Let a tile report a touch instead of blocking it (#160)

A tile could be solid, one-way and scaled, but not report without
blocking. Water, lava, spikes or a goal zone needed a trigger object
of their own laid over the map. Now a tile has a switch Trigger, and
the collision events name the tile and the cell they met.

- A tile has the switch "Trigger (reports, never blocks)" under
  Solid. Its cells hold nothing up: no Move, no falling body, no
  ray. They raise On collision enter, stay and exit with the tilemap
  as other.
- The three events have the outputs tile, cell x and cell y. Without
  a tile, tile is empty and the cell is -1, -1.
- The three events have the input per cell. Off, the default, gives
  one event per area: cells of the same tile that touch side by
  side. On gives one event per cell.
- The scene view outlines trigger cells in yellow like a trigger
  collider, merged to rectangles but never with solid cells.
- Is touching with the map counts trigger tiles, and Set tile works
  while the game runs.
- Both systems find the touches with the same overlap check in
  runtime/systems/tileTriggers.ts, not through the physics engine.

Decided here:

- Trigger wins over Solid. A tile with both does not block, as with
  a collider.
- Water beside lava is two areas, and so are two ponds that touch
  only at a corner.
- An area names the cell nearest to the centre of the collider, and
  on exit the cell it named last.
- The solid cells of a map still do not name their tile.
- Normal, impulse and point are 0, as with a trigger collider.
- Collision scale and outlines shape a trigger cell like a solid
  one. One-way, friction and bounciness have no effect.

Tests:
- Gate on stein2: 205 files, 3104 tests (3067 + 37), exit 0
- e2e 39,40: 106 checks. e2e 1-8,39-43: 490 checks. Both on
  melektaus-atx. No full run, the sum of 1648 (1645 + 3) is
  reckoned and CI measures it.
- 37 unit tests in tests/core/tile-triggers.test.ts, nearly all
  against both systems
- Three e2e checks with a real mouse in block 40l-iv
- 24 unit counter-probes fail tests. Two did not at first, tests
  were added for them.
- Two browser counter-probes fail their checks
- Two screenshots looked at (yellow outline, the node with per cell)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -45,7 +45,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1645 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1648 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs

src/core/graph/GraphInterpreter.ts

@@ -120,6 +120,17 @@ export class GraphInterpreter {
     this.follow(node, port, ctx, 0);
   }
 
+  /**
+   * A data input of an event node, read ahead of its pulse: what decides whether the event concerns
+   * the node at all (#160). Null for an input the node does not have.
+   */
+  eventInput(node: GraphNode, key: string, ctx: NodeContext): Value {
+    this.frame = this.emptyFrame(this.graph);
+    this.steps = 0;
+    const port = this.def(node)?.inputs.find((p) => p.key === key);
+    return port ? this.inputValue(node, port, ctx) : null;
+  }
+
   /**
    * The `On state entered` / `On state exited` nodes watching one machine. A node whose machine
    * input carries a running value rather than a pick has no state outputs at all, so it is never

src/core/graph/nodes/action/tileNodes.ts

@@ -7,7 +7,7 @@ import { cellToWorld, worldToCell } from '../../../model/liveTilemap';
 import { worldMatrixOf } from '../../NodeContext';
 import type { NodeDef, PortDef } from '../../NodeRegistry';
 import { asEntity, asNumber, asString, type Value } from '../../values';
-import { bool, exec, int, num, obj } from '../shared';
+import { bool, exec, int, num, obj, tilePort } from '../shared';
 
 /** The layer by name, picked from the layers of every map in the project; none = the first. */
 const layerPort = (): PortDef => ({
@@ -17,14 +17,6 @@ const layerPort = (): PortDef => ({
   field: { key: 'layer', label: 'Layer', kind: 'tileLayer', default: '' },
 });
 
-/** A tile asset; unconnected, it shows the tile picker, and none chosen empties the cell. */
-const tilePort = (key = 'tile', label = key): PortDef => ({
-  key,
-  label,
-  type: 'string',
-  field: { key, label: 'Tile', kind: 'asset', default: '', assetTypes: ['tile'] },
-});
-
 const CELL_INPUTS: readonly PortDef[] = [int('cellX', 0, 'cell x'), int('cellY', 0, 'cell y')];
 
 /** A cell counts in whole columns and rows, so a fraction arriving over an edge is floored. */

src/core/graph/nodes/events/OnCollisionEnter.ts

@@ -1,6 +1,6 @@
 import type { NodeDef } from '../../NodeRegistry';
 import { hiddenOrder, orderParam } from '../../eventOrder';
-import { collisionOutputs, contactOutputs } from '../shared';
+import { collisionOutputs, contactOutputs, perCellPort, tileOutputs } from '../shared';
 
 export const OnCollisionEnter: NodeDef = {
   type: 'OnCollisionEnter',
@@ -9,9 +9,11 @@ export const OnCollisionEnter: NodeDef = {
   description:
     'Runs once when a collider of the owner starts overlapping another collider. While playing, ' +
     'normal x/y point from the other collider towards the own one, impulse says how hard they met and ' +
-    'point x/y where (all 0 for a trigger).',
-  inputs: [],
-  outputs: [...collisionOutputs(), ...contactOutputs()],
+    'point x/y where (all 0 for a trigger). Walking onto a trigger tile of a tilemap runs it too, with ' +
+    'the tile and its cell: once for a whole stretch of touching cells of that tile, or with "per cell" ' +
+    'for every cell.',
+  inputs: [perCellPort()],
+  outputs: [...collisionOutputs(), ...contactOutputs(), ...tileOutputs()],
   params: [orderParam],
   hiddenParams: hiddenOrder,
 };

src/core/graph/nodes/events/OnCollisionExit.ts

@@ -1,14 +1,16 @@
 import type { NodeDef } from '../../NodeRegistry';
 import { hiddenOrder, orderParam } from '../../eventOrder';
-import { collisionOutputs } from '../shared';
+import { collisionOutputs, perCellPort, tileOutputs } from '../shared';
 
 export const OnCollisionExit: NodeDef = {
   type: 'OnCollisionExit',
   label: 'On collision exit',
   category: 'event',
-  description: 'Runs once when a collider of the owner stops overlapping another collider.',
-  inputs: [],
-  outputs: collisionOutputs(),
+  description:
+    'Runs once when a collider of the owner stops overlapping another collider – or leaves a trigger tile ' +
+    'of a tilemap: the whole stretch of touching cells of that tile, or with "per cell" each cell.',
+  inputs: [perCellPort()],
+  outputs: [...collisionOutputs(), ...tileOutputs()],
   params: [orderParam],
   hiddenParams: hiddenOrder,
 };

src/core/graph/nodes/events/OnCollisionStay.ts

@@ -1,6 +1,6 @@
 import type { NodeDef } from '../../NodeRegistry';
 import { hiddenOrder, orderParam } from '../../eventOrder';
-import { collisionOutputs, contactOutputs } from '../shared';
+import { collisionOutputs, contactOutputs, perCellPort, tileOutputs } from '../shared';
 
 export const OnCollisionStay: NodeDef = {
   type: 'OnCollisionStay',
@@ -9,9 +9,11 @@ export const OnCollisionStay: NodeDef = {
   description:
     'Runs while a collider of the owner overlaps another collider: every frame, or with an interval once ' +
     'each time that long has passed since they met. While playing, normal x/y, impulse and point x/y tell ' +
-    'how they touch right now (all 0 for a trigger and in the editor run).',
-  inputs: [],
-  outputs: [...collisionOutputs(), ...contactOutputs()],
+    'how they touch right now (all 0 for a trigger and in the editor run). On a trigger tile of a ' +
+    'tilemap, tile and cell x/y say which: the cell nearest to the own collider, or with "per cell" each ' +
+    'cell it lies on.',
+  inputs: [perCellPort()],
+  outputs: [...collisionOutputs(), ...contactOutputs(), ...tileOutputs()],
   params: [
     {
       key: 'interval',

src/core/graph/nodes/shared.ts

@@ -44,6 +44,34 @@ export const contactOutputs = (): PortDef[] => [
   num('pointY', 'point y'),
 ];
 
