuntime.java

@@ -0,0 +1,506 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import art.arcane.iris.generation.block.B;
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.mantle.CaveTerrainSnapshot;
+import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.IrisComplex;
+import art.arcane.iris.generation.terrain.Terrain3DColumn;
+import art.arcane.iris.world.history.FloatingBiomeOverlay;
+import art.arcane.volmlib.nativelib.terrain.NativeBiome;
+import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
+import art.arcane.volmlib.util.hunk.Hunk;
+import art.arcane.volmlib.util.hunk.storage.StorageHunk;
+import com.github.benmanes.caffeine.cache.Cache;
+import com.github.benmanes.caffeine.cache.Caffeine;
+
+import java.util.Arrays;
+import java.util.Objects;
+
+public final class TerrainTransformRuntime implements AutoCloseable {
+    private static final long CACHE_BYTES = 64L * 1024 * 1024;
+
+    private final Engine engine;
+    private final IrisComplex complex;
+    private final TerrainTransformer transformer;
+    private final TerrainColumnTransformer columnTransformer;
+    private final int radius;
+    private final int minY;
+    private final int height;
+    private final NativeBlockState air;
+    private final Cache<Long, Snapshot> originals;
+    private final Cache<Long, Snapshot> transformed;
+    private final Cache<Long, ColumnSnapshot> transformedColumns;
+    private volatile boolean closed;
+
+    public TerrainTransformRuntime(Options options, TerrainTransformer transformer) {
+        engine = Objects.requireNonNull(options, "terrain runtime options").engine();
+        complex = options.complex();
+        this.transformer = Objects.requireNonNull(transformer, "terrain transformer");
+        columnTransformer = transformer.columnTransformer();
+        radius = transformer.radius();
+        if (radius < 0 || radius > 16) {
+            throw new IllegalArgumentException("Terrain transform radius must be between 0 and 16 blocks");
+        }
+        minY = engine.getMinHeight();
+        height = engine.getHeight();
+        if (height <= 0) {
+            throw new IllegalArgumentException("Terrain height must be positive");
+        }
+        air = Objects.requireNonNull(B.getState("AIR"), "air block state");
+        originals = newCache();
+        transformed = newCache();
+        transformedColumns = columnTransformer == null ? null : Caffeine.newBuilder().maximumWeight(CACHE_BYTES)
+                .weigher((Long key, ColumnSnapshot snapshot) -> snapshot.weight()).build();
+    }
+
+    public void generate(int x, int z, Hunk<NativeBlockState> blocks, Hunk<NativeBiome> biomes,
+                         boolean multicore, ChunkContext target) {
+        requireOpen();
+        if ((x & 15) != 0 || (z & 15) != 0 || blocks.getWidth() != 16 || blocks.getDepth() != 16
+                || blocks.getHeight() != height || biomes.getWidth() != 16 || biomes.getDepth() != 16
+                || biomes.getHeight() != height || target.getComplex() != complex) {
+            throw new IllegalArgumentException("Terrain transform output must match its runtime and chunk");
+        }
+        Snapshot snapshot = transformed(x >> 4, z >> 4);
+        target.setTerrainBiomeOutput(biomes);
+        for (int y = 0; y < height; y++) {
+            for (int localZ = 0; localZ < 16; localZ++) {
+                for (int localX = 0; localX < 16; localX++) {
+                    int index = index(localX, y, localZ);
+                    blocks.setRaw(localX, y, localZ, snapshot.blocks[index]);
+                    NativeBiome biome = snapshot.biomes[index];
+                    if (biome != null) {
+                        biomes.setRaw(localX, y, localZ, biome);
+                    }
+                }
+            }
+        }
+        for (int localZ = 0; localZ < 16; localZ++) {
+            for (int localX = 0; localX < 16; localX++) {
+                target.setTerrainHeight(localX, localZ, snapshot.solidHeights[(localZ << 4) | localX]);
+            }
+        }
+        target.setCaveTerrain(snapshot.caves);
+        target.setFloatingBiomes(snapshot.floating == null ? null : snapshot.floating.copy());
+    }
+
+    public int height(int x, int z, boolean ignoreFluid) {
+        requireOpen();
+        Snapshot snapshot = fullForQuery(x, z);
+        if (snapshot != null) {
+            return snapshot.height(x & 15, z & 15, ignoreFluid);
+        }
+        ColumnSnapshot column = transformedColumn(x, z);
+        return ignoreFluid ? column.ground : column.top;
+    }
+
+    public int readyHeight(int x, int z, boolean ignoreFluid) {
+        requireOpen();
+        Snapshot snapshot = transformed.getIfPresent(key(x >> 4, z >> 4));
+        if (snapshot != null) {
+            return snapshot.height(x & 15, z & 15, ignoreFluid);
+        }
+        ColumnSnapshot column = readyTransformedColumn(x, z);
+        return column == null ? Integer.MIN_VALUE : ignoreFluid ? column.ground : column.top;
+    }
+
+    public NativeBlockState block(int worldX, int localY, int worldZ) {
+        requireOpen();
+        if (localY < 0 || localY >= height) {
+            return air;
+        }
+        return transformed(worldX >> 4, worldZ >> 4).blocks[index(worldX & 15, localY, worldZ & 15)];
+    }
+
+    public NativeBlockState readyBlock(int worldX, int localY, int worldZ) {
+        requireOpen();
+        if (localY < 0 || localY >= height) {
+            return air;
+        }
+        Snapshot snapshot = transformed.getIfPresent(key(worldX >> 4, worldZ >> 4));
+        return snapshot == null ? null : snapshot.blocks[index(worldX & 15, localY, worldZ & 15)];
+    }
+
+    public Terrain3DColumn column(int x, int z) {
+        requireOpen();
+        Snapshot snapshot = fullForQuery(x, z);
+        return snapshot == null ? transformedColumn(x, z).terrain : snapshot.column(x & 15, z & 15);
+    }
+
+    public Terrain3DColumn readyColumn(int x, int z) {
+        requireOpen();
+        Snapshot snapshot = transformed.getIfPresent(key(x >> 4, z >> 4));
+        if (snapshot != null) {
+            return snapshot.column(x & 15, z & 15);
+        }
+        ColumnSnapshot column = readyTransformedColumn(x, z);
+        return column == null ? null : column.terrain;
+    }
+
+    public int fluidHeight(int x, int z) {
+        requireOpen();
+        Snapshot snapshot = fullForQuery(x, z);
+        return snapshot == null ? transformedColumn(x, z).fluid : snapshot.fluidHeights[((z & 15) << 4) | (x & 15)];
+    }
+
+    public int readyFluidHeight(int x, int z) {
+        requireOpen();
+        Snapshot snapshot = transformed.getIfPresent(key(x >> 4, z >> 4));
+        if (snapshot != null) {
+            return snapshot.fluidHeights[((z & 15) << 4) | (x & 15)];
+        }
+        ColumnSnapshot column = readyTransformedColumn(x, z);
+        return column == null ? Integer.MIN_VALUE : column.fluid;
+    }
+
+    @Override
+    public void close() {
+        closed = true;
+        originals.invalidateAll();
+        transformed.invalidateAll();
+        originals.cleanUp();
+        transformed.cleanUp();
+        if (transformedColumns != null) {
+            transformedColumns.invalidateAll();
+            transformedColumns.cleanUp();
+        }
+    }
+
+    private Cache<Long, Snapshot> newCache() {
+        return Caffeine.newBuilder().maximumWeight(CACHE_BYTES)
+                .weigher((Long key, Snapshot snapshot) -> snapshot.weight()).build();
+    }
+
+    private Snapshot transformed(int chunkX, int chunkZ) {
+        Snapshot snapshot = transformed.get(key(chunkX, chunkZ), ignored -> transform(chunkX, chunkZ));
+        if (closed) {
+            transformed.invalidateAll();
+            requireOpen();
+        }
+        return snapshot;
+    }
+
+    private Snapshot original(int chunkX, int chunkZ) {
+        Snapshot snapshot = originals.get(key(chunkX, chunkZ), ignored -> materialize(chunkX, chunkZ));
+        if (closed) {
+            originals.invalidateAll();
+            requireOpen();
+        }
+        return snapshot;
+    }
+
+    private Snapshot fullForQuery(int x, int z) {
+        return columnTransformer == null ? transformed(x >> 4, z >> 4)
+                : transformed.getIfPresent(key(x >> 4, z >> 4));
+    }
+
+    private ColumnSnapshot readyTransformedColumn(int x, int z) {
+        return transformedColumns == null ? null : transformedColumns.getIfPresent(key(x, z));
+    }
+
+    private ColumnSnapshot transformedColumn(int x, int z) {
+        ColumnSnapshot snapshot = transformedColumns.get(key(x, z), ignored -> transformColumn(x, z));
+        if (closed) {
+            transformedColumns.invalidateAll();
+            requireOpen();
+        }
+        return snapshot;
+    }
+
+    private Snapshot materialize(int chunkX, int chunkZ) {
+        requireOpen();
+        int x = chunkX << 4;
+        int z = chunkZ << 4;
+        NativeBlockState[] states = new NativeBlockState[Math.multiplyExact(256, height)];
+        Arrays.fill(states, air);
+        NativeBiome[] biomeStates = new NativeBiome[states.length];
+        Hunk<NativeBlockState> blocks = new SnapshotHunk<>(states, air);
+        Hunk<NativeBiome> biomes = new SnapshotHunk<>(biomeStates, null);
+        long session = engine.getGenerationSessionId();
+        ChunkContext context = new ChunkContext(x, z, complex, session, true,
+                ChunkContext.PrefillPlan.NATURAL_TERRAIN, engine.getMetrics(), engine.getDimensionStackContext());
+        try (IrisContext.Scope scope = IrisContext.open(engine, session, context)) {
+            engine.getMode().generateTerrain(x, z, blocks, biomes, false, context);
+        }
+        FloatingBiomeOverlay floating = context.getFloatingBiomes();
+        return new Snapshot(new SnapshotData(states, biomeStates, context.getCaveTerrain(),
+                floating == null ? null : floating.copy()), false);
+    }
+
+    private Snapshot transform(int chunkX, int chunkZ) {
+        requireOpen();
+        Snapshot source = original(chunkX, chunkZ);
+        TransformContext context = new TransformContext(new TransformTarget(chunkX, chunkZ, false, 0, 0), source);
+        try {
+            transformer.transform(context);
+        } finally {
