Pin folder headers, cut layers and see through blocks in the Objects view

Folder headers stay pinned at the top of the object tree while scrolling,
one per nesting level; clicking one jumps to that folder. A faint slider
over the 3D view (or Page Up/Down) cuts away the top layers so floors of
large structures like the woodland mansion can be looked into. Clicking a
block in the overview list makes that block type see-through, so whatever
it hides shows behind it.

Model used: Claude Opus 5.5

core/src/main/java/com/volmit/iris/core/gui/MapWindow.java

@@ -270,7 +270,7 @@ public final class MapWindow implements MapCanvas.Listener {
                 viewport.setCoveredLeftWidth(sidebarIsland.getWidth());
             }
         });
-        content.add(viewport, OBJECTS_CARD);
+        content.add(objectExplorer.view(), OBJECTS_CARD);
         statusBars.add(objectExplorer.statusBar(), OBJECTS_CARD);
     }
 

core/src/main/java/com/volmit/iris/core/gui/ObjectBrowser.java

@@ -91,7 +91,7 @@ final class ObjectBrowser extends JPanel {
         countLabel.setFont(StudioTheme.smallFont());
         countLabel.setForeground(StudioTheme.SECONDARY_TEXT);
         add(StudioToolbar.padded(searchField, 8, 8, 8, 8), BorderLayout.NORTH);
-        add(StudioToolbar.scroll(tree), BorderLayout.CENTER);
+        add(StickyFolderHeaders.around(tree, StudioToolbar.scroll(tree)), BorderLayout.CENTER);
         add(StudioToolbar.padded(countLabel, 6, 12, 8, 12), BorderLayout.SOUTH);
     }
 

core/src/main/java/com/volmit/iris/core/gui/ObjectExplorer.java

@@ -27,12 +27,14 @@ import org.bukkit.util.BlockVector;
 
 import javax.swing.JComponent;
 import java.awt.EventQueue;
+import java.awt.event.KeyEvent;
 import java.awt.image.BufferedImage;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Set;
 import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutorService;
@@ -46,13 +48,15 @@ import java.util.concurrent.atomic.AtomicLong;
 final class ObjectExplorer {
     private static final NamespacedKey TINT_BIOME = NamespacedKey.minecraft("plains");
     private static final int UNKNOWN_BLOCK_COLOR = 0x8A8A8A;
-    private static final String CAMERA_HINT = "Drag to orbit · Shift-drag to pan · Scroll to zoom · R resets";
+    private static final String CAMERA_HINT = "Drag to orbit · Shift-drag to pan · Scroll to zoom · Page Up/Down cuts layers · R resets";
 
     private final Engine startEngine;
     private final UUID worldId;
     private final ObjectBrowser browser = new ObjectBrowser(this::showObject);
     private final ObjectViewport viewport = new ObjectViewport();
-    private final ObjectOverviewPanel overview = new ObjectOverviewPanel();
+    private final ObjectLayerCut layerCut = new ObjectLayerCut(shownLayers -> rebuildShownMesh());
+    private final JComponent viewWithLayerCut = layerCut.floatingOver(viewport);
+    private final ObjectOverviewPanel overview = new ObjectOverviewPanel(this::showSeeThrough);
     private final StudioStatusBar statusBar = new StudioStatusBar();
     private final ExecutorService loader = Executors.newSingleThreadExecutor(runnable -> {
         Thread thread = new Thread(runnable, "Iris Object Loader");
@@ -61,11 +65,14 @@ final class ObjectExplorer {
         return thread;
     });
     private final AtomicLong objectRevision = new AtomicLong();
+    private final AtomicLong meshRevision = new AtomicLong();
     private final Map<String, BlockModel> models = new ConcurrentHashMap<>();
     private final Runnable hotloadHook = this::reloadPack;
     private volatile BlockTextures textures;
     private volatile BiomeTintColors tintColors;
     private String shownKey;
+    private LoadedObject shown;
+    private Set<String> seeThroughTypes = Set.of();
 
     ObjectExplorer(Engine engine, UUID worldId) {
         this.startEngine = engine;
@@ -73,6 +80,8 @@ final class ObjectExplorer {
         GuiHost.interruptOnPluginShutdown(loader);
         GuiHost.registerHotloadHook(hotloadHook);
         statusBar.showPosition(CAMERA_HINT);
+        viewport.bindKey(KeyEvent.VK_PAGE_UP, "show-layer-above", () -> layerCut.stepBy(1));
+        viewport.bindKey(KeyEvent.VK_PAGE_DOWN, "hide-top-layer", () -> layerCut.stepBy(-1));
     }
 
     JComponent browser() {
@@ -83,6 +92,13 @@ final class ObjectExplorer {
         return viewport;
     }
 