+/** A tile asset; unconnected as an input, it shows the tile picker. */
+export const tilePort = (key = 'tile', label = key): PortDef => ({
+  key,
+  label,
+  type: 'string',
+  field: { key, label: 'Tile', kind: 'asset', default: '', assetTypes: ['tile'] },
+});
+
+export const PER_CELL_INPUT = 'perCell';
+
+/**
+ * Which pairs a collision event hears of a map's trigger tiles (#160): off, one per area of a tile –
+ * a pond is entered once –, on, one per cell.
+ */
+export const perCellPort = (): PortDef => ({
+  key: PER_CELL_INPUT,
+  label: 'per cell',
+  type: 'boolean',
+  default: false,
+  field: { key: PER_CELL_INPUT, label: 'Per cell (trigger tiles)', kind: 'boolean', default: false },
+});
+
+/**
+ * The trigger tile a collider lies on (#160) and its cell in the map, which is then `other`. No
+ * tile and cell -1, -1 for anything else: another collider, or a map's solid cells.
+ */
+export const tileOutputs = (): PortDef[] => [tilePort(), num('cellX', 'cell x'), num('cellY', 'cell y')];
+
 export const keyParam: PropertyDef = { key: 'key', label: 'Key', kind: 'key', default: 'ArrowRight' };
 
 /** No `assetTypes`: any placeable asset is a valid answer here. */

src/core/model/tile.ts

@@ -21,6 +21,8 @@ export const TILE_FPS_KEY = 'fps';
 export const TILE_PLAYBACK_KEY = 'playback';
 /** Whether a cell of this tile blocks movement in the running game (#132). */
 export const TILE_SOLID_KEY = 'solid';
+/** Whether a cell of this tile reports what overlaps it instead of blocking it: water, lava, a goal (#160). */
+export const TILE_TRIGGER_KEY = 'trigger';
 /** Whether a solid tile only blocks from above, as a platform one jumps through from below (#155). */
 export const TILE_ONE_WAY_KEY = 'oneWay';
 /** How far a contact may lean from straight up and still land on a one-way tile, in degrees (#159). */
@@ -59,8 +61,13 @@ export function tilePlayback(asset: TileAsset): TilePlayback {
   return (TILE_PLAYBACKS as readonly string[]).includes(value) ? (value as TilePlayback) : 'loop';
 }
 
+/** A trigger never blocks, whatever Solid says: it is the collider's rule, where Trigger wins too. */
 export function isTileSolid(asset: TileAsset): boolean {
-  return asset.props[TILE_SOLID_KEY] === true;
+  return asset.props[TILE_SOLID_KEY] === true && !isTileTrigger(asset);
+}
+
+export function isTileTrigger(asset: TileAsset): boolean {
+  return asset.props[TILE_TRIGGER_KEY] === true;
 }
 
 export function isTileOneWay(asset: TileAsset): boolean {

src/core/model/tilemapCollision.ts

@@ -22,6 +22,9 @@
  * each function takes `oneWay` – so a platform never merges with a wall into one rectangle, nor with
  * a platform of another angle (#159). The same holds for a tile's surface (#193): with a `material`,
  * a function looks only at the tiles of that friction and bounciness.
+ *
+ * A trigger tile (#160) blocks nothing. Its cells are worked out by the same rules, apart from the
+ * solid ones, and handed on cell by cell: the collision events tell which one was walked into.
  */
 import type { TileAsset, TilemapAsset } from './Asset';
 import { tileQuadrants } from './autotile';
@@ -33,6 +36,7 @@ import { findFrame } from './texture';
 import {
   isTileOneWay,
   isTileSolid,
+  isTileTrigger,
   readTile,
   TILE_ONE_WAY_ANGLE_KEY,
   tileCollisionScale,
@@ -244,6 +248,17 @@ function passesFilter(tile: TileAsset, oneWay: OneWayFilter, material?: TileMate
   return oneWay === true || tileOneWayAngle(tile) === oneWay;
 }
 
+/**
+ * Whose cells a function works out: the solid tiles passing `oneWay` and `material`, or the trigger
+ * tiles (#160) – all of them, or the one named. A trigger tile is never among the solid ones.
+ */
+type TilePick = { oneWay: OneWayFilter; material?: TileMaterial } | { trigger: string | null };
+
+function picks(tile: TileAsset, pick: TilePick): boolean {
+  if ('trigger' in pick) return isTileTrigger(tile) && (pick.trigger === null || pick.trigger === tile.id);
+  return isTileSolid(tile) && passesFilter(tile, pick.oneWay, pick.material);
+}
+
 /** The solid tiles painted on the map, each once, in the order the layers first name them. */
 function solidTiles(project: Project, asset: TilemapAsset): TileAsset[] {
   const ids = new Set(asset.layers.flatMap((layer) => cellsTileIds(layer.cells)));
@@ -286,8 +301,7 @@ function blockingCells(
   project: Project,
   props: PropertyBag,
   asset: TilemapAsset,
-  oneWay: OneWayFilter,
-  material?: TileMaterial,
+  pick: TilePick,
 ): readonly CellBlock[] {
   const tiles = new Map<string, BlockingTile>();
   for (const layer of asset.layers)
@@ -296,7 +310,7 @@ function blockingCells(
       const tile = findAssetOf(project, id, 'tile');
       tiles.set(
         id,
-        tile && isTileSolid(tile) && passesFilter(tile, oneWay, material)
+        tile && picks(tile, pick)
           ? { scale: tileCollisionScale(tile), shape: tileShape(project, tile) }
           : null,
       );
@@ -304,8 +318,9 @@ function blockingCells(
   const key = [
     tilemapColumns(props),
     tilemapRows(props),
-    oneWay,
-    material ? `${material.friction}/${material.bounciness}` : '',
+    'trigger' in pick
+      ? `trigger:${pick.trigger ?? ''}`
+      : `${pick.oneWay}|${pick.material ? `${pick.material.friction}/${pick.material.bounciness}` : ''}`,
     asset.layers.map((layer) => cellsVersion(layer.cells)).join(','),
     [...tiles]
       .map(([id, t]) => (t ? `${id}:${t.scale}:${t.shape ? identityToken(t.shape) : 'box'}` : id))
@@ -373,7 +388,7 @@ export function solidCells(
   oneWay: OneWayFilter = false,
   material?: TileMaterial,
 ): Required<SolidCell>[] {
-  return boxCells(blockingCells(project, props, asset, oneWay, material));
+  return boxCells(blockingCells(project, props, asset, { oneWay, material }));
 }
 