+            context.active = false;
+        }
+        return new Snapshot(new SnapshotData(context.blocks, source.biomes, source.caves, source.floating), true);
+    }
+
+    private ColumnSnapshot transformColumn(int x, int z) {
+        requireOpen();
+        Snapshot source = original(x >> 4, z >> 4);
+        TransformContext context = new TransformContext(
+                new TransformTarget(x >> 4, z >> 4, true, x & 15, z & 15), source);
+        try {
+            columnTransformer.transform(context, x & 15, z & 15);
+        } finally {
+            context.active = false;
+        }
+        return new ColumnSnapshot(context.blocks);
+    }
+
+    private void requireOpen() {
+        if (closed) {
+            throw new IllegalStateException("Terrain transform runtime is closed");
+        }
+    }
+
+    private static int index(int x, int y, int z) {
+        return (y << 8) | (z << 4) | x;
+    }
+
+    private static long key(int chunkX, int chunkZ) {
+        return ((long) chunkX << 32) | (chunkZ & 0xffffffffL);
+    }
+
+    public record Options(Engine engine, IrisComplex complex) {
+        public Options {
+            Objects.requireNonNull(engine, "terrain engine");
+            Objects.requireNonNull(complex, "terrain complex");
+        }
+    }
+
+    private record TransformTarget(int chunkX, int chunkZ, boolean columnOnly, int localX, int localZ) {
+    }
+
+    private record SnapshotData(NativeBlockState[] blocks, NativeBiome[] biomes,
+                                CaveTerrainSnapshot caves, FloatingBiomeOverlay floating) {
+    }
+
+    private static final class SnapshotHunk<T> extends StorageHunk<T> {
+        private final T[] values;
+        private final T fallback;
+
+        private SnapshotHunk(T[] values, T fallback) {
+            super(16, values.length / 256, 16);
+            this.values = values;
+            this.fallback = fallback;
+        }
+
+        @Override
+        public T getRaw(int x, int y, int z) {
+            return y < 0 || y >= getHeight() ? fallback : values[index(x, y, z)];
+        }
+
+        @Override
+        public void setRaw(int x, int y, int z, T value) {
+            if (value != null) {
+                values[index(x, y, z)] = value;
+            }
+        }
+    }
+
+    private final class TransformContext implements TerrainTransformContext {
+        private final int x;
+        private final int z;
+        private final NativeBlockState[] blocks;
+        private final TransformTarget target;
+        private final Snapshot[] sources = new Snapshot[9];
+        private volatile boolean active = true;
+
+        private TransformContext(TransformTarget target, Snapshot source) {
+            this.target = target;
+            x = target.chunkX() << 4;
+            z = target.chunkZ() << 4;
+            if (target.columnOnly()) {
+                blocks = new NativeBlockState[height];
+                for (int y = 0; y < height; y++) {
+                    blocks[y] = source.blocks[index(target.localX(), y, target.localZ())];
+                }
+            } else {
+                blocks = source.blocks.clone();
+            }
+            sources[4] = source;
+        }
+
+        @Override
+        public int blockX() {
+            return x;
+        }
+
+        @Override
+        public int blockZ() {
+            return z;
+        }
+
+        @Override
+        public int minY() {
+            return minY;
+        }
+
+        @Override
+        public int height() {
+            return height;
+        }
+
+        @Override
+        public NativeBlockState original(int worldX, int worldY, int worldZ) {
+            requireActive();
+            long localX = (long) worldX - x;
+            long localZ = (long) worldZ - z;
+            if (localX < -radius || localX >= 16L + radius || localZ < -radius || localZ >= 16L + radius) {
+                throw new IllegalArgumentException("Terrain transform read exceeds its declared radius");
+            }
+            long localY = (long) worldY - minY;
+            if (localY < 0 || localY >= height) {
+                return air;
+            }
+            int sourceX = worldX >> 4;
+            int sourceZ = worldZ >> 4;
+            int slot = (sourceZ - (z >> 4) + 1) * 3 + sourceX - (x >> 4) + 1;
+            Snapshot snapshot = sources[slot];
+            if (snapshot == null) {
+                snapshot = TerrainTransformRuntime.this.original(sourceX, sourceZ);
+                sources[slot] = snapshot;
+            }
+            return snapshot.blocks[index(worldX & 15, (int) localY, worldZ & 15)];
+        }
+
+        @Override
+        public void set(int localX, int localY, int localZ, NativeBlockState state) {
+            requireActive();
+            Objects.requireNonNull(state, "terrain transform block state");
+            if (localX < 0 || localX >= 16 || localZ < 0 || localZ >= 16 || localY < 0 || localY >= height) {
+                throw new IllegalArgumentException("Terrain transform write exceeds its output chunk");
+            }
+            if (target.columnOnly() && (localX != target.localX() || localZ != target.localZ())) {
+                throw new IllegalArgumentException("Terrain column transform write exceeds its output column");
+            }
+            if (complex.allowsMantleWrite(x + localX, z + localZ)) {
+                blocks[target.columnOnly() ? localY : index(localX, localY, localZ)] = state;
+            }
+        }
+
+        private void requireActive() {
+            requireOpen();
+            if (!active) {
+                throw new IllegalStateException("Terrain transform context is no longer active");
+            }
+        }
+    }
+
+    private static final class ColumnSnapshot {
+        private final Terrain3DColumn terrain;
+        private final int ground;
+        private final int top;
+        private final int fluid;
+
+        private ColumnSnapshot(NativeBlockState[] blocks) {
+            int solidHeight = -1;
+            int topHeight = -1;
+            int fluidHeight = -1;
+            for (int y = blocks.length - 1; y >= 0; y--) {
+                NativeBlockState state = blocks[y];
+                if (state.isAir()) {
+                    continue;
+                }
+                if (topHeight == -1) {
+                    topHeight = y;
+                }
+                if ((state.isFluid() || state.isWaterLogged()) && fluidHeight == -1) {
+                    fluidHeight = y;
+                }
+                if (!state.isFluid() && solidHeight == -1) {
+                    solidHeight = y;
+                }
+            }
+            ground = solidHeight;
+            top = topHeight;
+            fluid = fluidHeight;
+            terrain = Terrain3DColumn.fromOccupancy(ground, blocks.length,
+                    y -> !blocks[y].isAir() && !blocks[y].isFluid());
+        }
+
+        private int weight() {
+            return 192 + terrain.spanCount() * 8;
+        }
+    }
+
+    private static final class Snapshot {
+        private final NativeBlockState[] blocks;
+        private final NativeBiome[] biomes;
+        private final CaveTerrainSnapshot caves;
+        private final FloatingBiomeOverlay floating;
+        private final int[] solidHeights;
+        private final int[] topHeights;
+        private final int[] fluidHeights;
+        private final Terrain3DColumn[] columns;
+
+        private Snapshot(SnapshotData data, boolean effective) {
+            blocks = data.blocks();
+            biomes = data.biomes();
+            caves = data.caves();
+            floating = data.floating();
+            solidHeights = effective ? new int[256] : null;
+            topHeights = effective ? new int[256] : null;
+            fluidHeights = effective ? new int[256] : null;
+            columns = effective ? new Terrain3DColumn[256] : null;
+            if (!effective) {
+                return;
+            }
+            Arrays.fill(solidHeights, -1);
+            Arrays.fill(topHeights, -1);
+            Arrays.fill(fluidHeights, -1);
+            int height = blocks.length / 256;
+            for (int z = 0; z < 16; z++) {
+                for (int x = 0; x < 16; x++) {
+                    int column = (z << 4) | x;
+                    for (int y = height - 1; y >= 0; y--) {
+                        NativeBlockState state = blocks[index(x, y, z)];
+                        if (state.isAir()) {
+                            continue;
+                        }
+                        if (topHeights[column] == -1) {
+                            topHeights[column] = y;
+                        }
+                        if ((state.isFluid() || state.isWaterLogged()) && fluidHeights[column] == -1) {
+                            fluidHeights[column] = y;
+                        }
+                        if (!state.isFluid() && solidHeights[column] == -1) {
+                            solidHeights[column] = y;
+                        }
+                    }
+                }
+            }
+        }
+
+        private int height(int x, int z, boolean ignoreFluid) {
+            return (ignoreFluid ? solidHeights : topHeights)[(z << 4) | x];
+        }
+
+        private Terrain3DColumn column(int x, int z) {
+            int index = (z << 4) | x;
+            Terrain3DColumn column = columns[index];
+            if (column == null) {
+                column = Terrain3DColumn.fromOccupancy(solidHeights[index], blocks.length / 256, y -> {
+                    NativeBlockState state = blocks[index(x, y, z)];
+                    return !state.isAir() && !state.isFluid();
+                });
+                columns[index] = column;
+            }
+            return column;
+        }
+
+        private int weight() {
+            long blockReferences = (long) blocks.length * 20L;
+            long caveReferences = caves == null ? 0L : caves.estimatedRetainedBytes();
+            long floatingReferences = floating == null ? 0L : (long) blocks.length * 4L;
+            return (int) Math.min(Integer.MAX_VALUE, 16384L + blockReferences + caveReferences + floatingReferences);
+        }
+    }
+}

core/src/main/java/art/arcane/iris/generation/terrain/transform/TerrainTransformer.java

