auto-git:

[add] src/rendering/terrain-layer-material.ts
 [add] tests/domain/terrains.test.ts
 [change] src/app/App.tsx
 [change] src/core/terrain-brush.ts
 [change] src/document/migrate-scene-document.ts
 [change] src/document/scene-document-validation.ts
 [change] src/document/scene-document.ts
 [change] src/document/terrains.ts
 [change] src/geometry/terrain-brush.ts
 [change] src/geometry/terrain-mesh.ts
 [change] src/runtime-three/rapier-collision-world.ts
 [change] src/runtime-three/runtime-host.ts
 [change] src/runtime-three/runtime-scene-build.ts
 [change] src/viewport-three/ViewportCanvas.tsx
 [change] src/viewport-three/ViewportPanel.tsx
 [change] src/viewport-three/viewport-host.ts
 [change] tests/domain/build-runtime-scene.test.ts
 [change] tests/domain/rapier-collision-world.test.ts
 [change] tests/domain/terrain.command.test.ts
 [change] tests/domain/water-material.test.ts
 [change] tests/geometry/terrain-brush.test.ts
 [change] tests/geometry/terrain-mesh.test.ts
 [change] tests/serialization/scene-document-json.test.ts
 [change] tests/unit/terrain-foundation.integration.test.tsx
 [change] tests/unit/viewport-canvas.test.tsx

tests/geometry/terrain-brush.test.ts

@@ -1,7 +1,10 @@
 import { describe, expect, it } from "vitest";
 
 import { createDefaultTerrainBrushSettings } from "../../src/core/terrain-brush";
-import { createTerrain } from "../../src/document/terrains";
+import {
+  createTerrain,
+  getTerrainSampleLayerWeights
+} from "../../src/document/terrains";
 import {
   applyTerrainBrushStamp,
   getTerrainBrushWeight,
@@ -108,4 +111,52 @@ describe("terrain brush geometry", () => {
   it("returns zero influence outside the brush radius", () => {
     expect(getTerrainBrushWeight(2, 1, 0.5)).toBe(0);
   });
+
+  it("paints terrain layer weights toward the active layer while preserving a normalized blend", () => {
+    const terrain = createTerrain({
+      id: "terrain-paint",
+      position: { x: 0, y: 0, z: 0 },
+      sampleCountX: 3,
+      sampleCountZ: 3,
+      cellSize: 1
+    });
+
+    const paintedTerrain = applyTerrainBrushStamp({
+      terrain,
+      center: { x: 1, z: 1 },
+      settings: {
+        radius: 1.2,
+        strength: 0.5,
+        falloff: 0
+      },
+      tool: "paint",
+      layerIndex: 2
+    });
+
+    const paintedWeights = getTerrainSampleLayerWeights(paintedTerrain, 1, 1);
+
+    expect(paintedWeights[2]).toBeCloseTo(0.5);
+    expect(
+      paintedWeights[0] +
+        paintedWeights[1] +
+        paintedWeights[2] +
+        paintedWeights[3]
+    ).toBeCloseTo(1);
+
+    const repaintedBaseTerrain = applyTerrainBrushStamp({
+      terrain: paintedTerrain,
+      center: { x: 1, z: 1 },
+      settings: {
+        radius: 1.2,
+        strength: 0.5,
+        falloff: 0
+      },
+      tool: "paint",
+      layerIndex: 0
+    });
+
+    const baseWeights = getTerrainSampleLayerWeights(repaintedBaseTerrain, 1, 1);
+    expect(baseWeights[0]).toBeGreaterThan(paintedWeights[0]);
+    expect(baseWeights[2]).toBeLessThan(paintedWeights[2]);
+  });
 });

tests/geometry/terrain-mesh.test.ts

@@ -56,4 +56,46 @@ describe("terrain mesh generation", () => {
 
     expect(Array.from(derivedMesh.uvs)).toEqual([10, -6, 12, -6, 10, -4, 12, -4]);
   });
+
+  it("derives full per-vertex layer weights from the compact terrain paint data", () => {
+    const terrain = createTerrain({
+      sampleCountX: 2,
+      sampleCountZ: 2,
+      paintWeights: [
+        0.2,
+        0.3,
+        0.1,
+        0,
+        0.5,
+        0,
+        0.1,
+        0.1,
+        0.1,
+        0.25,
+        0.25,
+        0.25
+      ]
+    });
+
+    const derivedMesh = buildTerrainDerivedMeshData(terrain);
+
+    expect(Array.from(derivedMesh.layerWeights)).toEqual([
+      expect.closeTo(0.4, 5),
+      expect.closeTo(0.2, 5),
+      expect.closeTo(0.3, 5),
+      expect.closeTo(0.1, 5),
+      expect.closeTo(0.5, 5),
+      expect.closeTo(0, 5),
+      expect.closeTo(0.5, 5),
+      expect.closeTo(0, 5),
+      expect.closeTo(0.7, 5),
+      expect.closeTo(0.1, 5),
+      expect.closeTo(0.1, 5),
+      expect.closeTo(0.1, 5),
+      expect.closeTo(0.25, 5),
+      expect.closeTo(0.25, 5),
+      expect.closeTo(0.25, 5),
+      expect.closeTo(0.25, 5)
+    ]);
+  });
 });