 function boxCells(cells: readonly CellBlock[]): Required<SolidCell>[] {
@@ -393,7 +408,11 @@ export function solidPolygons(
   oneWay: OneWayFilter = false,
   material?: TileMaterial,
 ): readonly CellPolygon[] {
-  return derived(polygonsOf, blockingCells(project, props, asset, oneWay, material), (cells) => {
+  return polygonCells(blockingCells(project, props, asset, { oneWay, material }));
+}
+
+function polygonCells(blocks: readonly CellBlock[]): readonly CellPolygon[] {
+  return derived(polygonsOf, blocks, (cells) => {
     const polygons: CellPolygon[] = [];
     for (const cell of cells) {
       if (cell.box !== null) continue;
@@ -527,9 +546,126 @@ export function solidRects(
   oneWay: OneWayFilter = false,
   material?: TileMaterial,
 ): readonly CellRect[] {
-  return derived(rectsOf, blockingCells(project, props, asset, oneWay, material), (cells) =>
-    mergeCells(boxCells(cells)),
-  );
+  return rectCells(blockingCells(project, props, asset, { oneWay, material }));
+}
+
+function rectCells(blocks: readonly CellBlock[]): readonly CellRect[] {
+  return derived(rectsOf, blocks, (cells) => mergeCells(boxCells(cells)));
+}
+
+/** The trigger tiles painted on the map (#160), each once, in the order the layers first name them. */
+function triggerTiles(project: Project, asset: TilemapAsset): TileAsset[] {
+  const ids = new Set(asset.layers.flatMap((layer) => cellsTileIds(layer.cells)));
+  return [...ids].flatMap((id) => {
+    const tile = findAssetOf(project, id, 'tile');
+    return tile && isTileTrigger(tile) ? [tile] : [];
+  });
+}
+
+/**
+ * Where the map's trigger tiles lie (#160), as the scene view outlines it: merged into rectangles by
+ * the rules of the solid cells, but on their own – a pond never merges with its shore.
+ */
+export function triggerRects(project: Project, props: PropertyBag, asset: TilemapAsset): readonly CellRect[] {
+  return rectCells(blockingCells(project, props, asset, { trigger: null }));
+}
+
+/** The trigger cells that report with an outline, like `solidPolygons`. */
+export function triggerPolygons(
+  project: Project,
+  props: PropertyBag,
+  asset: TilemapAsset,
+): readonly CellPolygon[] {
+  return polygonCells(blockingCells(project, props, asset, { trigger: null }));
+}
+
+/**
+ * One cell of a trigger tile as the running game meets it (#160): what of the cell reports, in
+ * cells, and the area it is part of – the stretch of cells of the same tile touching side to side,
+ * named after its top-left one. Walking through a pond is one area, however many cells it crosses.
+ */
+export interface TriggerCell {
+  x: number;
+  y: number;
+  tileId: string;
+  area: string;
+  rects: readonly CellRect[];
+  polygons: readonly CellPolygon[];
+}
+
+const triggers = new KeyedMemo<ReadonlyMap<string, readonly TriggerCell[]>>();
+const NO_TRIGGERS: ReadonlyMap<string, readonly TriggerCell[]> = new Map();
+
+/**
+ * The trigger cells of a map by `x,y`. Each tile is worked out on its own, so a cell holding two
+ * trigger tiles on two layers reports both, and water next to lava is two areas.
+ */
+export function triggerCells(
+  project: Project,
+  props: PropertyBag,
+  asset: TilemapAsset,
+): ReadonlyMap<string, readonly TriggerCell[]> {
+  const tiles = triggerTiles(project, asset);
+  if (tiles.length === 0) return NO_TRIGGERS;
+  const blocks = tiles.map((tile) => blockingCells(project, props, asset, { trigger: tile.id }));
+  const key = tiles.map((tile, i) => `${tile.id}:${identityToken(blocks[i]!)}`).join(',');
+  return triggers.get(key, () => {
+    const byCell = new Map<string, TriggerCell[]>();
+    for (const [i, tile] of tiles.entries())
+      for (const cell of cellsOfTile(tile.id, blocks[i]!)) {
+        const at = `${cell.x},${cell.y}`;
+        const held = byCell.get(at);
+        if (held) held.push(cell);
+        else byCell.set(at, [cell]);
+      }
+    return byCell;
+  });
+}
+
+function cellsOfTile(tileId: string, blocks: readonly CellBlock[]): TriggerCell[] {
+  const boxes = new Map(boxCells(blocks).map((cell) => [`${cell.x},${cell.y}`, cell]));
+  const isBox = (x: number, y: number) => boxes.has(`${x},${y}`);
+  const areas = areasOf(blocks);
+  return blocks.map((block) => {
+    const at = `${block.x},${block.y}`;
+    const box = boxes.get(at);
+    return {
+      x: block.x,
+      y: block.y,
+      tileId,
+      area: areas.get(at)!,
+      rects: box
+        ? cellPieces(box, isBox).map((p) => ({ x: p.x0, y: p.y0, w: p.x1 - p.x0, h: p.y1 - p.y0 }))
+        : [],
+      polygons: box
+        ? []
+        : block.polygons.map(({ outline, pieces }) => ({ x: block.x, y: block.y, outline, pieces })),
+    };
+  });
+}
+
+/** Which area each cell is part of: cells joined side to side, the area named after its top-left cell. */
+function areasOf(cells: readonly { x: number; y: number }[]): Map<string, string> {
+  const open = new Set(cells.map((cell) => `${cell.x},${cell.y}`));
+  const areas = new Map<string, string>();
+  for (const first of [...cells].sort((a, b) => a.y - b.y || a.x - b.x)) {
+    const name = `${first.x},${first.y}`;
+    if (!open.delete(name)) continue;
+    const queue = [first];
+    for (let cell = queue.pop(); cell; cell = queue.pop()) {
+      areas.set(`${cell.x},${cell.y}`, name);
+      for (const [dx, dy] of [
+        [1, 0],
+        [-1, 0],
+        [0, 1],
+        [0, -1],
+      ] as const) {
+        const next = { x: cell.x + dx, y: cell.y + dy };
+        if (open.delete(`${next.x},${next.y}`)) queue.push(next);
+      }
+    }
+  }
+  return areas;
 }
 
 /**

src/core/properties/PropertySchema.ts

@@ -37,6 +37,7 @@ import {
   TILE_SORT_OFFSET_KEY,
   TILE_SORT_OFFSET_LIMIT,
   TILE_SOLID_KEY,
+  TILE_TRIGGER_KEY,
 } from '../model/tile';
 import { TILEMAP_COLUMNS_KEY, TILEMAP_MAX_CELLS, TILEMAP_ROWS_KEY, TILEMAP_TILE_KEY } from '../model/tilemap';
 import { PALETTE_KEY } from '../model/pixelart';
@@ -454,6 +455,8 @@ const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
     },
     // Solid is what the tile *is* – water, a wall – so it holds wherever the tile lies (#132).
     { key: TILE_SOLID_KEY, label: 'Solid', kind: 'boolean', default: false },
+    // A trigger tile blocks nothing and tells the collision events what lies on it (#160).
+    { key: TILE_TRIGGER_KEY, label: 'Trigger (reports, never blocks)', kind: 'boolean', default: false },
     // A solid tile that only blocks from above, like a platform one jumps through (#155).
     { key: TILE_ONE_WAY_KEY, label: 'One-way (from above)', kind: 'boolean', default: false },
     { ...ONE_WAY_ANGLE_ROW, key: TILE_ONE_WAY_ANGLE_KEY, shownWith: TILE_ONE_WAY_KEY },

src/editor/viewport/tilemapSolids.ts

@@ -23,7 +23,9 @@ function sameSolids(a: TilemapSolids, b: TilemapSolids): boolean {
     sameRects(a.solids, b.solids) &&
     samePolygons(a.outlines, b.outlines) &&
     sameRects(a.oneWay.solids, b.oneWay.solids) &&
-    samePolygons(a.oneWay.outlines, b.oneWay.outlines)
+    samePolygons(a.oneWay.outlines, b.oneWay.outlines) &&
+    sameRects(a.triggers.solids, b.triggers.solids) &&
+    samePolygons(a.triggers.outlines, b.triggers.outlines)
   );
 }
 
@@ -39,7 +41,7 @@ export class TilemapSolidOutlines {
    */
   private readonly boundaries = new Map<
     string,
-    { from: TilemapSolids; solid: BoundaryLine[]; oneWay: BoundaryLine[] }
+    { from: TilemapSolids; solid: BoundaryLine[]; oneWay: BoundaryLine[]; triggers: BoundaryLine[] }
   >();
   /** What each map's last draw tinted, or nothing: the painted layer's part, while it is selected. */
   private readonly tinted = new Map<string, TilemapSolids>();
@@ -64,7 +66,9 @@ export class TilemapSolidOutlines {
     oneWay: CellRect[];
     oneWayOutlines: Vec2[][];
     boundary: BoundaryLine[];
-    filled: { rects: CellRect[]; outlines: number; oneWay: CellRect[] } | null;
+    triggers: CellRect[];
+    triggerOutlines: Vec2[][];
+    filled: { rects: CellRect[]; outlines: number; oneWay: CellRect[]; triggers: CellRect[] } | null;
   } | null {
     const node = item && this.nodes.get(item.id);
     if (!node || !item.tilemap) return null;
@@ -75,6 +79,7 @@ export class TilemapSolidOutlines {
             rects: tinted.solids.map((rect) => ({ ...rect })),
             outlines: tinted.outlines.length,
             oneWay: tinted.oneWay.solids.map((rect) => ({ ...rect })),
+            triggers: tinted.triggers.solids.map((rect) => ({ ...rect })),
           }
         : null,
       visible: node.visible,
@@ -82,6 +87,10 @@ export class TilemapSolidOutlines {
       outlines: item.tilemap.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
       oneWay: item.tilemap.oneWay.solids.map((rect) => ({ ...rect })),
       oneWayOutlines: item.tilemap.oneWay.outlines.map((polygon) => polygon.outline.map((p) => ({ ...p }))),
+      triggers: item.tilemap.triggers.solids.map((rect) => ({ ...rect })),
+      triggerOutlines: item.tilemap.triggers.outlines.map((polygon) =>
+        polygon.outline.map((p) => ({ ...p })),
+      ),
       boundary: this.boundaryOf(item).solid.map((line) => ({
         points: line.points.map((p) => ({ ...p })),
         closed: line.closed,
@@ -89,7 +98,11 @@ export class TilemapSolidOutlines {
     };
   }
 
