Feat: Implement applying spline road to terrain

src/app/App.tsx

@@ -28,6 +28,7 @@ import { createImportAudioAssetCommand } from "../commands/import-audio-asset-co
 import { createImportBackgroundImageAssetCommand } from "../commands/import-background-image-asset-command";
 import { createImportModelAssetCommand } from "../commands/import-model-asset-command";
 import { createAddPathPointCommand } from "../commands/add-path-point-command";
+import { createApplySplineRoadToTerrainCommand } from "../commands/apply-spline-road-to-terrain-command";
 import { createDeletePathCommand } from "../commands/delete-path-command";
 import { createDeletePathPointCommand } from "../commands/delete-path-point-command";
 import { createDeletePathPointsCommand } from "../commands/delete-path-points-command";
@@ -9740,6 +9741,24 @@ export function App({
     }
   };
 
+  const handleApplyPathRoadToTerrain = () => {
+    if (selectedPath === null) {
+      setStatusMessage("Select a path before applying its road to terrain.");
+      return;
+    }
+
+    try {
+      store.executeCommand(
+        createApplySplineRoadToTerrainCommand({
+          pathId: selectedPath.id
+        })
+      );
+      setStatusMessage("Applied Path road to terrain.");
+    } catch (error) {
+      setStatusMessage(getErrorMessage(error));
+    }
+  };
+
   const handlePathPointDraftChange = (
     pointIndex: number,
     axis: keyof Vec3Draft,
@@ -22147,10 +22166,23 @@ export function App({
                         ))}
                       </select>
                     </label>
+                    <div className="inline-actions">
+                      <button
+                        className="toolbar__button"
+                        data-testid="apply-path-road-to-terrain"
+                        type="button"
+                        disabled={!selectedPath.road.enabled}
+                        onClick={handleApplyPathRoadToTerrain}
+                      >
+                        Apply Road to Terrain
+                      </button>
+                    </div>
                     <div className="material-summary">
                       Road preview draws a temporary corridor along the resolved
-                      spline. Width affects the visible strip; shoulder and
-                      falloff are stored for the terrain-apply command next.
+                      spline. Applying it patches terrain heights, terrain paint
+                      weights when the material already exists on a terrain
+                      layer, and foliage blocker masks. It does not generate a
+                      road mesh yet.
                     </div>
                   </div>
 

src/commands/apply-spline-road-to-terrain-command.ts

@@ -0,0 +1,119 @@
+import { createOpaqueId } from "../core/ids";
+import { cloneEditorSelection, type EditorSelection } from "../core/selection";
+import type { TerrainBrushPatch } from "../core/terrain-brush";
+import type { ToolMode } from "../core/tool-mode";
+import { getTerrains } from "../document/terrains";
+import { createSplineRoadTerrainPatches } from "../geometry/spline-road-terrain";
+
+import {
+  applyTerrainBrushPatchToDocument,
+  isTerrainBrushPatchEmpty
+} from "./apply-terrain-brush-patch-command";
+import type { CommandContext, EditorCommand } from "./command";
+
+interface ApplySplineRoadToTerrainCommandOptions {
+  pathId: string;
+  label?: string;
+}
+
+function clonePatch(patch: TerrainBrushPatch): TerrainBrushPatch {
+  return {
+    terrainId: patch.terrainId,
+    heightSamples: patch.heightSamples.map((entry) => ({ ...entry })),
+    paintWeights: patch.paintWeights.map((entry) => ({ ...entry })),
+    foliageMaskValues: patch.foliageMaskValues.map((entry) => ({ ...entry })),
+    foliageBlockerMaskValues: patch.foliageBlockerMaskValues.map((entry) => ({
+      ...entry
+    }))
+  };
+}
+
+function setPathSelection(pathId: string): EditorSelection {
+  return {
+    kind: "paths",
+    ids: [pathId]
+  };
+}
+
+function applyPatches(
+  context: CommandContext,
+  patches: readonly TerrainBrushPatch[],
+  direction: "forward" | "backward"
+) {
+  let nextDocument = context.getDocument();
+
+  for (const patch of patches) {
+    nextDocument = applyTerrainBrushPatchToDocument(
+      nextDocument,
+      patch,
+      direction
+    );
+  }
+
+  context.setDocument(nextDocument);
+}
+
+export function createApplySplineRoadToTerrainCommand(
+  options: ApplySplineRoadToTerrainCommandOptions
+): EditorCommand {
+  let patches: TerrainBrushPatch[] | null = null;
+  let previousSelection: EditorSelection | null = null;
+  let previousToolMode: ToolMode | null = null;
+
+  return {
+    id: createOpaqueId("command"),
+    label: options.label ?? "Apply spline road to terrain",
+    execute(context) {
+      const currentDocument = context.getDocument();
+      const path = currentDocument.paths[options.pathId];
+
+      if (path === undefined) {
+        throw new Error(`Path ${options.pathId} does not exist.`);
+      }
+
+      if (!path.road.enabled) {
+        throw new Error("Enable road preview before applying it to terrain.");
+      }
+
+      if (previousSelection === null) {
+        previousSelection = cloneEditorSelection(context.getSelection());
+      }
+
+      if (previousToolMode === null) {
+        previousToolMode = context.getToolMode();
+      }
+
+      if (patches === null) {
+        patches = createSplineRoadTerrainPatches({
+          path,
+          terrains: getTerrains(currentDocument.terrains)
+        })
+          .filter((patch) => !isTerrainBrushPatchEmpty(patch))
+          .map(clonePatch);
+      }
+
+      if (patches.length === 0) {
+        throw new Error("The spline road did not affect any enabled terrain.");
+      }
+
+      applyPatches(context, patches, "forward");
+      context.setSelection(setPathSelection(options.pathId));
+      context.setToolMode("select");
+    },
+    undo(context) {
+      if (patches === null) {
+        return;
+      }
+
+      applyPatches(context, [...patches].reverse(), "backward");
+
+      if (previousSelection !== null) {
+        context.setSelection(previousSelection);
+      }
+
+      if (previousToolMode !== null) {
+        context.setToolMode(previousToolMode);
+      }
+    }
+  };
+}