+    /**
+     * The 3D view with the slider that cuts away its top layers.
+     */
+    JComponent view() {
+        return viewWithLayerCut;
+    }
+
     JComponent overview() {
         return overview;
     }
@@ -133,6 +149,7 @@ final class ObjectExplorer {
         if (key == null) {
             viewport.showMessage("This pack has no objects");
             overview.clear();
+            layerCut.showHeight(1);
             return;
         }
         browser.select(key);
@@ -141,6 +158,8 @@ final class ObjectExplorer {
 
     private void showObject(String key) {
         shownKey = key;
+        shown = null;
+        meshRevision.incrementAndGet();
         long revision = objectRevision.incrementAndGet();
         viewport.showMessage("Loading " + key + "...");
         statusBar.showActivity("Loading " + key + "...");
@@ -170,18 +189,73 @@ final class ObjectExplorer {
         });
         ObjectMesh mesh = ObjectMesh.build(blocks, this::modelOf);
         ObjectSummary summary = ObjectSummary.of(key, mesh.bounds(), blocks.values(), this::iconOf);
-        return new LoadedObject(mesh, summary);
+        return new LoadedObject(blocks, mesh, summary);
     }
 
     private void showLoaded(long revision, LoadedObject loaded) {
         if (revision != objectRevision.get()) {
             return;
         }
+        shown = loaded;
+        seeThroughTypes = Set.of();
         statusBar.showActivity("");
         viewport.show(loaded.mesh());
         overview.show(loaded.summary());
-        statusBar.showSummary(String.format(Locale.ROOT, "%s · %,d blocks · %,d faces drawn",
-                loaded.summary().key(), loaded.summary().blockCount(), loaded.mesh().faceCount()));
+        layerCut.showHeight(loaded.mesh().bounds().sizeY());
+        showMeshSummary(loaded.mesh());
+    }
+
+    private void showMeshSummary(ObjectMesh mesh) {
+        ObjectSummary summary = shown.summary();
+        String cut = layerCut.isCutting()
+                ? String.format(Locale.ROOT, " · showing layers 1-%d of %d", layerCut.shownLayerCount(), layerCut.layerCount())
+                : "";
+        statusBar.showSummary(String.format(Locale.ROOT, "%s · %,d blocks · %,d faces drawn%s",
+                summary.key(), summary.blockCount(), mesh.faceCount(), cut));
+    }
+
+    private void showSeeThrough(Set<String> blockTypes) {
+        seeThroughTypes = blockTypes;
+        rebuildShownMesh();
+    }
+
+    // Blocks under the cut that were covered by the ones above get their hidden faces back, so each floor shows, and
+    // blocks behind see-through ones get theirs back too.
+    private void rebuildShownMesh() {
+        LoadedObject object = shown;
+        if (object == null) {
+            return;
+        }
+        long revision = meshRevision.incrementAndGet();
+        ObjectMesh.Bounds bounds = object.mesh().bounds();
+        boolean cutting = layerCut.isCutting();
+        int highestShownY = bounds.minY() + layerCut.shownLayerCount() - 1;
+        Set<String> seeThrough = seeThroughTypes;
+        loader.execute(() -> {
+            if (revision != meshRevision.get()) {
+                return;
+            }
+            ObjectMesh mesh = !cutting && seeThrough.isEmpty()
+                    ? object.mesh()
+                    : ObjectMesh.build(cutting ? blocksUpTo(object.blocks(), highestShownY) : object.blocks(), this::modelOf, bounds,
+                    block -> seeThrough.contains(block.getMaterial().getKey().getKey()));
+            EventQueue.invokeLater(() -> {
+                if (revision == meshRevision.get() && shown == object) {
+                    viewport.replaceMesh(mesh);
+                    showMeshSummary(mesh);
+                }
+            });
+        });
+    }
+
+    private static Map<BlockVector, BlockData> blocksUpTo(Map<BlockVector, BlockData> blocks, int highestY) {
+        Map<BlockVector, BlockData> below = new HashMap<>();
+        blocks.forEach((position, block) -> {
+            if (position.getBlockY() <= highestY) {
+                below.put(position, block);
+            }
+        });
+        return below;
     }
 
     private void showFailure(long revision, String key, Throwable error) {
@@ -191,6 +265,7 @@ final class ObjectExplorer {
         statusBar.showActivity("");
         viewport.showMessage("Could not load " + key);
         overview.clear();
+        layerCut.showHeight(1);
         statusBar.showError(error.getMessage() == null ? error.getClass().getSimpleName() : error.getMessage());
     }
 
@@ -238,6 +313,6 @@ final class ObjectExplorer {
         return texture.tinted(colors.colorAt(texture.biomeTint(), 0, 0));
     }
 
-    private record LoadedObject(ObjectMesh mesh, ObjectSummary summary) {
+    private record LoadedObject(Map<BlockVector, BlockData> blocks, ObjectMesh mesh, ObjectSummary summary) {
     }
 }