-  private boundaryOf(item: DrawItem): { solid: BoundaryLine[]; oneWay: BoundaryLine[] } {
+  private boundaryOf(item: DrawItem): {
+    solid: BoundaryLine[];
+    oneWay: BoundaryLine[];
+    triggers: BoundaryLine[];
+  } {
     const from: TilemapSolids = item.tilemap!;
     let boundary = this.boundaries.get(item.id);
     if (!boundary || !sameSolids(boundary.from, from)) {
@@ -97,6 +110,7 @@ export class TilemapSolidOutlines {
         from,
         solid: solidBoundary(from.solids, from.outlines),
         oneWay: solidBoundary(from.oneWay.solids, from.oneWay.outlines),
+        triggers: solidBoundary(from.triggers.solids, from.triggers.outlines),
       };
       this.boundaries.set(item.id, boundary);
     }
@@ -113,8 +127,10 @@ export class TilemapSolidOutlines {
     const rects = item.tilemap?.solids ?? [];
     const outlines = item.tilemap?.outlines ?? [];
     const oneWay = item.tilemap?.oneWay ?? { solids: [], outlines: [] };
+    const triggers = item.tilemap?.triggers ?? { solids: [], outlines: [] };
     this.tinted.delete(item.id);
-    if (!show || rects.length + outlines.length + oneWay.solids.length + oneWay.outlines.length === 0) {
+    const shapes = [rects, outlines, oneWay.solids, oneWay.outlines, triggers.solids, triggers.outlines];
+    if (!show || shapes.every((list) => list.length === 0)) {
       if (node) node.visible = false;
       return;
     }
@@ -132,7 +148,12 @@ export class TilemapSolidOutlines {
     const color = Theme.colors.collider;
     const boundary = this.boundaryOf(item);
     // Filled piece by piece, but stroked only along the outer edges, so no seam shows (#173).
-    const fill = (rects: readonly CellRect[], outlines: readonly CellPolygon[], alpha: number) => {
+    const fill = (
+      rects: readonly CellRect[],
+      outlines: readonly CellPolygon[],
+      alpha: number,
+      color: number = Theme.colors.collider,
+    ) => {
       if (rects.length + outlines.length === 0) return;
       for (const rect of rects) {
         const corner = cellOrigin(item.props, rect.x, rect.y);
@@ -145,7 +166,11 @@ export class TilemapSolidOutlines {
         );
       node.fill({ color, alpha });
     };
-    const stroke = (lines: readonly BoundaryLine[], lineWidth: number) => {
+    const stroke = (
+      lines: readonly BoundaryLine[],
+      lineWidth: number,
+      color: number = Theme.colors.collider,
+    ) => {
       if (lines.length === 0) return;
       for (const line of lines)
         node.poly(
@@ -158,9 +183,12 @@ export class TilemapSolidOutlines {
       this.tinted.set(item.id, filled);
       fill(filled.solids, filled.outlines, COLLIDER_FILL_ALPHA);
       fill(filled.oneWay.solids, filled.oneWay.outlines, COLLIDER_FILL_ALPHA / 2);
+      fill(filled.triggers.solids, filled.triggers.outlines, COLLIDER_FILL_ALPHA, Theme.colors.trigger);
     }
     stroke(boundary.solid, width);
     stroke(boundary.oneWay, width / 2);
+    // Trigger tiles in the trigger collider's colour (#160): they report, the others block.
+    stroke(boundary.triggers, width, Theme.colors.trigger);
     // A platform's top is what blocks, so that is what stands out, with the cone it lands within.
     const length = CONE_LENGTH / (scale * scaleOf(item.m));
     for (const rect of oneWay.solids) {

src/runtime/physics/PhysicsSystem.ts

@@ -48,6 +48,7 @@ import {
   surfaceOf,
 } from '../systems/solids';
 import type { System } from '../systems/System';
+import { TileTriggers } from '../systems/tileTriggers';
 import {
   angleBetween,
   blend,
@@ -148,6 +149,8 @@ export class PhysicsSystem implements System, CollisionSource {
   private passes = new Map<string, boolean>();
   private hooks: PhysicsHooks | null = null;
   private readonly solids = new Solids();
+  /** The maps' trigger tiles (#160): found by overlap where the objects are drawn, not by the engine. */
+  private readonly triggers = new TileTriggers();
   private backlog = 0;
   /** The length of the frame being played – the time a move of the graph's is spread over. */
   private frameDt = PHYSICS_STEP;
@@ -188,6 +191,7 @@ export class PhysicsSystem implements System, CollisionSource {
     this.enters = [];
     this.stays = [];
     this.exits = [];
+    this.triggers.stop();
     this.backlog = 0;
     this.frameDt = PHYSICS_STEP;
     this.hinted.clear();
@@ -218,13 +222,22 @@ export class PhysicsSystem implements System, CollisionSource {
     this.backlog = steps === MAX_STEPS_PER_TICK ? 0 : Math.max(0, this.backlog);
     this.writeBack(this.backlog / PHYSICS_STEP);
     this.stays = [...this.current].filter(([key]) => !entered.has(key)).map(([, pair]) => pair);
+    this.triggers.update(world);
+    this.enters.push(...this.triggers.enters);
+    this.stays.push(...this.triggers.stays);
+    this.exits.push(...this.triggers.exits);
   }
 
   /** Whether any collider of `a` is in contact with any collider of `b` as of the last step. */
   touching(world: World, a: Entity, b: Entity): boolean {
     for (const ca of partsOf(world, a))
       for (const cb of partsOf(world, b))
-        if (this.current.has(`${ca.id}|${cb.id}`) || this.current.has(`${cb.id}|${ca.id}`)) return true;
+        if (
+          this.current.has(`${ca.id}|${cb.id}`) ||
+          this.current.has(`${cb.id}|${ca.id}`) ||
+          this.triggers.touching(world, ca, cb)
+        )
+          return true;
     return false;
   }
 
@@ -253,6 +266,8 @@ export class PhysicsSystem implements System, CollisionSource {
 
