From 5b1b636539a0dcc7c2d37f8a963366966ee52d56 Mon Sep 17 00:00:00 2001
From: Brian Neumann-Fopiano <mypsn1550@gmail.com>
Date: Tue, 1 Sep 2026 19:21:25 -0400
Subject: [PATCH] Point basis samples carry terrain slope

Routing, validation and the valley solver sampled basis columns with slope 0; the forward slope now rides on every point sample.
---
 .../IrisHydrologyRoutingTerrainSampler.java    |  4 +++-
 .../runtime/IrisHydrologyRuntime.java          |  4 +---
 ...IrisHydrologyRoutingTerrainSamplerTest.java | 18 ++++++++++++++++++
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSampler.java b/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSampler.java
index 278a5e7fc..7ed8c87b9 100644
--- a/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSampler.java
+++ b/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSampler.java
@@ -106,7 +106,9 @@ final class IrisHydrologyRoutingTerrainSampler implements HydrologyNaturalTerrai
 
     @Override
     public HydrologyTerrainSample sampleBasis(int blockX, int blockZ) {
-        return basis(blockX, blockZ).terrain();
+        // Point samples carry the forward slope; the aligned grid derives its own from neighbouring bases.
+        TerrainBasis basis = basis(blockX, blockZ);
+        return basis.terrain().withSlope(localSlope(blockX, blockZ, basis.naturalHeight()));
     }
 
     TerrainBasis basis(int blockX, int blockZ) {
diff --git a/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java b/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java
index 0e53a6359..4cbab856a 100644
--- a/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java
+++ b/core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java
@@ -387,9 +387,7 @@ public final class IrisHydrologyRuntime implements AutoCloseable {
     }
 
     private HydrologyTerrainSample createDetailedTerrainSample(int x, int z) {
-        IrisHydrologyRoutingTerrainSampler.TerrainBasis basis = routingTerrainSampler.basis(x, z);
-        double slope = routingTerrainSampler.localSlope(x, z, basis.naturalHeight());
-        return basis.terrain().withSlope(slope);
+        return routingTerrainSampler.sampleBasis(x, z);
     }
 
     private IrisHydrologyRoutingTerrainSampler.TerrainBasis createTerrainBasis(
diff --git a/core/src/test/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSamplerTest.java b/core/src/test/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSamplerTest.java
index 66c140ab1..3fdbba635 100644
--- a/core/src/test/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSamplerTest.java
+++ b/core/src/test/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSamplerTest.java
@@ -93,6 +93,24 @@ public class IrisHydrologyRoutingTerrainSamplerTest {
         }
     }
 
+    @Test
+    public void pointBasisSamplesCarryTheForwardSlope() {
+        IrisHydrologyRoutingTerrainSampler sampler = sampler(64);
+        int[][] points = {{0, 0}, {-129, -257}, {37, 1024}};
+
+        for (int[] point : points) {
+            int x = point[0];
+            int z = point[1];
+            double expected = IrisHydrologyRoutingTerrainSampler.localSlope(
+                    height(x, z), height(x + 3, z), height(x, z + 3));
+
+            double actual = sampler.sampleBasis(x, z).slope();
+
+            assertTrue(expected > 0D);
+            assertEquals(Double.doubleToRawLongBits(expected), Double.doubleToRawLongBits(actual));
+        }
+    }
+
     @Test
     public void adjacentGridsShareBasesAndProduceIdenticalOverlap() {
         AtomicInteger calls = new AtomicInteger();