core/src/main/java/com/volmit/iris/core/gui/ObjectLayerCut.java

@@ -0,0 +1,179 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2022 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+package com.volmit.iris.core.gui;
+
+import javax.swing.JComponent;
+import javax.swing.JLabel;
+import javax.swing.JPanel;
+import javax.swing.JSlider;
+import javax.swing.SwingConstants;
+import java.awt.AlphaComposite;
+import java.awt.BorderLayout;
+import java.awt.Graphics;
+import java.awt.Graphics2D;
+import java.awt.event.MouseAdapter;
+import java.awt.event.MouseEvent;
+import java.util.function.IntConsumer;
+
+/**
+ * A slim slider floating on the right of the 3D view that hides an object's top layers, to look inside buildings
+ * floor by floor. Most objects never need it, so it stays faint until the mouse comes near or a cut is active.
+ */
+final class ObjectLayerCut extends JPanel {
+    private static final long serialVersionUID = 4473820211915526120L;
+    private static final int PANEL_WIDTH = 30;
+    private static final int MAX_HEIGHT = 320;
+    private static final double HEIGHT_SHARE_OF_VIEW = 0.5D;
+    private static final int EDGE_MARGIN = 8;
+    private static final float FAINT_OPACITY = 0.28F;
+
+    private final JSlider slider = new JSlider(SwingConstants.VERTICAL, 1, 1, 1);
+    private final JLabel shownLayers = new JLabel(" ", SwingConstants.CENTER);
+    private final IntConsumer onLayersChanged;
+    private boolean resetting;
+    private boolean hovered;
+
+    /**
+     * @param onLayersChanged told how many layers from the bottom stay visible whenever the user moves the slider
+     */
+    ObjectLayerCut(IntConsumer onLayersChanged) {
+        super(new BorderLayout(0, StudioTheme.scale(4)));
+        this.onLayersChanged = onLayersChanged;
+        setOpaque(false);
+        setToolTipText("Hide the top layers to look inside · Page Up / Page Down");
+        shownLayers.setFont(StudioTheme.smallFont());
+        shownLayers.setForeground(StudioTheme.SECONDARY_TEXT);
+        slider.setOpaque(false);
+        slider.setFocusable(false);
+        slider.setToolTipText(getToolTipText());
+        slider.addChangeListener(event -> {
+            showLayerLabel();
+            if (!resetting) {
+                onLayersChanged.accept(slider.getValue());
+            }
+        });
+        MouseAdapter hover = new MouseAdapter() {
+            @Override
+            public void mouseEntered(MouseEvent event) {
+                setHovered(true);
+            }
+
+            @Override
+            public void mouseExited(MouseEvent event) {
+                setHovered(slider.getValueIsAdjusting());
+            }
+
+            @Override
+            public void mouseReleased(MouseEvent event) {
+                setHovered(slider.contains(event.getPoint()));
+            }
+        };
+        slider.addMouseListener(hover);
+        addMouseListener(hover);
+        add(shownLayers, BorderLayout.NORTH);
+        add(slider, BorderLayout.CENTER);
+        showHeight(1);
+    }
+
+    /**
+     * The view with this slider floating over its right edge, halfway down.
+     */
+    JComponent floatingOver(JComponent view) {
+        ObjectLayerCut cut = this;
+        JPanel stack = new JPanel(null) {
+            private static final long serialVersionUID = -1950381405658102347L;
+
+            // The slider overlaps the view, so the view may not repaint without the slider on top of it.
+            @Override
+            public boolean isOptimizedDrawingEnabled() {
+                return false;
+            }
+
+            @Override
+            public void doLayout() {
+                view.setBounds(0, 0, getWidth(), getHeight());
+                int width = StudioTheme.scale(PANEL_WIDTH);
+                int height = Math.min(StudioTheme.scale(MAX_HEIGHT), (int) (getHeight() * HEIGHT_SHARE_OF_VIEW));
+                cut.setBounds(getWidth() - width - StudioTheme.scale(EDGE_MARGIN), (getHeight() - height) / 2, width, height);
+            }
+        };
+        stack.add(cut);
+        stack.add(view);
+        return stack;
+    }
+
+    /**
+     * Shows every layer of a newly chosen object without telling anyone, since nothing is hidden yet.
+     */
+    void showHeight(int layers) {
+        resetting = true;
+        try {
+            slider.setMaximum(Math.max(1, layers));
+            slider.setValue(slider.getMaximum());
+        } finally {
+            resetting = false;
+        }
+        showLayerLabel();
+        setVisible(layers > 1);
+    }
+
+    void stepBy(int layers) {
+        if (isVisible()) {
+            slider.setValue(slider.getValue() + layers);
+        }
+    }
+
+    boolean isCutting() {
+        return slider.getValue() < slider.getMaximum();
+    }
+
+    int shownLayerCount() {
+        return slider.getValue();
+    }
+
+    int layerCount() {
+        return slider.getMaximum();
+    }
+
+    @Override
+    protected void paintChildren(Graphics graphics) {
+        Graphics2D canvas = (Graphics2D) graphics.create();
+        try {
+            if (!hovered && !isCutting()) {
+                canvas.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, FAINT_OPACITY));
+            }
+            super.paintChildren(canvas);
+        } finally {
+            canvas.dispose();
+        }
+    }
+
+    private void setHovered(boolean nowHovered) {
+        if (hovered != nowHovered) {
+            hovered = nowHovered;
+            repaint();
+        }
+    }
+
+    private void showLayerLabel() {
+        shownLayers.setText(isCutting() ? Integer.toString(slider.getValue()) : " ");
+        repaint();
+    }
+}