   /** How a current pair touches as of the last step: read from the engine's contact. */
   contactOf(pair: CollisionPair): Contact | null {
+    // A trigger tile is no engine collider: nothing presses there.
+    if (pair.tile) return null;
     const handles = this.pairColliders.get(`${pair.a.id}|${pair.b.id}`);
     if (!handles || !this.parts.has(handles[0]) || !this.parts.has(handles[1])) return null;
     return this.measure(this.engine!.getCollider(handles[0]), this.engine!.getCollider(handles[1]));

src/runtime/systems/BehaviorSystem.ts

@@ -1,5 +1,6 @@
 import {
   adoptDeclarations,
+  asBoolean,
   buildSharedState,
   carriedForward,
   declaredVariables,
@@ -20,6 +21,7 @@ import {
   type Value,
 } from '../../core/graph';
 import { GraphCache } from '../../core/graph/graphIndex';
+import { PER_CELL_INPUT } from '../../core/graph/nodes/shared';
 import { keyName } from '../../core/keys';
 import { graphLookup } from '../../core/model/graphLookup';
 import { type PropertyBag, textOf } from '../../core/model/Properties';
@@ -39,6 +41,7 @@ import {
   collisionStayPulses,
   type Instance,
   keyDownPulses,
+  type PerCell,
   type Produced,
   type Pulse,
   pulsesOf,
@@ -537,6 +540,8 @@ export class BehaviorSystem implements System {
       return owner ? world.hitTest(owner, pointer.x, pointer.y) : pointer.inside;
     };
     const { input, collisions, animations } = this;
+    const perCell: PerCell = (i, node) =>
+      asBoolean(i.interpreter.eventInput(node, PER_CELL_INPUT, contextOf(i)));
     const kinds: (() => Pulse[])[] = [
       () =>
         ticking.flatMap((i) =>
@@ -546,9 +551,9 @@ export class BehaviorSystem implements System {
       () => pulsesOf(ticking, 'OnKeyHeld', { dt }, (n) => input.heldKeys.has(keyOf(n))),
       () => ticking.flatMap((i) => timerPulses(i, dt)),
       // Neither system ticked in a paused frame: what they hold is last frame's, already told.
-      () => (frozen ? [] : ticking.flatMap((i) => collisionEnterPulses(i, collisions))),
-      () => (frozen ? [] : ticking.flatMap((i) => collisionStayPulses(i, collisions, dt))),
-      () => (frozen ? [] : ticking.flatMap((i) => collisionExitPulses(i, collisions))),
+      () => (frozen ? [] : ticking.flatMap((i) => collisionEnterPulses(i, collisions, perCell))),
+      () => (frozen ? [] : ticking.flatMap((i) => collisionStayPulses(i, collisions, dt, perCell))),
+      () => (frozen ? [] : ticking.flatMap((i) => collisionExitPulses(i, collisions, perCell))),
       () => (frozen ? [] : ticking.flatMap((i) => animationFinishedPulses(i, animations))),
       () =>
         pointer.pressed ? pulsesOf(ticking.filter(pointed), 'OnClick', { x: pointer.x, y: pointer.y }) : [],

src/runtime/systems/CollisionSystem.ts

@@ -18,6 +18,7 @@ import {
   Solids,
 } from './solids';
 import type { System } from './System';
+import { TileTriggers } from './tileTriggers';
 
 /**
  * Sorts collider overlaps into enter / stay / exit per tick, detection only; colliders of one body never
@@ -26,7 +27,8 @@ import type { System } from './System';
  *
  * A placed tilemap is a solid body of its own: its blocking cells, merged into rectangles, stop a
  * move and report a collision with the tilemap as the other side. Two tilemaps never meet – they
- * are ground, not things that walk into each other.
+ * are ground, not things that walk into each other. Its trigger tiles block nothing and report
+ * the cells overlapped (`TileTriggers`, #160).
  */
 export class CollisionSystem implements System, CollisionSource {
   enters: CollisionPair[] = [];
@@ -34,6 +36,7 @@ export class CollisionSystem implements System, CollisionSource {
   exits: CollisionPair[] = [];
   private previous = new Map<string, CollisionPair>();
   private readonly solids = new Solids();
+  private readonly triggers = new TileTriggers();
 
   update(world: World): void {
     this.solids.reset();
@@ -56,11 +59,17 @@ export class CollisionSystem implements System, CollisionSource {
     for (const [key, pair] of current) (this.previous.has(key) ? this.stays : this.enters).push(pair);
     for (const [key, pair] of this.previous) if (!current.has(key)) this.exits.push(pair);
     this.previous = current;
+    // A map's trigger tiles (#160) are pairs of their own, told after the others.
+    this.triggers.update(world);
+    this.enters.push(...this.triggers.enters);
+    this.stays.push(...this.triggers.stays);
+    this.exits.push(...this.triggers.exits);
   }
 
   stop(): void {
     this.previous = new Map();
     this.solids.reset();
+    this.triggers.stop();
     this.enters = [];
     this.stays = [];
     this.exits = [];
@@ -80,7 +89,9 @@ export class CollisionSystem implements System, CollisionSource {
   touching(world: World, a: Entity, b: Entity): boolean {
     for (const ca of partsOf(world, a)) {
       for (const cb of partsOf(world, b)) {
-        if (ca !== cb && collidersMeet(ca, cb) && this.overlaps(world, ca, cb)) return true;
+        if (ca === cb) continue;
+        if (collidersMeet(ca, cb) && this.overlaps(world, ca, cb)) return true;
+        if (this.triggers.touching(world, ca, cb)) return true;
       }
     }
     return false;

src/runtime/systems/behaviorPulses.ts

@@ -11,7 +11,7 @@ import { parentChain } from '../../core/model/walk';
 import type { Entity } from '../Entity';
 import type { AnimationSystem } from './AnimationSystem';
 import type { Input } from './Input';
-import { bodyOf, type CollisionPair, type CollisionSource, type Contact } from './solids';
+import { bodyOf, type CollisionPair, type CollisionSource, type Contact, pairKey } from './solids';
 
 /** What one node last handed on, and when – the graph tab shows the newest of every instance. */
 export interface Produced {
@@ -116,11 +116,32 @@ function sides(instance: Instance, pairs: readonly CollisionPair[]): Side[] {
   });
 }
 
-export function collisionEnterPulses(instance: Instance, collisions: CollisionSource | null): Pulse[] {
+/** Reads an event node's "per cell" input for its instance (#160). */
+export type PerCell = (instance: Instance, node: GraphNode) => boolean;
+
+/**
+ * What of the pairs one node hears. A trigger tile is told twice, per cell and per area (#160), and
+ * the node's "per cell" input picks one; it is only read where a tile is among the pairs at all.
+ */
+function heardBy(instance: Instance, node: GraphNode, all: Side[], perCell: PerCell): Side[] {
+  if (!all.some((side) => side.pair.tile)) return all;
+  const cells = perCell(instance, node);
+  return all.filter((side) => !side.pair.tile || side.pair.tile.perCell === cells);
+}
+
+export function collisionEnterPulses(
+  instance: Instance,
+  collisions: CollisionSource | null,
+  perCell: PerCell,
+): Pulse[] {
   const entered = sides(instance, collisions?.enters ?? []);
-  return instance.interpreter
-    .eventNodes('OnCollisionEnter')
-    .flatMap((node) => entered.map((side) => ({ instance, node, outputs: sideOutputs(side, side.pair) })));
+  return instance.interpreter.eventNodes('OnCollisionEnter').flatMap((node) =>
+    heardBy(instance, node, entered, perCell).map((side) => ({
+      instance,
+      node,
+      outputs: sideOutputs(side, side.pair),
+    })),
+  );
 }
 
 /**
@@ -132,6 +153,7 @@ export function collisionStayPulses(
   instance: Instance,
   collisions: CollisionSource | null,
   dt: number,
+  perCell: PerCell,
 ): Pulse[] {
   if (!collisions) return [];
   const interpreter = instance.interpreter;
@@ -146,10 +168,10 @@ export function collisionStayPulses(
     const interval = Math.max(0, Number(node.params.interval ?? 0)) || 0;
     const clocks = interpreter.nodeState.get(node.id) ?? {};
     const running: Record<string, number> = {};
-    for (const side of stays) {
+    for (const side of heardBy(instance, node, stays, perCell)) {
       let fires = 1;
       if (interval > 0) {
-        const key = `${side.pair.a.id}|${side.pair.b.id}`;
+        const key = pairKey(side.pair);
         let elapsed = (clocks[key] ?? 0) + dt;
         for (fires = 0; elapsed >= interval - 1e-9; fires++) elapsed -= interval;
         running[key] = elapsed;
@@ -163,11 +185,17 @@ export function collisionStayPulses(
   return pulses;
 }
 
-export function collisionExitPulses(instance: Instance, collisions: CollisionSource | null): Pulse[] {
+export function collisionExitPulses(
+  instance: Instance,
+  collisions: CollisionSource | null,
+  perCell: PerCell,
+): Pulse[] {
   const left = sides(instance, collisions?.exits ?? []);
   return instance.interpreter
     .eventNodes('OnCollisionExit')
-    .flatMap((node) => left.map((side) => ({ instance, node, outputs: sideOutputs(side) })));
+    .flatMap((node) =>
+      heardBy(instance, node, left, perCell).map((side) => ({ instance, node, outputs: sideOutputs(side) })),
+    );
 }
 