@@ -0,0 +1,11 @@
+package art.arcane.iris.generation.terrain.transform;
+
+public interface TerrainTransformer {
+    int radius();
+
+    void transform(TerrainTransformContext context);
+
+    default TerrainColumnTransformer columnTransformer() {
+        return null;
+    }
+}

core/src/main/java/art/arcane/iris/world/history/FloatingBiomeOverlay.java

@@ -64,6 +64,13 @@ public final class FloatingBiomeOverlay {
         return height;
     }
 
+    public FloatingBiomeOverlay copy() {
+        FloatingBiomeOverlay copy = new FloatingBiomeOverlay(height);
+        System.arraycopy(volume, 0, copy.volume, 0, volume.length);
+        System.arraycopy(surfaces, 0, copy.surfaces, 0, surfaces.length);
+        return copy;
+    }
+
     private int index(int localX, int y, int localZ) {
         return ((localX >> 2) * 4 + (localZ >> 2)) * quartHeight + (y >> 2);
     }

core/src/test/java/art/arcane/iris/generation/mantle/CaveTerrainSnapshotTest.java

@@ -81,6 +81,23 @@ public class CaveTerrainSnapshotTest {
         assertNull(snapshot.cavern(4, 22, 6));
     }
 
+    @Test
+    public void retainedEstimateCountsOnlyAllocatedSectionsAndTheirCells() {
+        Matter[] sections = new Matter[SECTIONS];
+        long emptyBytes = CaveTerrainSnapshot.capture(chunk(sections), 0, 0).estimatedRetainedBytes();
+        assertTrue(emptyBytes < 1024);
+        sections[1] = new IrisMatter(16, 16, 16);
+        sections[1].slice(MatterCavern.class).set(0, 0, 0, new MatterCavern(true, "cave", (byte) 0));
+        CaveTerrainSnapshot sparse = CaveTerrainSnapshot.capture(chunk(sections), 0, 0);
+        long sparseBytes = sparse.estimatedRetainedBytes();
+        assertTrue(sparseBytes >= emptyBytes + 4096L * 8L);
+        assertTrue(sparseBytes < emptyBytes + 4096L * 16L);
+        sections[1].slice(MatterCavern.class).set(1, 0, 0, new MatterCavern(true, "other", (byte) 0));
+        long moreCells = CaveTerrainSnapshot.capture(chunk(sections), 0, 0).estimatedRetainedBytes();
+        assertTrue(moreCells > sparseBytes);
+        assertEquals(sparseBytes, sparse.estimatedRetainedBytes());
+    }
+
     private static Map<String, String> legacyResolverInputs(MantleChunk<Matter> chunk) {
         Map<String, String> inputs = new HashMap<>();
         for (int x = 0; x < 16; x++) {

core/src/test/java/art/arcane/iris/generation/mode/ModeOverworldTerrainTransformTest.java

@@ -0,0 +1,67 @@
+package art.arcane.iris.generation.mode;
+
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.mantle.EngineMantle;
+import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.IrisComplex;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRuntime;
+import org.junit.Test;
+
+import static org.junit.Assert.assertSame;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+public class ModeOverworldTerrainTransformTest {
+    @Test
+    public void studioRawSnapshotsKeepTerrainMatterOnTheBoundCallingThread() {
+        Fixture fixture = new Fixture();
+        doReturn(mock(TerrainTransformRuntime.class)).when(fixture.complex).getTerrainTransform();
+        Thread owner = Thread.currentThread();
+        doAnswer(invocation -> {
+            assertSame(owner, Thread.currentThread());
+            assertSame(fixture.context, IrisContext.require().getChunkContext());
+            return null;
+        }).when(fixture.mantle).generateTerrainMatter(eq(-2), eq(3), anyBoolean(), eq(fixture.context));
+        try (IrisContext.Scope scope = IrisContext.open(fixture.engine, 41L, fixture.context)) {
+            fixture.mode.generateTerrainMatterForChunk(-2, 3, false, fixture.context);
+        }
+        verify(fixture.mantle).generateTerrainMatter(-2, 3, false, fixture.context);
+    }
+
+    @Test
+    public void ordinaryStudioTerrainStillUsesParallelMatter() {
+        Fixture fixture = new Fixture();
+        fixture.mode.generateTerrainMatterForChunk(-2, 3, false, fixture.context);
+        verify(fixture.mantle).generateTerrainMatter(-2, 3, true, fixture.context);
+    }
+
+    @Test
+    public void explicitParallelGenerationRemainsParallelWithATransformer() {
+        Fixture fixture = new Fixture();
+        doReturn(mock(TerrainTransformRuntime.class)).when(fixture.complex).getTerrainTransform();
+        fixture.mode.generateTerrainMatterForChunk(-2, 3, true, fixture.context);
+        verify(fixture.mantle).generateTerrainMatter(-2, 3, true, fixture.context);
+    }
+
+    private static final class Fixture {
+        private final ModeOverworld mode = mock(ModeOverworld.class, CALLS_REAL_METHODS);
+        private final Engine engine = mock(Engine.class);
+        private final EngineMantle mantle = mock(EngineMantle.class);
+        private final IrisComplex complex = mock(IrisComplex.class);
+        private final ChunkContext context = mock(ChunkContext.class);
+
+        private Fixture() {
+            doReturn(engine).when(mode).getEngine();
+            doReturn(mantle).when(engine).getMantle();
+            doReturn(true).when(engine).isStudio();
+            doReturn(complex).when(context).getComplex();
+            doReturn(true).when(context).isNaturalTerrain();
+        }
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/DimensionTerrainContextFallbackTest.java

@@ -1,6 +1,8 @@
 package art.arcane.iris.generation.runtime;
 
 import art.arcane.iris.generation.image.IrisImageMapRuntime;
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.context.IrisContext;
 import art.arcane.iris.generation.biome.IrisBiome;
 import art.arcane.iris.generation.terrain.IrisDimension;
 import art.arcane.iris.generation.terrain.IrisRegion;
@@ -54,10 +56,10 @@ public class DimensionTerrainContextFallbackTest {
         when(engine.getHeight()).thenReturn(128);
         when(dimension.getLoadKey()).thenReturn("root");
         when(complex.hasTerrain3D()).thenReturn(true);
-        when(complex.getSlopeStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 40D));
-        when(complex.terrainColumn(-17, 8)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 5, 20, 40));
-        when(complex.terrainColumn(-14, 8)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 8, 20, 60));
-        when(complex.terrainColumn(-17, 11)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 9, 20, 80));
+        when(complex.getRawSlopeStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 40D));
+        when(complex.originalTerrainColumn(-17, 8)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 5, 20, 40));
+        when(complex.originalTerrainColumn(-14, 8)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 8, 20, 60));
+        when(complex.originalTerrainColumn(-17, 11)).thenReturn(Terrain3DColumnFixtures.spans(20, 0, 9, 20, 80));
         DimensionTerrainContext context = DimensionTerrainContext.forStack(engine, dimension);
 
         assertEquals(5D, context.getSurfaceSlopeStream(5).getDouble(-17, 8), 0D);
@@ -90,7 +92,7 @@ public class DimensionTerrainContextFallbackTest {
         when(dimension.getLoadKey()).thenReturn("root");
         when(dimension.getFluidHeight()).thenReturn(12);
         when(complex.getNaturalHeightStream()).thenReturn(naturalHeight);
-        when(complex.getHeightStream()).thenReturn(resolvedHeight);
+        when(complex.getRawHeightStream()).thenReturn(resolvedHeight);
         when(complex.getRiverWaterSurfaceStream()).thenReturn(resolvedFluidHeight);
         when(complex.getNaturalTrueBiomeStream()).thenReturn(naturalBiome);
         when(complex.getTrueBiomeStream()).thenReturn(resolvedBiome);
@@ -132,7 +134,7 @@ public class DimensionTerrainContextFallbackTest {
         Terrain3DColumn natural = Terrain3DColumnFixtures.spans(30, 0, 15, 25, 40);
         Terrain3DColumn resolved = Terrain3DColumnFixtures.spans(30, 0, 12, 28, 35);
         when(complex.naturalTerrainColumn(-17, 8)).thenReturn(natural);
-        when(complex.terrainColumn(-17, 8)).thenReturn(resolved);
+        when(complex.originalTerrainColumn(-17, 8)).thenReturn(resolved);
         DimensionTerrainContext context = DimensionTerrainContext.forStack(engine, dimension);
 
         assertSame(resolved, context.terrainColumn(-17, 8));
@@ -141,4 +143,34 @@ public class DimensionTerrainContextFallbackTest {
         when(engine.answersFromNaturalTerrain(-17, 8)).thenReturn(false);
         assertSame(resolved, context.terrainColumn(-17, 8));
     }
+
+    @Test
+    public void stackInputsRemainOriginalWhenTheCallingChunkBeginsContent() {
+        Engine engine = mock(Engine.class);
+        IrisDimension dimension = mock(IrisDimension.class);
+        IrisComplex complex = mock(IrisComplex.class);
+        Terrain3DColumn original = Terrain3DColumnFixtures.spans(30, 0, 12, 28, 35);
+        when(engine.getDimension()).thenReturn(dimension);
+        when(engine.getComplex()).thenReturn(complex);
+        when(engine.getHeight()).thenReturn(128);
+        when(dimension.getLoadKey()).thenReturn("root");
+        when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 35D));
+        when(complex.getRawSlopeStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 4D));
+        when(complex.originalTerrainColumn(0, 0)).thenReturn(original);
+        when(complex.getHeightStream()).thenThrow(new AssertionError("Layout read final terrain height"));
+        when(complex.getSlopeStream()).thenThrow(new AssertionError("Layout read final terrain slope"));
+        when(complex.terrainColumn(0, 0)).thenThrow(new AssertionError("Layout read final terrain occupancy"));
+        DimensionTerrainContext terrain = DimensionTerrainContext.forStack(engine, dimension);
+        ChunkContext chunk = new ChunkContext(0, 0, complex, 7L, false, ChunkContext.PrefillPlan.NONE, null);
+
+        try (IrisContext.Scope ignored = IrisContext.open(engine, 7L, chunk)) {
+            assertEquals(35D, terrain.getNormalTerrainHeight(0, 0), 0D);
+            assertEquals(4D, terrain.getSlopeStream().getDouble(0, 0), 0D);
+            assertSame(original, terrain.terrainColumn(0, 0));
+            chunk.beginContent();
+            assertEquals(35D, terrain.getNormalTerrainHeight(0, 0), 0D);
+            assertEquals(4D, terrain.getSlopeStream().getDouble(0, 0), 0D);
+            assertSame(original, terrain.terrainColumn(0, 0));
+        }
+    }
 }

core/src/test/java/art/arcane/iris/generation/runtime/IrisComplexTerrainTransformTest.java