core/src/main/java/com/volmit/iris/core/gui/ObjectMesh.java

@@ -27,6 +27,7 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 /**
  * The faces of an object's blocks that can be seen from outside. A face is left out when the neighbour on its side
@@ -34,22 +35,36 @@ import java.util.function.Function;
  * kept apart because they are drawn last, blended over everything behind them.
  */
 record ObjectMesh(List<Quad> solidQuads, List<Quad> translucentQuads, Bounds bounds) {
+    private static final double SEE_THROUGH_OPACITY = 0.22D;
+    private static final double FULL_OPACITY = 1D;
+
     static ObjectMesh build(Map<BlockVector, BlockData> blocks, Function<BlockData, BlockModel> modelOf) {
+        return build(blocks, modelOf, Bounds.of(blocks.keySet()), block -> false);
+    }
+
+    /**
+     * @param bounds       the whole object's extent, kept when only part of it is drawn so the view does not jump
+     * @param isSeeThrough blocks drawn faintly, which hide nothing behind them, to look past them into the object
+     */
+    static ObjectMesh build(Map<BlockVector, BlockData> blocks, Function<BlockData, BlockModel> modelOf, Bounds bounds,
+                            Predicate<BlockData> isSeeThrough) {
         List<Quad> solid = new ArrayList<>();
         List<Quad> translucent = new ArrayList<>();
         for (Map.Entry<BlockVector, BlockData> entry : blocks.entrySet()) {
             BlockVector position = entry.getKey();
             BlockData block = entry.getValue();
             BlockModel model = modelOf.apply(block);
+            boolean seeThrough = isSeeThrough.test(block);
             for (Face face : model.faces()) {
-                if (isHiddenByNeighbour(blocks, modelOf, position, block, model, face)) {
+                if (isHiddenByNeighbour(blocks, modelOf, isSeeThrough, position, block, model, face)) {
                     continue;
                 }
-                Quad quad = new Quad(face, position.getBlockX(), position.getBlockY(), position.getBlockZ());
-                (face.texture().isTranslucent() ? translucent : solid).add(quad);
+                Quad quad = new Quad(face, position.getBlockX(), position.getBlockY(), position.getBlockZ(),
+                        seeThrough ? SEE_THROUGH_OPACITY : FULL_OPACITY);
+                (seeThrough || face.texture().isTranslucent() ? translucent : solid).add(quad);
             }
         }
-        return new ObjectMesh(List.copyOf(solid), List.copyOf(translucent), Bounds.of(blocks.keySet()));
+        return new ObjectMesh(List.copyOf(solid), List.copyOf(translucent), bounds);
     }
 
     boolean isEmpty() {
@@ -60,9 +75,11 @@ record ObjectMesh(List<Quad> solidQuads, List<Quad> translucentQuads, Bounds bou
         return solidQuads.size() + translucentQuads.size();
     }
 
-    // Glass next to glass and water next to water hide the side between them too, as they do in the game.
+    // Glass next to glass and water next to water hide the side between them too, as they do in the game. A see-through
+    // block only hides sides of other see-through blocks, so whatever it covers shows through it.
     private static boolean isHiddenByNeighbour(Map<BlockVector, BlockData> blocks, Function<BlockData, BlockModel> modelOf,
-                                               BlockVector position, BlockData block, BlockModel model, Face face) {
+                                               Predicate<BlockData> isSeeThrough, BlockVector position, BlockData block,
+                                               BlockModel model, Face face) {
         CubeFace side = face.cullFace();
         if (side == null) {
             return false;
@@ -71,7 +88,7 @@ record ObjectMesh(List<Quad> solidQuads, List<Quad> translucentQuads, Bounds bou
                 position.getBlockX() + side.stepX(),
                 position.getBlockY() + side.stepY(),
                 position.getBlockZ() + side.stepZ()));
-        if (neighbour == null) {
+        if (neighbour == null || (isSeeThrough.test(neighbour) && !isSeeThrough.test(block))) {
             return false;
         }
         BlockModel neighbourModel = modelOf.apply(neighbour);
@@ -82,9 +99,9 @@ record ObjectMesh(List<Quad> solidQuads, List<Quad> translucentQuads, Bounds bou
     }
 
     /**
-     * A model face placed at a block in the object.
+     * A model face placed at a block in the object, drawn at the given opacity on top of its texture's own.
      */
-    record Quad(Face face, int x, int y, int z) {
+    record Quad(Face face, int x, int y, int z, double opacity) {
         double originX() {
             return x + face.origin().x();
         }

core/src/main/java/com/volmit/iris/core/gui/ObjectOverviewPanel.java

@@ -20,6 +20,8 @@ package com.volmit.iris.core.gui;
 
 import javax.swing.BorderFactory;
 import javax.swing.BoxLayout;
+import javax.swing.JButton;
+import javax.swing.SwingUtilities;
 import javax.swing.DefaultListCellRenderer;
 import javax.swing.DefaultListModel;
 import javax.swing.JLabel;
@@ -28,18 +30,28 @@ import javax.swing.JPanel;
 import javax.swing.JTextField;
 import javax.swing.event.DocumentEvent;
 import javax.swing.event.DocumentListener;
+import java.awt.AlphaComposite;
 import java.awt.BorderLayout;
 import java.awt.Component;
+import java.awt.Composite;
+import java.awt.Cursor;
+import java.awt.Insets;
+import java.awt.event.MouseAdapter;
+import java.awt.event.MouseEvent;
 import java.awt.Graphics;
 import java.awt.Graphics2D;
 import java.awt.GridLayout;
 import java.awt.RenderingHints;
 import java.awt.image.BufferedImage;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Set;
+import java.util.function.Consumer;
 