 /**
@@ -189,7 +217,14 @@ export function animationFinishedPulses(instance: Instance, animations: Animatio
 
 /** What a collision event hands on, seen from the behavior's side; `contact` only for enter and stay. */
 function sideOutputs({ own, other, pair }: Side, contact?: Partial<Contact> | null): Record<string, Value> {
-  const outputs: Record<string, Value> = { other: bodyOf(other), otherCollider: other, collider: own };
+  const outputs: Record<string, Value> = {
+    other: bodyOf(other),
+    otherCollider: other,
+    collider: own,
+    tile: pair.tile?.tileId ?? '',
+    cellX: pair.tile?.cell.x ?? -1,
+    cellY: pair.tile?.cell.y ?? -1,
+  };
   if (contact !== undefined) {
     // The pair's normal points towards `a`; seen from `b` it points the other way.
     const sign = own === pair.a ? 1 : -1;

src/runtime/systems/solids.ts

@@ -72,6 +72,26 @@ export interface CollisionPair {
   normal?: Vec2;
   impulse?: number;
   point?: Vec2;
+  /** Set where `b` is a map and `a` overlaps a trigger tile of it (#160) – a pair of its own, next to the map's. */
+  tile?: TileContact;
+}
+
+/**
+ * Which trigger tile of a map a collider overlaps (#160). A pair stands either for one cell or for a
+ * whole area of the tile; an area names the cell of it nearest to the collider.
+ */
+export interface TileContact {
+  tileId: string;
+  cell: { x: number; y: number };
+  area: string;
+  perCell: boolean;
+}
+
+/** What a pair is known by from one tick to the next. */
+export function pairKey({ a, b, tile }: CollisionPair): string {
+  const key = `${a.id}|${b.id}`;
+  if (!tile) return key;
+  return `${key}|${tile.tileId}|${tile.perCell ? `cell ${tile.cell.x},${tile.cell.y}` : `area ${tile.area}`}`;
 }
 
 /**

src/runtime/systems/tileTriggers.ts

@@ -0,0 +1,128 @@
+import { isHandleEnabled, worldMatrixOf } from '../../core/graph';
+import { liveTilemap } from '../../core/model/liveTilemap';
+import { apply } from '../../core/model/matrix';
+import { findAssetOf } from '../../core/model/Project';
+import { cellOrigin, tilemapColumns, tilemapRows } from '../../core/model/tilemap';
+import {
+  cellPolygonShape,
+  cellRectShape,
+  cellsWithin,
+  type TriggerCell,
+  triggerCells,
+} from '../../core/model/tilemapCollision';
+import { shapeBounds, shapesOverlap } from '../../core/properties/collider';
+import type { Entity } from '../Entity';
+import type { World } from '../World';
+import { bodyOf, collidersMeet, type CollisionPair, pairKey, shapeOf } from './solids';
+
+/** A collision scale above 1 lets a cell reach this far into its neighbours, in cells. */
+const REACH = 1;
+
+/**
+ * The trigger tiles of the placed maps (#160): which colliders overlap which of their cells, sorted
+ * into enter / stay / exit like any other pair. Both collision systems ask this one, with the
+ * editor's own overlap test, so a run in the editor and a played game tell the same.
+ *
+ * Every overlap is told twice, as two pairs: once per cell and once per area of the tile. An event
+ * node hears the one it asks for.
+ */
+export class TileTriggers {
+  enters: CollisionPair[] = [];
+  stays: CollisionPair[] = [];
+  exits: CollisionPair[] = [];
+  private previous = new Map<string, CollisionPair>();
+
+  update(world: World): void {
+    const current = new Map<string, CollisionPair>();
+    const maps = world.entities.filter((e) => e.type === 'tilemap' && isHandleEnabled(e));
+    const colliders = maps.length > 0 ? world.entities.filter((e) => e.type === 'collider') : [];
+    for (const map of maps) {
+      const cells = cellsOf(world, map);
+      if (cells.size === 0) continue;
+      for (const collider of colliders)
+        for (const pair of pairsOf(collider, map, cells)) current.set(pairKey(pair), pair);
+    }
+    this.enters = [];
+    this.stays = [];
+    this.exits = [];
+    for (const [key, pair] of current) (this.previous.has(key) ? this.stays : this.enters).push(pair);
+    for (const [key, pair] of this.previous) if (!current.has(key)) this.exits.push(pair);
+    this.previous = current;
+  }
+
+  stop(): void {
+    this.previous = new Map();
+    this.enters = [];
+    this.stays = [];
+    this.exits = [];
+  }
+
+  /** Whether a collider lies on any trigger tile of a map right now, whichever of the two is `a`. */
+  touching(world: World, a: Entity, b: Entity): boolean {
+    if (a.type === 'tilemap') return b.type !== 'tilemap' && this.touching(world, b, a);
+    if (b.type !== 'tilemap' || !isHandleEnabled(b)) return false;
+    return touched(a, b, cellsOf(world, b)).length > 0;
+  }
+}
+
+function cellsOf(world: World, map: Entity): ReadonlyMap<string, readonly TriggerCell[]> {
+  const stored = findAssetOf(world.project, map.assetId, 'tilemap');
+  return stored ? triggerCells(world.project, map.props, liveTilemap(stored, map.tileEdits)) : new Map();
+}
+
+/** The trigger cells of the map the collider overlaps – none for one switched off, or on layers that do not meet. */
+function touched(
+  collider: Entity,
+  map: Entity,
+  cells: ReadonlyMap<string, readonly TriggerCell[]>,
+): TriggerCell[] {
+  if (cells.size === 0 || !isHandleEnabled(collider)) return [];
+  if (bodyOf(collider) === map || !collidersMeet(collider, map)) return [];
+  const m = worldMatrixOf(map);
+  const shape = shapeOf(collider);
+  // Only the cells the collider can reach are looked at, as for a move or a ray (#184).
+  const range = cellsWithin(map.props, m, shapeBounds(shape));
+  const from = (edge: number) => Math.max(0, Math.floor(edge) - REACH);
+  const to = (edge: number, count: number) => Math.min(count - 1, Math.floor(edge) + REACH);
+  const found: TriggerCell[] = [];
+  for (let y = from(range.top); y <= to(range.bottom, tilemapRows(map.props)); y++)
+    for (let x = from(range.left); x <= to(range.right, tilemapColumns(map.props)); x++)
+      for (const cell of cells.get(`${x},${y}`) ?? []) {
+        const over =
+          cell.rects.some((rect) => shapesOverlap(shape, cellRectShape(map.props, rect, m))) ||
+          cell.polygons.some((polygon) => shapesOverlap(shape, cellPolygonShape(map.props, polygon, m)));
+        if (over) found.push(cell);
+      }
+  return found;
+}
+
+/** A pair per cell overlapped, and one per area: that one with the cell nearest to the collider's middle. */
+function pairsOf(
+  collider: Entity,
+  map: Entity,
+  cells: ReadonlyMap<string, readonly TriggerCell[]>,
+): CollisionPair[] {
+  const found = touched(collider, map, cells);
+  if (found.length === 0) return [];
+  const m = worldMatrixOf(map);
+  const middle = shapeOf(collider);
+  const distance = (cell: TriggerCell) => {
+    const at = apply(m, cellOrigin(map.props, cell.x + 0.5, cell.y + 0.5));
+    return Math.hypot(at.x - middle.x, at.y - middle.y);
+  };
+  const pair = (cell: TriggerCell, perCell: boolean): CollisionPair => ({
+    a: collider,
+    b: map,
+    tile: { tileId: cell.tileId, cell: { x: cell.x, y: cell.y }, area: cell.area, perCell },
+  });
+  const nearest = new Map<string, TriggerCell>();
+  for (const cell of found) {
+    const key = `${cell.tileId}|${cell.area}`;
+    const known = nearest.get(key);
+    if (!known || distance(cell) < distance(known)) nearest.set(key, cell);
+  }
+  return [
+    ...found.map((cell) => pair(cell, true)),
+    ...[...nearest.values()].map((cell) => pair(cell, false)),
+  ];
+}