@@ -0,0 +1,341 @@
+package art.arcane.iris.generation.runtime;
+
+import art.arcane.iris.configuration.IrisSettings;
+import art.arcane.iris.generation.block.B;
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.terrain.IrisDimension;
+import art.arcane.iris.generation.terrain.transform.IrisTerrainTransform;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRegistry;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRuntime;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformer;
+import art.arcane.iris.pack.loading.IrisData;
+import art.arcane.iris.spi.IrisPlatform;
+import art.arcane.iris.spi.IrisPlatforms;
+import art.arcane.iris.spi.IrisServices;
+import art.arcane.iris.testsupport.PlatformLeakGuard;
+import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
+import art.arcane.volmlib.nativelib.terrain.NativeBiome;
+import art.arcane.volmlib.util.collection.KList;
+import art.arcane.volmlib.util.hunk.Hunk;
+import art.arcane.volmlib.util.stream.ProceduralStream;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.ClassRule;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+import org.mockito.MockedConstruction;
+import org.mockito.MockedStatic;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Answers.CALLS_REAL_METHODS;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.doReturn;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockConstruction;
+import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.when;
+
+public class IrisComplexTerrainTransformTest {
+    @ClassRule public static final PlatformLeakGuard PLATFORM_GUARD = PlatformLeakGuard.clean();
+    @Rule public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+    private IrisSettings previousSettings;
+
+    @Before
+    public void prepareRuntime() {
+        previousSettings = IrisSettings.settings;
+        IrisSettings.settings = new IrisSettings();
+        IrisServices.register(PreservationRegistry.class, mock(PreservationRegistry.class));
+        IrisPlatform platform = mock(IrisPlatform.class);
+        when(platform.platformName()).thenReturn("bukkit");
+        IrisPlatforms.bind(platform);
+    }
+
+    @After
+    public void releaseRuntime() {
+        IrisPlatforms.unbind();
+        IrisServices.remove(PreservationRegistry.class);
+        IrisSettings.settings = previousSettings;
+    }
+
+    @Test
+    public void absentProviderRetainsTheOriginalStreamsAndStackHeights() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), false)) {
+            ProceduralStream<Double> heights = ProceduralStream.ofDouble((x, z) -> 80D);
+            ProceduralStream<Integer> rounded = heights.round();
+            ProceduralStream<Double> tops = ProceduralStream.ofDouble((x, z) -> 90D);
+            ProceduralStream<Double> slopes = ProceduralStream.ofDouble((x, z) -> 4D);
+            fixture.complex.setHeightStream(heights);
+            fixture.complex.setRoundedHeighteightStream(rounded);
+            fixture.complex.setHeightFluidStream(tops);
+            fixture.complex.setSlopeStream(slopes);
+
+            assertNull(fixture.complex.getTerrainTransform());
+            assertSame(heights, fixture.complex.getHeightStream());
+            assertSame(rounded, fixture.complex.getRoundedHeighteightStream());
+            assertSame(tops, fixture.complex.getHeightFluidStream());
+            assertSame(slopes, fixture.complex.getSlopeStream());
+            assertSame(slopes, fixture.complex.getRawSlopeStream());
+            fixture.installStack();
+            assertEquals(40, fixture.engine.getHeight(0, 0, true));
+            assertEquals(50, fixture.engine.getHeight(0, 0, false));
+        }
+    }
+
+    @Test
+    public void capturedStreamsSwitchAtSamplingTimeWhenContentBegins() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.complex.setHeightStream(ProceduralStream.ofDouble((x, z) -> 80D + 2D * x + 3D * z));
+            fixture.complex.setRoundedHeighteightStream(ProceduralStream.ofDouble((x, z) -> 81D).round());
+            fixture.complex.setHeightFluidStream(ProceduralStream.ofDouble((x, z) -> 90D));
+            fixture.complex.setSlopeStream(ProceduralStream.ofDouble((x, z) -> 9D));
+            ProceduralStream<Double> heights = fixture.complex.getHeightStream();
+            ProceduralStream<Integer> rounded = fixture.complex.getRoundedHeighteightStream();
+            ProceduralStream<Double> tops = fixture.complex.getHeightFluidStream();
+            ProceduralStream<Double> slopes = fixture.complex.getSlopeStream();
+            ChunkContext context = fixture.context();
+
+            try (IrisContext.Scope ignored = IrisContext.open(fixture.engine, 7L, context)) {
+                assertEquals(80D, heights.getDouble(0, 0), 0D);
+                assertEquals(Integer.valueOf(81), rounded.get(0, 0));
+                assertEquals(90D, tops.getDouble(0, 0), 0D);
+                assertEquals(9D, slopes.getDouble(0, 0), 0D);
+                assertEquals(0, fixture.materializations.get());
+
+                context.beginContent();
+
+                assertEquals(120D, heights.getDouble(0, 0), 0D);
+                assertEquals(Integer.valueOf(120), rounded.get(0, 0));
+                assertEquals(125D, tops.getDouble(0, 0), 0D);
+                assertEquals(0D, slopes.getDouble(0, 0), 0D);
+            }
+        }
+    }
+
+    @Test
+    public void absentProviderRunsTerrainTransitionAndContentInTheirOriginalOrder() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), false)) {
+            EngineMode mode = mock(EngineMode.class, CALLS_REAL_METHODS);
+            doReturn(fixture.engine).when(mode).getEngine();
+            doReturn(fixture.complex).when(mode).getComplex();
+            when(fixture.engine.getPlatformHooks().shouldDisableChunkContextCache(fixture.engine)).thenReturn(true);
+            List<String> stages = new ArrayList<>();
+            NativeBlockState terrain = mock(NativeBlockState.class);
+            NativeBlockState transitioned = mock(NativeBlockState.class);
+            EngineStage natural = (x, z, blocks, biomes, multicore, context) -> {
+                assertTrue(context.isNaturalTerrain());
+                blocks.set(0, 1, 0, terrain);
+                stages.add("terrain");
+            };
+            EngineStage transition = (x, z, blocks, biomes, multicore, context) -> {
+                assertTrue(context.isNaturalTerrain());
+                assertSame(terrain, blocks.get(0, 1, 0));
+                blocks.set(0, 1, 0, transitioned);
+                stages.add("transition");
+            };
+            EngineStage content = (x, z, blocks, biomes, multicore, context) -> {
+                assertFalse(context.isNaturalTerrain());
+                assertSame(transitioned, blocks.get(0, 1, 0));
+                stages.add("content");
+            };
+            doReturn(new KList<>(natural)).when(mode).getTerrainStages();
+            doReturn(transition).when(mode).getTransitionStage();
+            doReturn(new KList<>(content)).when(mode).getStages();
+            Hunk<NativeBlockState> blocks = Hunk.newArrayHunk(16, 8, 16);
+            Hunk<NativeBiome> biomes = Hunk.newArrayHunk(16, 8, 16);
+
+            mode.generate(0, 0, blocks, biomes, false, 7L);
+
+            assertEquals(List.of("terrain", "transition", "content"), stages);
+            assertSame(transitioned, blocks.get(0, 1, 0));
+        }
+    }
+
+    @Test
+    public void transformedHeightsOverrideTheStackOnlyOutsideNaturalGeneration() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.installStack();
+            assertEquals(120, fixture.engine.getHeight(0, 0, true));
+            assertEquals(125, fixture.engine.getHeight(0, 0, false));
+            assertEquals(120, Engine.hostHeight(fixture.engine, 0, 0, true));
+            assertEquals(125, Engine.hostHeight(fixture.engine, 0, 0, false));
+
+            try (IrisContext.Scope ignored = IrisContext.open(fixture.engine, 7L, fixture.context())) {
+                assertEquals(40, fixture.engine.getHeight(0, 0, true));
+                assertEquals(50, fixture.engine.getHeight(0, 0, false));
+            }
+        }
+    }
+
+    @Test
+    public void mainThreadColdQueriesUseRawHeightsWithoutMaterializingTerrain() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.mainThread.set(true);
+            doReturn(7L).when(fixture.engine).getGenerationSessionId();
+            fixture.complex.setHeightStream(ProceduralStream.ofDouble((x, z) -> 80D));
+            fixture.complex.setHeightFluidStream(ProceduralStream.ofDouble((x, z) -> 90D));
+            assertEquals(Integer.MIN_VALUE, fixture.complex.transformedHeight(0, 0, true));
+            assertEquals(80D, fixture.complex.getHeightStream().getDouble(0, 0), 0D);
+            assertEquals(90D, fixture.complex.getHeightFluidStream().getDouble(0, 0), 0D);
+            fixture.ready.set(true);
+            assertEquals(120D, fixture.complex.getHeightStream().getDouble(0, 0), 0D);
+            assertEquals(125D, fixture.complex.getHeightFluidStream().getDouble(0, 0), 0D);
+            assertEquals(0, fixture.materializations.get());
+        }
+    }
+
+    @Test
+    public void mainThreadContentQueriesMaterializeUncachedNeighborTerrain() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.mainThread.set(true);
+            ChunkContext context = fixture.context();
+            context.beginContent();
+
+            try (IrisContext.Scope ignored = IrisContext.open(fixture.engine, 0L, context)) {
+                assertEquals(120, fixture.complex.transformedHeight(16, 0, true));
+                assertEquals(125, fixture.complex.transformedHeight(16, 0, false));
+                assertEquals(2, fixture.materializations.get());
+            }
+        }
+    }
+
+    @Test
+    public void originalSlopeIgnoresContentHeightOverrides() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.complex.setHeightStream(ProceduralStream.ofDouble((x, z) -> 80D + x + z));
+            ProceduralStream<Double> originalSlope = fixture.complex.getRawSlopeStream();
+            ChunkContext context = fixture.context();
+            context.setTerrainHeight(0, 0, 200);
+            context.setTerrainHeight(3, 0, 250);
+            context.setTerrainHeight(0, 3, 300);
+            context.beginContent();
+
+            try (IrisContext.Scope ignored = IrisContext.open(fixture.engine, 7L, context)) {
+                assertEquals(Math.sqrt(18D), originalSlope.getDouble(0, 0), 0.000001D);
+                assertEquals(0, fixture.materializations.get());
+            }
+        }
+    }
+
+    @Test
+    public void admittedMainThreadQueriesWaitUntilTheirComplexAndModeArePublished() throws Exception {
+        try (Fixture fixture = new Fixture(pack(), true)) {
+            fixture.mainThread.set(true);
+            doReturn(null).when(fixture.engine).getComplex();
+            try (IrisContext.Scope ignored = IrisContext.open(fixture.engine, 7L, null)) {
+                assertEquals(Integer.MIN_VALUE, fixture.complex.transformedHeight(16, 0, true));
+                doReturn(fixture.complex).when(fixture.engine).getComplex();
+                assertEquals(Integer.MIN_VALUE, fixture.complex.transformedHeight(16, 0, true));
+                assertEquals(0, fixture.materializations.get());
+
+                doReturn(mock(EngineMode.class)).when(fixture.engine).getMode();
+                assertEquals(120, fixture.complex.transformedHeight(16, 0, true));
+                assertEquals(1, fixture.materializations.get());
+            }
+        }
+    }
+
+    private Path pack() throws Exception {
+        Path pack = temporaryFolder.newFolder().toPath();
+        Files.createDirectories(pack.resolve("dimensions"));
+        Files.createDirectories(pack.resolve("regions"));
+        Files.createDirectories(pack.resolve("biomes"));
+        Files.createDirectories(pack.resolve("generators"));
+        Files.writeString(pack.resolve("dimensions/main.json"), """
+                {"focus":"plain","regions":["plain"],"landChance":1,
+                 "carvingEnabled":false,"hydrology":{"rivers":{"enabled":false}}}
+                """);
+        Files.writeString(pack.resolve("regions/plain.json"), "{\"landBiomes\":[\"plain\"]}");
+        Files.writeString(pack.resolve("generators/flat.json"), "{\"composite\":[]}");
+        Files.writeString(pack.resolve("biomes/plain.json"),
+                "{\"generators\":[{\"generator\":\"flat\",\"min\":10,\"max\":10}]}");
+        return pack;
+    }
+
+    private static final class Fixture implements AutoCloseable {
+        private final Engine engine = mock(Engine.class, CALLS_REAL_METHODS);
+        private final AtomicBoolean mainThread = new AtomicBoolean();
+        private final AtomicBoolean ready = new AtomicBoolean();
+        private final AtomicInteger materializations = new AtomicInteger();
+        private final MockedStatic<B> blocks = mockStatic(B.class);
+        private final MockedStatic<TerrainTransformRegistry> registries = mockStatic(TerrainTransformRegistry.class);
+        private final MockedConstruction<TerrainTransformRuntime> runtimes;
+        private final IrisData data;
+        private final IrisComplex complex;
+
+        private Fixture(Path pack, boolean transformed) {
+            runtimes = mockConstruction(TerrainTransformRuntime.class, (runtime, context) -> {
+                when(runtime.height(anyInt(), anyInt(), anyBoolean())).thenAnswer(invocation -> {
+                    materializations.incrementAndGet();
+                    return invocation.<Boolean>getArgument(2) ? 120 : 125;
+                });
+                when(runtime.readyHeight(anyInt(), anyInt(), anyBoolean())).thenAnswer(invocation ->
+                        ready.get() ? invocation.<Boolean>getArgument(2) ? 120 : 125 : Integer.MIN_VALUE);
+            });
+            NativeBlockState block = mock(NativeBlockState.class);
+            when(block.isFluid()).thenReturn(true);
+            blocks.when(() -> B.getState(anyString())).thenReturn(block);
+            blocks.when(() -> B.getStateOrNull(anyString(), eq(false))).thenReturn(block);
+            data = IrisData.openRuntime(pack.toFile());
+            IrisDimension dimension = data.getDimensionLoader().load("main");
+            if (transformed) {
+                IrisTerrainTransform descriptor = new IrisTerrainTransform().setId("test").setVersion("1");
+                dimension.setTerrainTransform(descriptor);
+                TerrainTransformRegistry registry = mock(TerrainTransformRegistry.class);
+                when(registry.resolve(descriptor)).thenReturn(mock(TerrainTransformer.class));
+                registries.when(TerrainTransformRegistry::discover).thenReturn(registry);
+            }
+            doReturn(data).when(engine).getData();
+            doReturn(dimension).when(engine).getDimension();
+            doReturn(data.getBiomeLoader().load("plain")).when(engine).getFocus();
+            doReturn(384).when(engine).getHeight();
+            doReturn(-64).when(engine).getMinHeight();
+            doReturn(320).when(engine).getMaxHeight();
+            EnginePlatformHooks hooks = mock(EnginePlatformHooks.class);
+            when(hooks.isMainThread()).thenAnswer(ignored -> mainThread.get());
+            doReturn(hooks).when(engine).getPlatformHooks();
+            doReturn(new SeedManager(1337L)).when(engine).getSeedManager();
+            complex = new IrisComplex(engine);
+            doReturn(complex).when(engine).getComplex();
+        }
+
+        private ChunkContext context() {
+            return new ChunkContext(0, 0, complex, 7L, false, ChunkContext.PrefillPlan.NONE, null);
+        }
+
+        private void installStack() {
+            DimensionStackContext stack = mock(DimensionStackContext.class);
+            when(stack.getStackTerrainHeight(0, 0)).thenReturn(40);
+            when(stack.getStackTopHeight(0, 0)).thenReturn(50);
+            doReturn(stack).when(engine).getDimensionStackContext();
+        }
+
+        @Override
+        public void close() {
+            try {
+                complex.close();
+                data.close();
+            } finally {
+                runtimes.close();
+                registries.close();
+                blocks.close();
+            }
+        }
+    }
+}

