diff --git a/src/commands/apply-terrain-brush-patch-command.ts b/src/commands/apply-terrain-brush-patch-command.ts
index ce59ebf7..c9122e65 100644
--- a/src/commands/apply-terrain-brush-patch-command.ts
+++ b/src/commands/apply-terrain-brush-patch-command.ts
@@ -81,6 +81,101 @@ function mergeTerrainSampleIndexIntoBounds(
   };
 }
 
+export function applyTerrainBrushPatchToDocument(
+  currentDocument: ReturnType<CommandContext["getDocument"]>,
+  patch: TerrainBrushPatch,
+  direction: "forward" | "backward"
+): ReturnType<CommandContext["getDocument"]> {
+  const terrain = currentDocument.terrains[patch.terrainId];
+
+  if (terrain === undefined) {
+    throw new Error(`Terrain ${patch.terrainId} does not exist.`);
+  }
+
+  const heightPatchForBounds = patch.heightSamples.map((entry) => ({
+    index: entry.index,
+    before: direction === "forward" ? entry.before : entry.after,
+    after: direction === "forward" ? entry.after : entry.before
+  }));
+
+  let renderDirtyBounds: TerrainSampleBounds | null = null;
+
+  for (const entry of patch.heightSamples) {
+    assertValidPatchEntry(entry, terrain.heights.length, "Terrain height");
+    terrain.heights[entry.index] =
+      direction === "forward" ? entry.after : entry.before;
+    renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
+      renderDirtyBounds,
+      terrain,
+      entry.index
+    );
+  }
+
+  updateTerrainBoundsCacheAfterHeightPatch(terrain, heightPatchForBounds);
+
+  for (const entry of patch.paintWeights) {
+    assertValidPatchEntry(
+      entry,
+      terrain.paintWeights.length,
+      "Terrain paint weight"
+    );
+    terrain.paintWeights[entry.index] =
+      direction === "forward" ? entry.after : entry.before;
+    renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
+      renderDirtyBounds,
+      terrain,
+      Math.floor(entry.index / (TERRAIN_LAYER_COUNT - 1))
+    );
+  }
+
+  for (const entry of patch.foliageMaskValues) {
+    if (currentDocument.foliageLayers[entry.layerId] === undefined) {
+      throw new Error(
+        `Foliage layer ${entry.layerId} does not exist for terrain mask patch.`
+      );
+    }
+
+    const mask = getOrCreateTerrainFoliageMask(terrain, entry.layerId);
+    assertValidPatchEntry(entry, mask.values.length, "Terrain foliage mask");
+    mask.values[entry.index] =
+      direction === "forward" ? entry.after : entry.before;
+    renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
+      renderDirtyBounds,
+      terrain,
+      entry.index
+    );
+
+    if (isTerrainFoliageMaskEmpty(mask)) {
+      delete terrain.foliageMasks[entry.layerId];
+    }
+  }
+
+  for (const entry of patch.foliageBlockerMaskValues) {
+    assertValidPatchEntry(
+      entry,
+      terrain.foliageBlockerMask.values.length,
+      "Terrain foliage blocker mask"
+    );
+    terrain.foliageBlockerMask.values[entry.index] =
+      direction === "forward" ? entry.after : entry.before;
+    renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
+      renderDirtyBounds,
+      terrain,
+      entry.index
+    );
+  }
+
+  markTerrainRenderSamplesDirty(terrain, renderDirtyBounds);
+
+  return {
+    ...currentDocument,
+    terrains: {
+      ...currentDocument.terrains,
+      [patch.terrainId]: terrain
+    }
+  };
+}
+
 export function createApplyTerrainBrushPatchCommand(
   options: ApplyTerrainBrushPatchCommandOptions
 ): EditorCommand {
@@ -102,99 +197,13 @@ export function createApplyTerrainBrushPatchCommand(
     context: CommandContext,
     direction: "forward" | "backward"
   ) => {
-    const currentDocument = context.getDocument();
-    const terrain = currentDocument.terrains[patch.terrainId];
-
-    if (terrain === undefined) {
-      throw new Error(`Terrain ${patch.terrainId} does not exist.`);
-    }
-
-    const heightPatchForBounds = patch.heightSamples.map((entry) => ({
-      index: entry.index,
-      before: direction === "forward" ? entry.before : entry.after,
-      after: direction === "forward" ? entry.after : entry.before
-    }));
-
-    let renderDirtyBounds: TerrainSampleBounds | null = null;
-
-    for (const entry of patch.heightSamples) {
-      assertValidPatchEntry(entry, terrain.heights.length, "Terrain height");
-      terrain.heights[entry.index] =
-        direction === "forward" ? entry.after : entry.before;
-      renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
-        renderDirtyBounds,
-        terrain,
-        entry.index
-      );
-    }
-
-    updateTerrainBoundsCacheAfterHeightPatch(terrain, heightPatchForBounds);
-
-    for (const entry of patch.paintWeights) {
-      assertValidPatchEntry(
-        entry,
-        terrain.paintWeights.length,
-        "Terrain paint weight"
-      );
-      terrain.paintWeights[entry.index] =
-        direction === "forward" ? entry.after : entry.before;
-      renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
-        renderDirtyBounds,
-        terrain,
-        Math.floor(entry.index / (TERRAIN_LAYER_COUNT - 1))
-      );
-    }
-
-    for (const entry of patch.foliageMaskValues) {
-      if (currentDocument.foliageLayers[entry.layerId] === undefined) {
-        throw new Error(
-          `Foliage layer ${entry.layerId} does not exist for terrain mask patch.`
-        );
-      }
-
-      const mask = getOrCreateTerrainFoliageMask(terrain, entry.layerId);
-      assertValidPatchEntry(
-        entry,
-        mask.values.length,
-        "Terrain foliage mask"
-      );
-      mask.values[entry.index] =
-        direction === "forward" ? entry.after : entry.before;
-      renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
-        renderDirtyBounds,
-        terrain,
-        entry.index
-      );
-
-      if (isTerrainFoliageMaskEmpty(mask)) {
-        delete terrain.foliageMasks[entry.layerId];
-      }
-    }
-
-    for (const entry of patch.foliageBlockerMaskValues) {
-      assertValidPatchEntry(
-        entry,
-        terrain.foliageBlockerMask.values.length,
-        "Terrain foliage blocker mask"
-      );
-      terrain.foliageBlockerMask.values[entry.index] =
-        direction === "forward" ? entry.after : entry.before;
-      renderDirtyBounds = mergeTerrainSampleIndexIntoBounds(
-        renderDirtyBounds,
-        terrain,
-        entry.index
-      );
-    }
-
-    markTerrainRenderSamplesDirty(terrain, renderDirtyBounds);
-
-    context.setDocument({
-      ...currentDocument,
-      terrains: {
-        ...currentDocument.terrains,
-        [patch.terrainId]: terrain
-      }
-    });
+    context.setDocument(
+      applyTerrainBrushPatchToDocument(
+        context.getDocument(),
+        patch,
+        direction
+      )
+    );
   };
 
   return {