 /**
  * The chosen object's size and the blocks it is made of, most used first, like the overview in Procedural Objects.
+ * Clicking a block makes it see-through in the 3D view, to look past walls into the object.
  */
 final class ObjectOverviewPanel extends JPanel {
     private static final long serialVersionUID = -8811407613645718290L;
@@ -50,6 +62,8 @@ final class ObjectOverviewPanel extends JPanel {
     private static final int ROW_PADDING = 10;
     private static final int SHARE_WIDTH = 92;
     private static final int SHARE_BAR_HEIGHT = 4;
+    private static final float SEE_THROUGH_ICON_OPACITY = 0.3F;
+    private static final String SEE_THROUGH_HINT = "Click a block to see through it";
 
     private final JLabel title = new JLabel(" ");
     private final JLabel path = new JLabel(" ");
@@ -65,10 +79,18 @@ final class ObjectOverviewPanel extends JPanel {
             return true;
         }
     };
+    private final JLabel seeThroughHint = new JLabel(SEE_THROUGH_HINT);
+    private final JButton showAllSolid = new JButton("Show all solid");
+    private final Set<String> seeThroughTypes = new LinkedHashSet<>();
+    private final Consumer<Set<String>> onSeeThroughChanged;
     private ObjectSummary summary;
 
-    ObjectOverviewPanel() {
+    /**
+     * @param onSeeThroughChanged told which block types to draw see-through whenever the user changes them
+     */
+    ObjectOverviewPanel(Consumer<Set<String>> onSeeThroughChanged) {
         super(new BorderLayout());
+        this.onSeeThroughChanged = onSeeThroughChanged;
         setPreferredSize(StudioTheme.scaledSize(PANEL_WIDTH, 0));
         setBorder(BorderFactory.createMatteBorder(0, 1, 0, 0, StudioTheme.SEPARATOR));
         setBackground(StudioTheme.PANEL_BACKGROUND);
@@ -85,6 +107,8 @@ final class ObjectOverviewPanel extends JPanel {
         blockCount.setText(formatCount(nextSummary.blockCount()));
         blockTypes.setText(formatCount(nextSummary.blockUses().size()));
         filled.setText(String.format(Locale.ROOT, "%.1f%%", nextSummary.filledFraction() * 100D));
+        seeThroughTypes.clear();
+        showSeeThroughState();
         refilter();
     }
 
@@ -96,6 +120,8 @@ final class ObjectOverviewPanel extends JPanel {
             value.setText("—");
         }
         rows.clear();
+        seeThroughTypes.clear();
+        showSeeThroughState();
     }
 
     private JPanel buildHeader() {
@@ -147,11 +173,68 @@ final class ObjectOverviewPanel extends JPanel {
         list.setCellRenderer(new BlockUseRenderer());
         list.setBackground(StudioTheme.PANEL_BACKGROUND);
         list.setBorder(StudioTheme.emptyBorder(0, 6, 4, 6));
-        blocks.add(StudioToolbar.padded(filterField, 4, 12, 6, 12), BorderLayout.NORTH);
+        list.setCursor(Cursor.getPredefinedCursor(Cursor.HAND_CURSOR));
+        list.addMouseListener(new MouseAdapter() {
+            @Override
+            public void mouseClicked(MouseEvent event) {
+                int row = list.locationToIndex(event.getPoint());
+                if (SwingUtilities.isLeftMouseButton(event) && row >= 0 && list.getCellBounds(row, row).contains(event.getPoint())) {
+                    toggleSeeThrough(rows.get(row).blockType());
+                }
+            }
+        });
+        JPanel top = new JPanel();
+        top.setLayout(new BoxLayout(top, BoxLayout.Y_AXIS));
+        top.setOpaque(false);
+        top.add(leftAligned(StudioToolbar.padded(filterField, 4, 12, 4, 12)));
+        top.add(leftAligned(StudioToolbar.padded(seeThroughRow(), 0, 14, 6, 12)));
+        blocks.add(top, BorderLayout.NORTH);
         blocks.add(StudioToolbar.scroll(list), BorderLayout.CENTER);
         return blocks;
     }
 
+    private JPanel seeThroughRow() {
+        seeThroughHint.setFont(StudioTheme.smallFont());
+        seeThroughHint.setForeground(StudioTheme.MUTED_TEXT);
+        showAllSolid.setFont(StudioTheme.smallFont());
+        showAllSolid.setForeground(StudioTheme.LINK);
+        showAllSolid.setBorderPainted(false);
+        showAllSolid.setContentAreaFilled(false);
+        showAllSolid.setFocusable(false);
+        showAllSolid.setMargin(new Insets(0, 0, 0, 0));
+        showAllSolid.setBorder(BorderFactory.createEmptyBorder());
+        showAllSolid.setCursor(Cursor.getPredefinedCursor(Cursor.HAND_CURSOR));
+        showAllSolid.addActionListener(event -> {
+            seeThroughTypes.clear();
+            seeThroughChanged();
+        });
+        JPanel row = new JPanel(new BorderLayout());
+        row.setOpaque(false);
+        row.add(seeThroughHint, BorderLayout.CENTER);
+        row.add(showAllSolid, BorderLayout.EAST);
+        showSeeThroughState();
+        return row;
+    }
+
+    private void toggleSeeThrough(String blockType) {
+        if (!seeThroughTypes.remove(blockType)) {
+            seeThroughTypes.add(blockType);
+        }
+        seeThroughChanged();
+    }
+
+    private void seeThroughChanged() {
+        showSeeThroughState();
+        list.repaint();
+        onSeeThroughChanged.accept(Set.copyOf(seeThroughTypes));
+    }
+
+    private void showSeeThroughState() {
+        int count = seeThroughTypes.size();
+        seeThroughHint.setText(count == 0 ? SEE_THROUGH_HINT : count == 1 ? "1 block see-through" : count + " blocks see-through");
+        showAllSolid.setVisible(count > 0);
+    }
+
     private void refilter() {
         rows.clear();
         if (summary == null) {
@@ -199,6 +282,7 @@ final class ObjectOverviewPanel extends JPanel {
     private final class BlockUseRenderer extends DefaultListCellRenderer {
         private static final long serialVersionUID = 2785337045812613941L;
         private transient BufferedImage icon;
+        private boolean seeThrough;
         private double share;
         private String percent = "";
 