core/src/test/java/art/arcane/iris/generation/runtime/UpperDimensionTransitionTest.java

@@ -42,7 +42,7 @@ public class UpperDimensionTransitionTest {
         Terrain3DColumn terrain = Terrain3DColumnFixtures.spans(80, 0, 30, 40, 60, 100, 110);
         when(complex.naturalTerrainColumn(-17, 8)).thenReturn(terrain);
         when(complex.getNaturalHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 110D));
-        when(complex.getHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 64D));
+        when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 64D));
         Engine engine = mock(Engine.class);
         when(engine.getDimension()).thenReturn(dimension);
         when(engine.getComplex()).thenReturn(complex);
@@ -63,7 +63,7 @@ public class UpperDimensionTransitionTest {
         when(dimension.getUpperDimensionGap()).thenReturn(32);
         IrisComplex complex = mock(IrisComplex.class);
         when(complex.getNaturalHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 139D));
-        when(complex.getHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 75D + 4D * x));
+        when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 75D + 4D * x));
         TransitionGenerationPlan plan = mock(TransitionGenerationPlan.class);
         when(complex.getTransitionGenerationPlan()).thenReturn(plan);
         Engine engine = mock(Engine.class);
@@ -86,7 +86,7 @@ public class UpperDimensionTransitionTest {
         when(dimension.getUpperDimensionGap()).thenReturn(32);
         IrisComplex complex = mock(IrisComplex.class);
         when(complex.getNaturalHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 80D));
-        when(complex.getHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 64D));
+        when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 64D));
         TransitionGenerationPlan plan = mock(TransitionGenerationPlan.class);
         when(complex.getTransitionGenerationPlan()).thenReturn(plan);
         Engine engine = mock(Engine.class);
@@ -126,4 +126,23 @@ public class UpperDimensionTransitionTest {
         assertThrows(IllegalArgumentException.class, () -> new UpperDimensionContext.CeilingLayout(256, -1));
     }
 