src/viewport/items.ts

@@ -20,6 +20,8 @@ import {
   oneWayAngles,
   solidPolygons,
   solidRects,
+  triggerPolygons,
+  triggerRects,
 } from '../core/model/tilemapCollision';
 import { findAsset, findAssetOf, type ObjectHolder, type Project } from '../core/model/Project';
 import { objectName } from '../core/model/SceneObject';
@@ -113,6 +115,8 @@ export interface TilemapDraw {
    * its tiles land things within (#159), since tiles of two angles never merge.
    */
   oneWay: { solids: (CellRect & { angle: number })[]; outlines: CellPolygon[] };
+  /** Where its trigger tiles lie (#160): they block nothing and are outlined like a trigger collider. */
+  triggers: { solids: readonly CellRect[]; outlines: readonly CellPolygon[] };
   /**
    * Where one layer alone blocks, the same way – the scene view fills only the painted layer's
    * part. Worked out when asked for, since only the selected map ever asks.
@@ -121,7 +125,7 @@ export interface TilemapDraw {
 }
 
 /** Where a map, or one of its layers, blocks. */
-export type TilemapSolids = Pick<TilemapDraw, 'solids' | 'outlines' | 'oneWay'>;
+export type TilemapSolids = Pick<TilemapDraw, 'solids' | 'outlines' | 'oneWay' | 'triggers'>;
 
 function solidsOf(project: Project, props: PropertyBag, asset: TilemapAsset): TilemapSolids {
   const angles = oneWayAngles(project, asset);
@@ -134,6 +138,10 @@ function solidsOf(project: Project, props: PropertyBag, asset: TilemapAsset): Ti
       ),
       outlines: angles.flatMap((angle) => solidPolygons(project, props, asset, angle)),
     },
+    triggers: {
+      solids: triggerRects(project, props, asset),
+      outlines: triggerPolygons(project, props, asset),
+    },
   };
 }
 