@@ -206,9 +290,12 @@ final class ObjectOverviewPanel extends JPanel {
         public Component getListCellRendererComponent(JList<?> owner, Object value, int index, boolean selected, boolean focused) {
             ObjectSummary.BlockUse use = (ObjectSummary.BlockUse) value;
             super.getListCellRendererComponent(owner, use.blockType(), index, false, false);
+            seeThrough = seeThroughTypes.contains(use.blockType());
             setFont(StudioTheme.font());
+            setForeground(seeThrough ? StudioTheme.MUTED_TEXT : StudioTheme.TEXT);
             setBorder(StudioTheme.emptyBorder(0, ROW_PADDING + ICON_SIZE + ICON_GAP, 0, ROW_PADDING + SHARE_WIDTH));
-            setToolTipText(use.blockType() + " · " + formatCount(use.count()) + " blocks");
+            setToolTipText(use.blockType() + " · " + formatCount(use.count()) + " blocks · click to "
+                    + (seeThrough ? "make solid again" : "see through it"));
             icon = use.icon();
             share = summary == null || summary.blockCount() == 0 ? 0D : use.count() / (double) summary.blockCount();
             percent = share > 0D && share < 0.001D ? "<0.1%" : String.format(Locale.ROOT, "%.1f%%", share * 100D);
@@ -225,7 +312,12 @@ final class ObjectOverviewPanel extends JPanel {
                 int iconY = (getHeight() - iconSize) / 2;
                 if (icon != null) {
                     canvas.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_NEAREST_NEIGHBOR);
+                    Composite solid = canvas.getComposite();
+                    if (seeThrough) {
+                        canvas.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, SEE_THROUGH_ICON_OPACITY));
+                    }
                     canvas.drawImage(icon, iconX, iconY, iconSize, iconSize, null);
+                    canvas.setComposite(solid);
                 } else {
                     canvas.setColor(StudioTheme.CONTROL_BORDER);
                     canvas.drawRect(iconX, iconY, iconSize - 1, iconSize - 1);
@@ -248,7 +340,7 @@ final class ObjectOverviewPanel extends JPanel {
             int barY = (getHeight() - barHeight) / 2;
             canvas.setColor(StudioTheme.HOVER_BACKGROUND);
             canvas.fillRoundRect(barX, barY, barWidth, barHeight, barHeight, barHeight);
-            canvas.setColor(StudioTheme.ACCENT);
+            canvas.setColor(seeThrough ? StudioTheme.CONTROL_BORDER : StudioTheme.ACCENT);
             canvas.fillRoundRect(barX, barY, Math.max(barHeight, (int) Math.round(barWidth * share)), barHeight, barHeight, barHeight);
             canvas.setColor(StudioTheme.SECONDARY_TEXT);
             int textWidth = canvas.getFontMetrics().stringWidth(percent);

core/src/main/java/com/volmit/iris/core/gui/ObjectRenderer.java

@@ -193,7 +193,7 @@ final class ObjectRenderer {
                 }
                 int color = BlockPreviewColors.shade(texels[texel], shade);
                 if (blended) {
-                    pixels[index] = blend(color, pixels[index], alphas == null ? 1D : alphas[texel] / FULL_ALPHA);
+                    pixels[index] = blend(color, pixels[index], (alphas == null ? 1D : alphas[texel] / FULL_ALPHA) * quad.opacity());
                     continue;
                 }
                 depths[index] = depth;

core/src/main/java/com/volmit/iris/core/gui/ObjectViewport.java

@@ -86,6 +86,26 @@ final class ObjectViewport extends JComponent {
         repaint();
     }
 
+    /**
+     * Swaps in another mesh of the same object, like one with its top layers cut away, keeping the camera.
+     */
+    void replaceMesh(ObjectMesh nextMesh) {
+        mesh = nextMesh;
+        message = nextMesh.isEmpty() ? "No visible blocks" : null;
+        requestFrame();
+        repaint();
+    }
+
+    void bindKey(int keyCode, String name, Runnable action) {
+        getInputMap(WHEN_FOCUSED).put(KeyStroke.getKeyStroke(keyCode, 0), name);
+        getActionMap().put(name, new AbstractAction() {
+            @Override
+            public void actionPerformed(ActionEvent event) {
+                action.run();
+            }
+        });
+    }
+
     void showMessage(String text) {
         mesh = null;
         message = text;
@@ -218,12 +238,6 @@ final class ObjectViewport extends JComponent {
     }
 
     private void bindKey(int keyCode, String name, UnaryOperator<ObjectCamera> move) {
-        getInputMap(WHEN_FOCUSED).put(KeyStroke.getKeyStroke(keyCode, 0), name);
-        getActionMap().put(name, new AbstractAction() {
-            @Override
-            public void actionPerformed(ActionEvent event) {
-                moveCamera(move);
-            }
-        });
+        bindKey(keyCode, name, () -> moveCamera(move));
     }
 }