+    @Test
+    public void upperGapIgnoresTheFinalTransformedHostHeight() {
+        IrisDimension dimension = mock(IrisDimension.class);
+        when(dimension.getLoadKey()).thenReturn("main");
+        when(dimension.getUpperDimensionGap()).thenReturn(32);
+        IrisComplex complex = mock(IrisComplex.class);
+        when(complex.getNaturalHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 80D));
+        when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 64D));
+        when(complex.getHeightStream()).thenThrow(new AssertionError("Upper gap read final terrain height"));
+        Engine engine = mock(Engine.class);
+        when(engine.getDimension()).thenReturn(dimension);
+        when(engine.getComplex()).thenReturn(complex);
+        when(engine.getHeight()).thenReturn(256);
+
+        UpperDimensionContext context = UpperDimensionContext.create(engine, dimension);
+
+        assertEquals(175, context.getEffectiveSurfaceY(0, 0));
+    }
+
 }

core/src/test/java/art/arcane/iris/generation/stage/IrisNaturalCaveScanTest.java

@@ -5,6 +5,8 @@ import art.arcane.iris.generation.decoration.IrisCeilingDecorator;
 import art.arcane.iris.generation.decoration.IrisSurfaceDecorator;
 import art.arcane.iris.generation.runtime.Engine;
 import art.arcane.iris.generation.runtime.SeedManager;
+import art.arcane.iris.generation.terrain.transform.TerrainTransformRuntime;
+import art.arcane.iris.generation.terrain.Terrain3DColumn;
 import art.arcane.iris.generation.context.ChunkContext;
 import art.arcane.iris.generation.mantle.CaveTerrainSnapshot;
 import art.arcane.iris.generation.mantle.EngineMantle;
@@ -162,6 +164,64 @@ public class IrisNaturalCaveScanTest {
         }
     }
 
+    @Test
+    public void transformedCavesDecorateOnlyTheirRemainingFloorAndCeiling() throws Exception {
+        Fixture fixture = new Fixture();
+        fixture.transform();
+        Hunk<NativeBlockState> output = fixture.cavity();
+        output.setRaw(0, 4, 0, fixture.stone);
+        output.setRaw(0, 5, 0, fixture.stone);
+        fixture.modifier.decorateNaturalCaves(-32, 48, output, null);
+        assertEquals(List.of("floor:0:0:5:1", "ceiling:0:0:8:1"), fixture.calls);
+        assertSame(fixture.stone, output.getRaw(0, 4, 0));
+        assertSame(fixture.stone, output.getRaw(0, 5, 0));
+        assertSame(fixture.floor, output.getRaw(0, 6, 0));
+        assertSame(fixture.ceiling, output.getRaw(0, 8, 0));
+    }
+
+    @Test
+    public void transformedFilledCavesAndRemovedBoundariesRemainUndecorated() throws Exception {
+        Fixture filled = new Fixture();
+        filled.transform();
+        Hunk<NativeBlockState> filledOutput = filled.cavity();
+        for (int y = 4; y <= 8; y++) {
+            filledOutput.setRaw(0, y, 0, filled.stone);
+        }
+        filled.modifier.decorateNaturalCaves(-32, 48, filledOutput, null);
+        assertTrue(filled.calls.isEmpty());
+        assertTrue(filled.markers.isEmpty());
+
+        Fixture openRoof = new Fixture();
+        openRoof.transform();
+        Hunk<NativeBlockState> openRoofOutput = openRoof.cavity();
+        for (int y = 9; y < 32; y++) {
+            openRoofOutput.setRaw(0, y, 0, openRoof.air);
+        }
+        openRoof.modifier.decorateNaturalCaves(-32, 48, openRoofOutput, null);
+        assertTrue(openRoof.calls.isEmpty());
+
+        Fixture openFloor = new Fixture();
+        openFloor.transform();
+        Hunk<NativeBlockState> openFloorOutput = openFloor.cavity();
+        for (int y = 0; y < 4; y++) {
+            openFloorOutput.setRaw(0, y, 0, openFloor.air);
+        }
+        openFloor.modifier.decorateNaturalCaves(-32, 48, openFloorOutput, null);
+        assertTrue(openFloor.calls.isEmpty());
+    }
+
+    @Test
+    public void transformKeepsNaturalTerrainOpeningsOutOfCaveDecoration() throws Exception {
+        Fixture fixture = new Fixture();
+        fixture.transform();
+        doReturn(true).when(fixture.complex).hasTerrain3D();
+        Terrain3DColumn natural = Terrain3DColumn.fromOccupancy(31, 32, y -> y < 4 || y > 8);
+        doReturn(natural).when(fixture.complex).terrainColumn(anyInt(), anyInt(), any());
+        fixture.modifier.decorateNaturalCaves(-32, 48, fixture.cavity(), null);
+        assertTrue(fixture.calls.isEmpty());
+        assertTrue(fixture.markers.isEmpty());
+    }
+
     private static void verifyScan(int width, int depth) throws Exception {
         Fixture fixture = new Fixture();
         Hunk<NativeBlockState> output = fixture.output(width, depth);
@@ -236,6 +296,7 @@ public class IrisNaturalCaveScanTest {
         private final NativeBlockState floor = block("minecraft:moss_block", true);
         private final NativeBlockState ceiling = block("minecraft:calcite", true);
         private final IrisCarveModifier modifier = mock(IrisCarveModifier.class, CALLS_REAL_METHODS);
+        private final IrisComplex complex = mock(IrisComplex.class);
         private final Mantle<Matter> mantle;
         private final MantleChunk<Matter> chunk;
         private final List<String> calls = new ArrayList<>();
@@ -252,7 +313,7 @@ public class IrisNaturalCaveScanTest {
             mantle = mock(Mantle.class);
             chunk = mock(MantleChunk.class);
             doReturn(engine).when(modifier).getEngine();
-            doReturn(mock(IrisComplex.class)).when(modifier).getComplex();
+            doReturn(complex).when(modifier).getComplex();
             doReturn(32).when(engine).getHeight();
             doReturn(new SeedManager(1337L)).when(engine).getSeedManager();
             doReturn(engineMantle).when(engine).getMantle();
@@ -317,6 +378,20 @@ public class IrisNaturalCaveScanTest {
             return output;
         }
 
+        private void transform() {
+            doReturn(mock(TerrainTransformRuntime.class)).when(complex).getTerrainTransform();
+            doReturn(true).when(complex).isTerrain3DOpening(anyInt(), anyInt(), anyInt());
+        }
+
+        private Hunk<NativeBlockState> cavity() {
+            Hunk<NativeBlockState> output = Hunk.newArrayHunk(1, 32, 1);
+            output.fill(stone);
+            for (int y = 4; y <= 8; y++) {
+                output.setRaw(0, y, 0, air);
+            }
+            return output;
+        }
+
         private Hunk<NativeBlockState> output(int width, int depth) {
             Hunk<NativeBlockState> output = Hunk.newArrayHunk(width, 32, depth);
             for (int x = 0; x < width; x++) {

core/src/test/java/art/arcane/iris/generation/terrain/Terrain3DColumnTest.java

@@ -8,6 +8,27 @@ import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 
 public class Terrain3DColumnTest {
+    @Test
+    public void occupancyPreservesCavitiesAndIsolatedBlocks() {
+        Terrain3DColumn column = Terrain3DColumn.fromOccupancy(7, 9,
+                y -> y < 3 || y == 5 || y >= 7);
+        assertEquals(3, column.spanCount());
+        assertEquals(8, column.topY());
+        assertEquals(2, column.surfaceY(1));
+        assertEquals(5, column.surfaceY(5));
+        assertFalse(column.isSolid(3));
+        assertFalse(column.isSolid(6));
+        assertTrue(column.isSolid(8));
+    }
+
+    @Test
+    public void emptyOccupancyHasNoSurface() {
+        Terrain3DColumn column = Terrain3DColumn.fromOccupancy(3, 9, y -> false);
+        assertEquals(-1, column.topY());
+        assertEquals(-1, column.highestSolidY(8));
+        assertFalse(column.isSolid(0));
+    }
+
     @Test
     public void displacementPreservesCavitiesAndUpperShelvesWhileKeepingBedrockGrounded() {
         Terrain3DColumn original = new Terrain3DColumn(40D, 25, true, new int[]{0, 30, 36, 42, 50, 54});

core/src/test/java/art/arcane/iris/generation/terrain/transform/TerrainTransformRegistryTest.java

@@ -0,0 +1,107 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import art.arcane.volmlib.util.collection.KMap;
+import org.junit.Test;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+
+public class TerrainTransformRegistryTest {
+    @Test
+    public void absentConfigurationDisablesTransformation() {
+        assertNull(new TerrainTransformRegistry(List.of()).resolve(null));
+    }
+
+    @Test
+    public void selectsThePinnedProviderVersion() {
+        TestProvider first = new TestProvider("1", 0);
+        TestProvider second = new TestProvider("2", 16);
+        TerrainTransformRegistry registry = new TerrainTransformRegistry(List.of(first, second));
+
+        assertSame(first.transformer, registry.resolve(descriptor("1")));
+        assertSame(second.transformer, registry.resolve(descriptor("2")));
+    }
+
+    @Test
+    public void rejectsUnavailableProvidersAndVersions() {
+        TerrainTransformRegistry registry = new TerrainTransformRegistry(List.of(new TestProvider("1", 1)));
+
+        assertThrows(IllegalStateException.class, () -> registry.resolve(descriptor("2")));
+        assertThrows(IllegalStateException.class, () -> registry.resolve(descriptor("1").setId("missing")));
+    }
+
+    @Test
+    public void freezesSettingsBeforeCreatingTheTransformer() {
+        TestProvider provider = new TestProvider("1", 1);
+        KMap<String, String> settings = new KMap<>();
+        settings.put("strength", "2");
+        IrisTerrainTransform descriptor = descriptor("1").setSettings(settings);
+
+        new TerrainTransformRegistry(List.of(provider)).resolve(descriptor);
+        settings.put("strength", "9");
+
+        assertEquals("2", provider.settings.get("strength"));
+        assertThrows(UnsupportedOperationException.class, () -> provider.settings.put("strength", "3"));
+    }
+
+    @Test
+    public void rejectsDuplicateProviderIdentities() {
+        assertThrows(IllegalArgumentException.class, () -> new TerrainTransformRegistry(
+                List.of(new TestProvider("1", 1), new TestProvider("1", 2))));
+    }
+
+    @Test
+    public void rejectsMissingIdentityAndInvalidNeighborhoodSizes() {
+        assertThrows(IllegalArgumentException.class,
+                () -> new TerrainTransformRegistry(List.of(new TestProvider(" ", 1))));
+        TerrainTransformRegistry registry = new TerrainTransformRegistry(List.of(new TestProvider("1", 1)));
+        assertThrows(IllegalArgumentException.class, () -> registry.resolve(descriptor(" ")));
+        assertThrows(IllegalArgumentException.class, () -> registry.resolve(descriptor("1").setId("")));
+        assertThrows(IllegalArgumentException.class, () -> new TerrainTransformRegistry(
+                List.of(new TestProvider("1", -1))).resolve(descriptor("1")));
+        assertThrows(IllegalArgumentException.class, () -> new TerrainTransformRegistry(
+                List.of(new TestProvider("1", 17))).resolve(descriptor("1")));
+    }
+
+    private static IrisTerrainTransform descriptor(String version) {
+        return new IrisTerrainTransform().setId("test").setVersion(version);
+    }
+
+    private static final class TestProvider implements TerrainTransformProvider {
+        private final String version;
+        private final TerrainTransformer transformer;
+        private Map<String, String> settings;
+
+        private TestProvider(String version, int radius) {
+            this.version = version;
+            this.transformer = new TestTransformer(radius);
+        }
+
+        @Override
+        public String id() {
+            return "test";
+        }
+
+        @Override
+        public String version() {
+            return version;
+        }
+
+        @Override
+        public TerrainTransformer create(Map<String, String> settings) {
+            this.settings = settings;
+            return transformer;
+        }
+    }
+
+    private record TestTransformer(int radius) implements TerrainTransformer {
+        @Override
+        public void transform(TerrainTransformContext context) {
+        }
+    }
+}

core/src/test/java/art/arcane/iris/generation/terrain/transform/TerrainTransformRuntimeTest.java