core/src/main/java/com/volmit/iris/core/gui/StickyFolderHeaders.java

@@ -0,0 +1,268 @@
+/*
+ * Iris is a World Generator for Minecraft Bukkit Servers
+ * Copyright (c) 2022 Arcane Arts (Volmit Software)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+package com.volmit.iris.core.gui;
+
+import javax.swing.CellRendererPane;
+import javax.swing.Icon;
+import javax.swing.JComponent;
+import javax.swing.JLayer;
+import javax.swing.JScrollPane;
+import javax.swing.JTree;
+import javax.swing.JViewport;
+import javax.swing.SwingUtilities;
+import javax.swing.UIManager;
+import javax.swing.plaf.LayerUI;
+import javax.swing.plaf.basic.BasicTreeUI;
+import javax.swing.tree.TreePath;
+import java.awt.AWTEvent;
+import java.awt.Color;
+import java.awt.Component;
+import java.awt.Cursor;
+import java.awt.GradientPaint;
+import java.awt.Graphics;
+import java.awt.Graphics2D;
+import java.awt.Point;
+import java.awt.Rectangle;
+import java.awt.event.MouseEvent;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Keeps the folders around the top row of a scrolled tree pinned at the top, so a long folder never hides where it
+ * is. The next folder pushes the last header up as it arrives, and clicking a header jumps to its folder.
+ */
+final class StickyFolderHeaders extends LayerUI<JScrollPane> {
+    private static final long serialVersionUID = -2205474187531254372L;
+    private static final int SHADOW_HEIGHT = 4;
+    private static final Color SHADOW = new Color(0, 0, 0, 90);
+    private static final Color NO_SHADOW = new Color(0, 0, 0, 0);
+    // The folder handle sits one indent to the left of the folder's name, as the tree draws it.
+    private static final int HANDLE_NUDGE = 1;
+
+    private final JTree tree;
+    private final CellRendererPane rendererPane = new CellRendererPane();
+    private transient JLayer<JScrollPane> layer;
+    private int hoveredHeader = -1;
+
+    private StickyFolderHeaders(JTree tree) {
+        this.tree = tree;
+    }
+
+    static JLayer<JScrollPane> around(JTree tree, JScrollPane scrollPane) {
+        // Copying the scrolled pixels would drag the pinned headers along, so every scroll repaints instead.
+        scrollPane.getViewport().setScrollMode(JViewport.SIMPLE_SCROLL_MODE);
+        StickyFolderHeaders headers = new StickyFolderHeaders(tree);
+        JLayer<JScrollPane> layer = new JLayer<>(scrollPane, headers);
+        scrollPane.getViewport().addChangeListener(event -> layer.repaint());
+        return layer;
+    }
+
+    @Override
+    @SuppressWarnings("unchecked")
+    public void installUI(JComponent component) {
+        super.installUI(component);
+        layer = (JLayer<JScrollPane>) component;
+        layer.setLayerEventMask(AWTEvent.MOUSE_EVENT_MASK | AWTEvent.MOUSE_MOTION_EVENT_MASK);
+        tree.add(rendererPane);
+    }
+
+    @Override
+    public void uninstallUI(JComponent component) {
+        layer.setLayerEventMask(0);
+        tree.remove(rendererPane);
+        layer = null;
+        super.uninstallUI(component);
+    }
+
+    @Override
+    public void paint(Graphics graphics, JComponent component) {
+        super.paint(graphics, component);
+        List<Header> headers = pinnedHeaders();
+        if (headers.isEmpty()) {
+            return;
+        }
+        Graphics2D canvas = (Graphics2D) graphics.create();
+        try {
+            Rectangle viewport = viewportBounds();
+            canvas.clipRect(viewport.x, viewport.y, viewport.width, viewport.height);
+            for (int index = 0; index < headers.size(); index++) {
+                paintHeader(canvas, headers.get(index), index == hoveredHeader, viewport);
+            }
+            Header last = headers.get(headers.size() - 1);
+            int shadowTop = viewport.y + last.top() + tree.getRowHeight();
+            canvas.setPaint(new GradientPaint(0, shadowTop, SHADOW, 0, shadowTop + StudioTheme.scale(SHADOW_HEIGHT), NO_SHADOW));
+            canvas.fillRect(viewport.x, shadowTop, viewport.width, StudioTheme.scale(SHADOW_HEIGHT));
+        } finally {
+            canvas.dispose();
+        }
+    }
+
+    private void paintHeader(Graphics2D canvas, Header header, boolean hovered, Rectangle viewport) {
+        int rowHeight = tree.getRowHeight();
+        int top = viewport.y + header.top();
+        Rectangle row = tree.getRowBounds(header.row());
+        int left = viewport.x + row.x - scrolledView().x;
+        canvas.setColor(hovered ? StudioTheme.HOVER_BACKGROUND : tree.getBackground());
+        canvas.fillRect(viewport.x, top, viewport.width, rowHeight);
+        paintFolderHandle(canvas, left, top + rowHeight / 2);
+        Component renderer = tree.getCellRenderer().getTreeCellRendererComponent(
+                tree, header.path().getLastPathComponent(), false, true, false, header.row(), false);
+        rendererPane.paintComponent(canvas, renderer, tree, left, top, viewport.x + viewport.width - left, rowHeight, true);
+    }
+
+    private void paintFolderHandle(Graphics2D canvas, int folderLeft, int middleY) {
+        Icon handle = UIManager.getIcon("Tree.expandedIcon");
+        if (handle == null || !(tree.getUI() instanceof BasicTreeUI treeUI)) {
+            return;