@@ -0,0 +1,338 @@
+package art.arcane.iris.generation.terrain.transform;
+
+import art.arcane.iris.generation.block.B;
+import art.arcane.iris.generation.context.ChunkContext;
+import art.arcane.iris.generation.context.IrisContext;
+import art.arcane.iris.generation.runtime.Engine;
+import art.arcane.iris.generation.runtime.EngineMode;
+import art.arcane.iris.generation.runtime.IrisComplex;
+import art.arcane.iris.generation.terrain.Terrain3DColumn;
+import art.arcane.iris.world.history.FloatingBiomeOverlay;
+import art.arcane.volmlib.nativelib.terrain.NativeBiome;
+import art.arcane.volmlib.nativelib.terrain.NativeBlockState;
+import art.arcane.volmlib.util.hunk.Hunk;
+import art.arcane.volmlib.util.stream.ProceduralStream;
+import org.junit.Test;
+import org.mockito.MockedStatic;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Consumer;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotSame;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyBoolean;
+import static org.mockito.ArgumentMatchers.anyInt;
+import static org.mockito.Mockito.doAnswer;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.when;
+
+public class TerrainTransformRuntimeTest {
+    @Test
+    public void reusesTerrainAndCopiesOutputMetadata() {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger transforms = new AtomicInteger();
+            TerrainTransformRuntime runtime = fixture.runtime(0, context -> {
+                transforms.incrementAndGet();
+                context.set(0, 2, 0, fixture.air);
+                context.set(0, 5, 0, fixture.stone);
+                context.set(0, 6, 0, fixture.water);
+            });
+            assertEquals(Integer.MIN_VALUE, runtime.readyHeight(0, 0, true));
+            assertEquals(Integer.MIN_VALUE, runtime.readyFluidHeight(0, 0));
+            assertNull(runtime.readyBlock(0, 0, 0));
+            assertNull(runtime.readyColumn(0, 0));
+            assertEquals(5, runtime.height(0, 0, true));
+            assertEquals(6, runtime.height(0, 0, false));
+            assertEquals(6, runtime.fluidHeight(0, 0));
+            assertEquals(6, runtime.readyFluidHeight(0, 0));
+            assertSame(fixture.air, runtime.block(0, 2, 0));
+            assertSame(fixture.stone, runtime.readyBlock(0, 5, 0));
+            assertEquals(5, runtime.column(0, 0).topY());
+            assertSame(runtime.column(0, 0), runtime.readyColumn(0, 0));
+            Hunk<NativeBlockState> blocks = Hunk.newArrayHunk(16, 8, 16);
+            Hunk<NativeBiome> biomes = Hunk.newArrayHunk(16, 8, 16);
+            ChunkContext first = fixture.context(0);
+            runtime.generate(0, 0, blocks, biomes, false, first);
+            assertEquals(5, first.getRoundedHeight(0, 0));
+            assertSame(fixture.stone, blocks.getRaw(0, 5, 0));
+            assertSame(fixture.biome, biomes.getRaw(0, 2, 0));
+            blocks.setRaw(0, 5, 0, fixture.air);
+            first.getFloatingBiomes().retainHighestSurfaces((x, z) -> -1);
+            ChunkContext second = fixture.context(0);
+            runtime.generate(0, 0, blocks, biomes, false, second);
+            assertSame(fixture.stone, blocks.getRaw(0, 5, 0));
+            assertNotSame(first.getFloatingBiomes(), second.getFloatingBiomes());
+            assertEquals(2, second.getFloatingBiomes().surfaceYAt(1, 1));
+            assertEquals(1, fixture.generations.get());
+            assertEquals(1, transforms.get());
+            assertEquals(5, runtime.readyHeight(0, 0, true));
+            runtime.close();
+            assertThrows(IllegalStateException.class, () -> runtime.height(0, 0, true));
+        }
+    }
+
+    @Test
+    public void neighboringReadsAlwaysUseOriginalTerrainInEitherOrder() {
+        try (Fixture fixture = new Fixture()) {
+            TerrainTransformer transformer = fixture.transformer(16, context -> {
+                int neighbor = context.blockX() == 0 ? 16 : 0;
+                assertSame(fixture.stone, context.original(neighbor, -2, 0));
+                assertSame(fixture.air, context.original(neighbor, -5, 0));
+                context.set(0, 2, 0, fixture.air);
+            });
+            TerrainTransformRuntime forward = new TerrainTransformRuntime(new TerrainTransformRuntime.Options(fixture.engine, fixture.complex), transformer);
+            TerrainTransformRuntime backward = new TerrainTransformRuntime(new TerrainTransformRuntime.Options(fixture.engine, fixture.complex), transformer);
+            assertEquals(1, forward.height(0, 0, true));
+            assertEquals(1, forward.height(16, 0, true));
+            assertEquals(1, backward.height(16, 0, true));
+            assertEquals(1, backward.height(0, 0, true));
+            forward.close();
+            backward.close();
+        }
+    }
+
+    @Test
+    public void rejectsReadsBeyondDeclaredRadiusAndWritesBeyondChunk() {
+        try (Fixture fixture = new Fixture()) {
+            TerrainTransformRuntime runtime = fixture.runtime(1, context -> {
+                assertThrows(IllegalArgumentException.class, () -> context.original(17, -2, 0));
+                assertThrows(IllegalArgumentException.class, () -> context.original(-2, -2, 0));
+                assertThrows(IllegalArgumentException.class, () -> context.set(16, 0, 0, fixture.stone));
+                assertThrows(IllegalArgumentException.class, () -> context.set(0, 8, 0, fixture.stone));
+                assertSame(fixture.stone, context.original(-1, -2, 0));
+                assertEquals(-4, context.minY());
+                assertEquals(8, context.height());
+            });
+            assertEquals(2, runtime.height(0, 0, true));
+            runtime.close();
+        }
+    }
+
+    @Test
+    public void preservesHistoricalColumnsAndCountsWaterloggedFluid() {
+        try (Fixture fixture = new Fixture()) {
+            when(fixture.complex.allowsMantleWrite(0, 0)).thenReturn(false);
+            NativeBlockState wetSlab = mock(NativeBlockState.class);
+            when(wetSlab.isWaterLogged()).thenReturn(true);
+            TerrainTransformRuntime runtime = fixture.runtime(0, context -> {
+                context.set(0, 5, 0, fixture.stone);
+                context.set(1, 5, 0, wetSlab);
+            });
+            assertEquals(2, runtime.height(0, 0, true));
+            assertEquals(5, runtime.height(1, 0, true));
+            assertEquals(5, runtime.fluidHeight(1, 0));
+            assertEquals(-1, runtime.fluidHeight(0, 0));
+            runtime.close();
+        }
+    }
+
+    @Test
+    public void concurrentNeighborTransformsDoNotDependOnEachOther() throws Exception {
+        try (Fixture fixture = new Fixture()) {
+            CountDownLatch entered = new CountDownLatch(2);
+            TerrainTransformRuntime runtime = fixture.runtime(16, context -> {
+                entered.countDown();
+                try {
+                    assertTrue(entered.await(5, TimeUnit.SECONDS));
+                } catch (InterruptedException failure) {
+                    Thread.currentThread().interrupt();
+                    throw new IllegalStateException(failure);
+                }
+                int neighbor = context.blockX() == 0 ? 16 : 0;
+                context.set(0, 4, 0, context.original(neighbor, -2, 0));
+            });
+            ExecutorService workers = Executors.newFixedThreadPool(2);
+            try {
+                Future<Integer> first = workers.submit(() -> runtime.height(0, 0, true));
+                Future<Integer> second = workers.submit(() -> runtime.height(16, 0, true));
+                assertEquals(Integer.valueOf(4), first.get(10, TimeUnit.SECONDS));
+                assertEquals(Integer.valueOf(4), second.get(10, TimeUnit.SECONDS));
+                assertEquals(2, fixture.generations.get());
+            } finally {
+                workers.shutdownNow();
+                runtime.close();
+            }
+        }
+    }
+
+    @Test
+    public void sparseColumnQueriesAvoidFullTransformationAndReuseOccupancy() {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger fullCalls = new AtomicInteger();
+            AtomicInteger columnCalls = new AtomicInteger();
+            TerrainTransformRuntime runtime = fixture.columnRuntime(0, (context, x, z) -> {
+                columnCalls.incrementAndGet();
+                context.set(x, 2, z, fixture.air);
+                context.set(x, 5, z, fixture.stone);
+                context.set(x, 6, z, fixture.water);
+                assertThrows(IllegalArgumentException.class, () -> context.set((x + 1) & 15, 0, z, fixture.stone));
+                assertThrows(IllegalArgumentException.class, () -> context.set(x, 0, (z + 1) & 15, fixture.stone));
+            }, context -> fullCalls.incrementAndGet());
+            assertEquals(Integer.MIN_VALUE, runtime.readyHeight(3, 5, true));
+            assertEquals(5, runtime.height(3, 5, true));
+            assertEquals(6, runtime.height(3, 5, false));
+            assertEquals(6, runtime.fluidHeight(3, 5));
+            assertEquals(5, runtime.readyHeight(3, 5, true));
+            assertEquals(6, runtime.readyHeight(3, 5, false));
+            assertEquals(6, runtime.readyFluidHeight(3, 5));
+            assertEquals(5, runtime.column(3, 5).topY());
+            assertSame(runtime.column(3, 5), runtime.readyColumn(3, 5));
+            assertNull(runtime.readyBlock(3, 5, 5));
+            assertEquals(Integer.MIN_VALUE, runtime.readyHeight(4, 5, true));
+            assertEquals(1, columnCalls.get());
+            assertEquals(0, fullCalls.get());
+            assertEquals(1, fixture.generations.get());
+            runtime.close();
+        }
+    }
+
+    @Test
+    public void columnPredictionMatchesBoundaryGenerationAndFullOutputTakesPrecedence() {
+        try (Fixture fixture = new Fixture()) {
+            AtomicInteger fullCalls = new AtomicInteger();
+            AtomicInteger columnCalls = new AtomicInteger();
+            TerrainColumnTransformer operation = (context, x, z) -> {
+                NativeBlockState neighbor = context.original(context.blockX() + x + 1,
+                        context.minY() + 2, context.blockZ() + z + 1);
+                context.set(x, 2, z, fixture.air);
+                context.set(x, 4, z, neighbor);
+                context.set(x, 5, z, fixture.water);
+            };
+            TerrainTransformRuntime runtime = fixture.columnRuntime(1, (context, x, z) -> {
+                columnCalls.incrementAndGet();
+                operation.transform(context, x, z);
+            }, context -> {
+                fullCalls.incrementAndGet();
+                for (int z = 0; z < 16; z++) {
+                    for (int x = 0; x < 16; x++) {
+                        operation.transform(context, x, z);
+                    }
+                }
+            });
+            assertEquals(4, runtime.height(-1, 15, true));
+            assertEquals(5, runtime.height(-1, 15, false));
+            Terrain3DColumn predicted = runtime.column(-1, 15);
+            assertEquals(2, fixture.generations.get());
+            assertEquals(0, fullCalls.get());
+            Hunk<NativeBlockState> blocks = Hunk.newArrayHunk(16, 8, 16);
+            runtime.generate(-16, 0, blocks, Hunk.newArrayHunk(16, 8, 16), false, fixture.context(-16));
+            assertEquals(1, fullCalls.get());
+            assertEquals(1, columnCalls.get());
+            assertEquals(4, runtime.height(-1, 15, true));
+            assertEquals(5, runtime.fluidHeight(-1, 15));
+            assertEquals(predicted, runtime.column(-1, 15));
+            assertNotSame(predicted, runtime.column(-1, 15));
+            assertSame(runtime.column(-1, 15), runtime.readyColumn(-1, 15));
+            assertSame(fixture.stone, runtime.readyBlock(-1, 4, 15));
+            assertSame(fixture.water, blocks.getRaw(15, 5, 15));
+            runtime.close();
+        }
+    }
+
+    private static final class Fixture implements AutoCloseable {
+        private final Engine engine = mock(Engine.class);
+        private final IrisComplex complex = mock(IrisComplex.class);
+        private final EngineMode mode = mock(EngineMode.class);
+        private final NativeBlockState air = mock(NativeBlockState.class);
+        private final NativeBlockState stone = mock(NativeBlockState.class);
+        private final NativeBlockState water = mock(NativeBlockState.class);
+        private final NativeBiome biome = mock(NativeBiome.class);
+        private final AtomicInteger generations = new AtomicInteger();
+        private final MockedStatic<B> states = mockStatic(B.class);
+
+        private Fixture() {
+            states.when(() -> B.getState("AIR")).thenReturn(air);
+            when(air.isAir()).thenReturn(true);
+            when(water.isFluid()).thenReturn(true);
+            when(engine.getComplex()).thenReturn(complex);
+            when(engine.getMode()).thenReturn(mode);
+            when(engine.getHeight()).thenReturn(8);
+            when(engine.getMinHeight()).thenReturn(-4);
+            when(engine.getGenerationSessionId()).thenReturn(7L);
+            when(complex.getRawHeightStream()).thenReturn(ProceduralStream.ofDouble((x, z) -> 2));
+            when(complex.getTrueBiomeStream()).thenReturn(mock(ProceduralStream.class));
+            when(complex.getRockStream()).thenReturn(mock(ProceduralStream.class));
+            when(complex.getFluidStream()).thenReturn(mock(ProceduralStream.class));
+            when(complex.getRegionStream()).thenReturn(mock(ProceduralStream.class));
+            when(complex.allowsMantleWrite(anyInt(), anyInt())).thenReturn(true);
+            doAnswer(invocation -> {
+                generations.incrementAndGet();
+                Hunk<NativeBlockState> blocks = invocation.getArgument(2);
+                Hunk<NativeBiome> biomes = invocation.getArgument(3);
+                ChunkContext context = invocation.getArgument(5);
+                assertTrue(context.isNaturalTerrain());
+                assertSame(context, IrisContext.require().getChunkContext());
+                assertEquals(7L, IrisContext.require().getGenerationSessionId());
+                for (int x = 0; x < 16; x++) {
+                    for (int z = 0; z < 16; z++) {
+                        for (int y = 0; y <= 2; y++) {
+                            blocks.setRaw(x, y, z, stone);
+                            biomes.setRaw(x, y, z, biome);
+                        }
+                    }
+                }
+                context.floatingBiomes(8).record(1, 2, 1, new FloatingBiomeOverlay.Identity("island", "region"));
+                return null;
+            }).when(mode).generateTerrain(anyInt(), anyInt(), any(), any(), anyBoolean(), any());
+        }
+
+        private TerrainTransformRuntime runtime(int radius, Consumer<TerrainTransformContext> action) {
+            return new TerrainTransformRuntime(new TerrainTransformRuntime.Options(engine, complex), transformer(radius, action));
+        }
+
+        private TerrainTransformer transformer(int radius, Consumer<TerrainTransformContext> action) {
+            return new TerrainTransformer() {
+                @Override
+                public int radius() {
+                    return radius;
+                }
+
+                @Override
+                public void transform(TerrainTransformContext context) {
+                    action.accept(context);
+                }
+            };
+        }
+
+        private TerrainTransformRuntime columnRuntime(int radius, TerrainColumnTransformer column,
+                                                       Consumer<TerrainTransformContext> full) {
+            TerrainTransformer transformer = new TerrainTransformer() {
+                @Override
+                public int radius() {
+                    return radius;
+                }
+
+                @Override
+                public void transform(TerrainTransformContext context) {
+                    full.accept(context);
+                }
+
+                @Override
+                public TerrainColumnTransformer columnTransformer() {
+                    return column;
+                }
+            };
+            return new TerrainTransformRuntime(new TerrainTransformRuntime.Options(engine, complex), transformer);
+        }
+
+        private ChunkContext context(int x) {
+            return new ChunkContext(x, 0, complex, 7L, true, ChunkContext.PrefillPlan.NONE, null);
+        }
+
+        @Override
+        public void close() {
+            states.close();
+        }
+    }
+}

core/src/test/java/art/arcane/iris/pack/schema/SchemaBuilderIdentityTest.java

@@ -78,7 +78,7 @@ public class SchemaBuilderIdentityTest {
     @ClassRule
     public static final PlatformLeakGuard PLATFORM_GUARD = PlatformLeakGuard.clean();
 
-    private static final String EXPECTED_SCHEMA_DIGEST = "9c118c18c2f757f08ffb71d52d8549c53e4dae4939e0e030d36e4e9115794497";
+    private static final String EXPECTED_SCHEMA_DIGEST = "4d37a4cf8bb02c6e334545b549433a1b75ff5b8aae0bc91cd8b21cd8f62307c0";
 
     private static final List<Class<?>> SCHEMA_ROOTS = List.of(
             IrisDimension.class,

core/src/test/java/art/arcane/iris/structure/placement/IrisStructureLocatorContractTest.java

@@ -420,7 +420,7 @@ public class IrisStructureLocatorContractTest {
 
         assertEquals(IrisStructureLocator.LocateStatus.SEARCH_LIMIT_REACHED, result.status());
         assertFalse(result.found());
-        verify(engine, times(8_192)).getComplex();
+        verify(engine, times(16_384)).getComplex();
     }
 
     @Test