+        }
+        int middleX = folderLeft - treeUI.getRightChildIndent() + HANDLE_NUDGE;
+        handle.paintIcon(tree, canvas, middleX - handle.getIconWidth() / 2, middleY - handle.getIconHeight() / 2);
+    }
+
+    @Override
+    protected void processMouseEvent(MouseEvent event, JLayer<? extends JScrollPane> source) {
+        if (event.getID() == MouseEvent.MOUSE_EXITED) {
+            hover(-1);
+            return;
+        }
+        int index = headerAt(event);
+        if (index < 0) {
+            return;
+        }
+        event.consume();
+        if (event.getID() == MouseEvent.MOUSE_CLICKED && SwingUtilities.isLeftMouseButton(event)) {
+            jumpTo(pinnedHeaders().get(index), index);
+        }
+    }
+
+    @Override
+    protected void processMouseMotionEvent(MouseEvent event, JLayer<? extends JScrollPane> source) {
+        hover(headerAt(event));
+    }
+
+    private void hover(int index) {
+        if (index == hoveredHeader) {
+            return;
+        }
+        hoveredHeader = index;
+        tree.setCursor(index >= 0 ? Cursor.getPredefinedCursor(Cursor.HAND_CURSOR) : null);
+        layer.repaint();
+    }
+
+    // The folder lands just below the headers of the folders around it.
+    private void jumpTo(Header header, int level) {
+        Rectangle row = tree.getRowBounds(header.row());
+        JViewport viewport = layer.getView().getViewport();
+        int top = Math.max(0, row.y - level * tree.getRowHeight());
+        int bottomLimit = Math.max(0, tree.getHeight() - viewport.getExtentSize().height);
+        viewport.setViewPosition(new Point(viewport.getViewPosition().x, Math.min(top, bottomLimit)));
+    }
+
+    private int headerAt(MouseEvent event) {
+        if (layer == null || event.getComponent() == null) {
+            return -1;
+        }
+        Point point = SwingUtilities.convertPoint(event.getComponent(), event.getPoint(), layer);
+        Rectangle viewport = viewportBounds();
+        if (!viewport.contains(point)) {
+            return -1;
+        }
+        List<Header> headers = pinnedHeaders();
+        for (int index = headers.size() - 1; index >= 0; index--) {
+            int top = viewport.y + headers.get(index).top();
+            if (point.y >= top && point.y < top + tree.getRowHeight()) {
+                return index;
+            }
+        }
+        return -1;
+    }
+
+    /**
+     * For each depth, the folder holding the row just under the headers above it, as long as that folder's own row
+     * has scrolled out of sight.
+     */
+    private List<Header> pinnedHeaders() {
+        List<Header> headers = new ArrayList<>();
+        int rowHeight = tree.getRowHeight();
+        if (layer == null || rowHeight <= 0 || tree.getRowCount() == 0) {
+            return headers;
+        }
+        Rectangle view = scrolledView();
+        for (int level = 0; ; level++) {
+            int slotTop = level * rowHeight;
+            int row = tree.getClosestRowForLocation(view.x, view.y + slotTop);
+            TreePath path = row < 0 ? null : tree.getPathForRow(row);
+            if (path == null || !isInsideFolderAt(path, level)) {
+                return headers;
+            }
+            TreePath folder = ancestorAt(path, level);
+            Rectangle lastRow = tree.getRowBounds(lastRowInside(folder, row));
+            int top = Math.min(slotTop, lastRow.y + lastRow.height - view.y - rowHeight);
+            headers.add(new Header(folder, tree.getRowForPath(folder), top));
+            if (top < slotTop) {
+                return headers;
+            }
+        }
+    }
+
+    // The hidden root is the path's first element, so the folder at depth 0 is its second.
+    private static boolean isInsideFolderAt(TreePath path, int level) {
+        return path.getPathCount() > level + 2;
+    }
+
+    private static TreePath ancestorAt(TreePath path, int level) {
+        TreePath ancestor = path;
+        while (ancestor.getPathCount() > level + 2) {
+            ancestor = ancestor.getParentPath();
+        }
+        return ancestor;
+    }
+
+    // A folder's rows sit together, so the last one is found by halving the rows after a row known to be inside.
+    private int lastRowInside(TreePath folder, int rowInside) {
+        int low = rowInside;
+        int high = tree.getRowCount() - 1;
+        while (low < high) {
+            int middle = (low + high + 1) >>> 1;
+            if (folder.isDescendant(tree.getPathForRow(middle))) {
+                low = middle;
+            } else {
+                high = middle - 1;
+            }
+        }
+        return low;
+    }
+
+    private Rectangle scrolledView() {
+        return layer.getView().getViewport().getViewRect();
+    }
+
+    private Rectangle viewportBounds() {
+        JViewport viewport = layer.getView().getViewport();
+        Point origin = SwingUtilities.convertPoint(viewport, 0, 0, layer);
+        return new Rectangle(origin.x, origin.y, viewport.getWidth(), viewport.getHeight());
+    }
+
+    /**
+     * A pinned folder, its row in the tree and how far below the top of the visible rows it is drawn.
+     */
+    private record Header(TreePath path, int row, int top) {
+    }
+}