tests/core/tile-triggers.test.ts

@@ -0,0 +1,568 @@
+import { beforeAll, describe, expect, it } from 'vitest';
+import { CreateAssetCommand, SetAssetPropertyCommand } from '../../src/core/commands/assetCommands';
+import { AddColliderCommand, AddRigidbodyCommand } from '../../src/core/commands/colliderCommands';
+import { AddFrameCommand, SetFrameCollisionCommand } from '../../src/core/commands/frameCommands';
+import { SetOverrideCommand } from '../../src/core/commands/sceneObjectCommands';
+import { AddTilePictureCommand } from '../../src/core/commands/tileCommands';
+import { AddTilemapLayerCommand, PaintTilesCommand } from '../../src/core/commands/tilemapCommands';
+import type { TextureAsset, TilemapAsset } from '../../src/core/model/Asset';
+import { ALL_LAYERS, COLLIDES_WITH_KEY } from '../../src/core/model/collisionLayers';
+import { findAsset } from '../../src/core/model/Project';
+import {
+  isTileSolid,
+  isTileTrigger,
+  TILE_COLLISION_SCALE_KEY,
+  TILE_SOLID_KEY,
+  TILE_TRIGGER_KEY,
+} from '../../src/core/model/tile';
+import { TILEMAP_COLUMNS_KEY, TILEMAP_ROWS_KEY } from '../../src/core/model/tilemap';
+import {
+  solidRects,
+  type TriggerCell,
+  triggerCells,
+  triggerPolygons,
+  triggerRects,
+} from '../../src/core/model/tilemapCollision';
+import { schemaFor } from '../../src/core/properties/PropertySchema';
+import { loadPhysics } from '../../src/runtime/physics/engine';
+import type { CollisionPair, CollisionSource } from '../../src/runtime/systems/solids';
+import type { System } from '../../src/runtime/systems/System';
+import {
+  COLLISION_SYSTEMS,
+  graphEditor,
+  makeBehavior,
+  makeSpriteScene,
+  place,
+  roundTrip,
+  startWorld,
+} from '../helpers';
+
+/**
+ * A scene with a 10 × 10 map of 48 px cells placed at the origin – cell (x, y) spans
+ * x·48 − 240 … x·48 − 192 – and a 40 × 40 hero with a collider, standing in row 4 left of a pond:
+ * water in the cells 6, 7 and 8 of that row and lava in cell 9, both trigger tiles.
+ */
+function setup() {
+  const f = makeSpriteScene('Hero');
+  f.stack.execute(new SetAssetPropertyCommand(f.spriteId, 'width', 40));
+  f.stack.execute(new SetAssetPropertyCommand(f.spriteId, 'height', 40));
+  const water = new CreateAssetCommand('tile', 'Water');
+  const lava = new CreateAssetCommand('tile', 'Lava');
+  const wall = new CreateAssetCommand('tile', 'Wall');
+  const map = new CreateAssetCommand('tilemap', 'Ground');
+  for (const command of [water, lava, wall, map]) f.stack.execute(command);
+  f.stack.execute(new SetAssetPropertyCommand(water.asset.id, TILE_TRIGGER_KEY, true));
+  f.stack.execute(new SetAssetPropertyCommand(lava.asset.id, TILE_TRIGGER_KEY, true));
+  f.stack.execute(new SetAssetPropertyCommand(wall.asset.id, TILE_SOLID_KEY, true));
+  f.stack.execute(new SetAssetPropertyCommand(map.asset.id, TILEMAP_COLUMNS_KEY, 10));
+  f.stack.execute(new SetAssetPropertyCommand(map.asset.id, TILEMAP_ROWS_KEY, 10));
+  const mapObject = place(f, map.asset.id, { x: 0, y: 0 });
+  const hero = place(f, f.spriteId, { x: 0, y: -24 });
+  const collider = new AddColliderCommand(f.sceneId, hero);
+  f.stack.execute(collider);
+  const assetOf = () => findAsset(f.project, map.asset.id) as TilemapAsset;
+  const paint = (at: [number, number][], tileId: string, layerId = assetOf().layers[0]!.id) =>
+    f.stack.execute(
+      new PaintTilesCommand(
+        map.asset.id,
+        layerId,
+        at.map(([x, y]) => ({ x, y, tileId })),
+      ),
+    );
+  const s = {
+    ...f,
+    waterId: water.asset.id,
+    lavaId: lava.asset.id,
+    wallId: wall.asset.id,
+    mapId: map.asset.id,
+    mapObject,
+    hero,
+    heroCollider: collider.objectId,
+    assetOf,
+    paint,
+  };
+  paint(
+    [
+      [6, 4],
+      [7, 4],
+      [8, 4],
+    ],
+    s.waterId,
+  );
+  paint([[9, 4]], s.lavaId);
+  return s;
+}
+
+type Setup = ReturnType<typeof setup>;
+
+const cellsOf = (s: Setup) => triggerCells(s.project, s.assetOf().props, s.assetOf());
+const flat = (cells: ReadonlyMap<string, readonly TriggerCell[]>) => [...cells.values()].flat();
+
+/** A pair as a word: `area` or `cell`, the tile's name and the cell. */
+function told(s: Setup, pairs: readonly CollisionPair[]): string[] {
+  const name = (id: string) => (id === s.waterId ? 'water' : id === s.lavaId ? 'lava' : id);
+  return pairs
+    .filter((pair) => pair.tile)
+    .map(
+      ({ tile }) =>
+        `${tile!.perCell ? 'cell' : 'area'} ${name(tile!.tileId)} ${tile!.cell.x},${tile!.cell.y}`,
+    )
+    .sort();
+}
+
+describe('trigger tiles: which cells report (#160)', () => {
+  it('a tile reports once Trigger is on, the switch survives a round trip, and it wins over Solid', () => {
+    const s = setup();
+    expect(schemaFor('tile').find((d) => d.key === TILE_TRIGGER_KEY)).toMatchObject({
+      kind: 'boolean',
+      default: false,
+    });
+    const tile = (id: string, project = s.project) => {
+      const found = findAsset(project, id);
+      if (found?.type !== 'tile') throw new Error('no tile');
+      return found;
+    };
+    expect(isTileTrigger(tile(s.wallId))).toBe(false);
+    expect(isTileTrigger(tile(s.waterId, roundTrip(s.project)))).toBe(true);
+    s.stack.execute(new SetAssetPropertyCommand(s.waterId, TILE_SOLID_KEY, true));
+    expect(isTileTrigger(tile(s.waterId))).toBe(true);
+    expect(isTileSolid(tile(s.waterId))).toBe(false);
+    expect(solidRects(s.project, s.assetOf().props, s.assetOf())).toEqual([]);
+  });
+
+  it('merges into rectangles like solid cells, but never with them', () => {
+    const s = setup();
+    s.paint([[5, 4]], s.wallId);
+    const props = s.assetOf().props;
+    // Water and lava are both triggers, so the outline runs round the two together.
+    expect(triggerRects(s.project, props, s.assetOf())).toEqual([{ x: 6, y: 4, w: 4, h: 1 }]);
+    expect(solidRects(s.project, props, s.assetOf())).toEqual([{ x: 5, y: 4, w: 1, h: 1 }]);
+  });
+
+  it('an area is the cells of one tile touching side to side, named after its top-left cell', () => {
+    const s = setup();
+    s.paint(
+      [
+        [7, 5],
+        [9, 5],
+        [2, 2],
+        [3, 3],
+      ],
+      s.waterId,
+    );
+    const areas = Object.fromEntries(
+      flat(cellsOf(s)).map((cell) => [
+        `${cell.tileId === s.waterId ? 'w' : 'l'} ${cell.x},${cell.y}`,
+        cell.area,
+      ]),
+    );
+    expect(areas).toEqual({
+      'w 6,4': '6,4',
+      'w 7,4': '6,4',
+      'w 8,4': '6,4',
+      'w 7,5': '6,4',
+      // Lava lies between this one and the pond, and lava is not water.
+      'w 9,5': '9,5',
+      'l 9,4': '9,4',
+      // Corner to corner is not side to side.
+      'w 2,2': '2,2',
+      'w 3,3': '3,3',
+    });
+  });
+
+  it('a cell holding two trigger tiles on two layers reports both; outside the grid nothing does', () => {
+    const s = setup();
+    const add = new AddTilemapLayerCommand(s.mapId);
+    s.stack.execute(add);
+    s.paint(
+      [
+        [6, 4],
+        [12, 4],
+      ],
+      s.lavaId,
+      add.layerId,
+    );
+    const cells = cellsOf(s);
+    expect(
+      cells
+        .get('6,4')!
+        .map((cell) => cell.tileId)
+        .sort(),
+    ).toEqual([s.lavaId, s.waterId].sort());
+    expect(cells.has('12,4')).toBe(false);
+  });
+
+  it('the collision scale draws a cell in on its open sides only, as for a solid tile', () => {
+    const s = setup();
+    s.stack.execute(new SetAssetPropertyCommand(s.waterId, TILE_COLLISION_SCALE_KEY, 0.5));
+    const pieces = (x: number) =>
+      cellsOf(s)
+        .get(`${x},4`)!
+        .find((cell) => cell.tileId === s.waterId)!.rects;
+    const span = (x: number) => {
+      const rects = pieces(x);
+      return [Math.min(...rects.map((r) => r.x)), Math.max(...rects.map((r) => r.x + r.w))];
+    };
+    // The left end is drawn in, the side towards the next water cell is flush; lava is no water.
+    expect(span(6)).toEqual([6.25, 7]);
+    expect(span(7)).toEqual([7, 8]);
+    expect(span(8)).toEqual([8, 8.75]);
+    expect(pieces(7).every((r) => r.y === 4.25 && r.h === 0.5)).toBe(true);
+  });
+
+  it('a trigger tile with an outline reports with that polygon', () => {
+    const s = setup();
+    const texture = new CreateAssetCommand('texture', 'Sheet');
+    s.stack.execute(texture);
+    const sheet = findAsset(s.project, texture.asset.id) as TextureAsset;
+    sheet.file = `${'a'.repeat(64)}.png`;
+    sheet.width = 32;
+    sheet.height = 32;
+    const frame = new AddFrameCommand(texture.asset.id, { x: 0, y: 0, w: 32, h: 32 });
+    s.stack.execute(frame);
+    s.stack.execute(new AddTilePictureCommand(s.lavaId, frame.frameId));
+    // The lower half of the tile.
+    const half = [
+      { x: 0, y: 16 },
+      { x: 32, y: 16 },
+      { x: 32, y: 32 },
+      { x: 0, y: 32 },
+    ];
+    s.stack.execute(new SetFrameCollisionCommand(texture.asset.id, frame.frameId, [half]));
+    const lava = cellsOf(s)
+      .get('9,4')!
+      .find((cell) => cell.tileId === s.lavaId)!;
+    expect(lava.rects).toEqual([]);
+    expect(lava.polygons.map((p) => p.outline)).toEqual([
+      [
+        { x: 9, y: 4.5 },
+        { x: 10, y: 4.5 },
+        { x: 10, y: 5 },
+        { x: 9, y: 5 },
+      ],
+    ]);
+    const props = s.assetOf().props;
+    expect(triggerPolygons(s.project, props, s.assetOf())).toHaveLength(1);
+    expect(triggerRects(s.project, props, s.assetOf())).toEqual([{ x: 6, y: 4, w: 3, h: 1 }]);
+  });
+});
+
+describe.each(COLLISION_SYSTEMS)('$system: trigger tiles report and never block (#160)', ({ physics }) => {
+  beforeAll(async () => {
+    await loadPhysics();
+  });
+
+  const run = (s: Setup) => {
+    const started = startWorld(s, { physics, collisions: true });
+    const hero = started.world.findByObjectId(s.hero)!;
+    const map = started.world.findByObjectId(s.mapObject)!;
+    const goTo = (x: number, y = -24) => {
+      hero.x = x;
+      hero.y = y;
+      started.world.tick(1 / 60);
+    };
+    return { ...started, hero, map, goTo };
+  };
+
+  it('walking through the pond: each cell and each area enters, stays and exits on its own', () => {
+    const s = setup();
+    const { collisions, goTo } = run(s);
+    goTo(0);
+    expect(collisions.enters).toEqual([]);
+    // 40 … 80: over the first water cell (48 … 96).
+    goTo(60);
+    expect(told(s, collisions.enters)).toEqual(['area water 6,4', 'cell water 6,4']);
+    expect(told(s, collisions.stays)).toEqual([]);
+    // 80 … 120: over cells 6 and 7, nearer to 7. The area is the one entered before.
+    goTo(100);
+    expect(told(s, collisions.enters)).toEqual(['cell water 7,4']);
+    expect(told(s, collisions.stays)).toEqual(['area water 7,4', 'cell water 6,4']);
+    expect(told(s, collisions.exits)).toEqual([]);
+    // 160 … 200: over the last water cell and the lava next to it.
+    goTo(180);
+    expect(told(s, collisions.enters)).toEqual(['area lava 9,4', 'cell lava 9,4', 'cell water 8,4']);
+    expect(told(s, collisions.stays)).toEqual(['area water 8,4']);
+    expect(told(s, collisions.exits)).toEqual(['cell water 6,4', 'cell water 7,4']);
+    goTo(0);
+    expect(told(s, collisions.exits)).toEqual([
+      'area lava 9,4',
+      'area water 8,4',
+      'cell lava 9,4',
+      'cell water 8,4',
+    ]);
+    goTo(0);
+    expect(collisions.enters.length + collisions.stays.length + collisions.exits.length).toBe(0);
+  });
+
+  it('the pair is the collider and the map, apart from the pair of the map’s solid cells', () => {
+    const s = setup();
+    const add = new AddTilemapLayerCommand(s.mapId);
+    s.stack.execute(add);
+    s.paint([[6, 4]], s.wallId, add.layerId);
+    // The same map once more, placed after the hero: the order in the scene has no say in the pair.
+    const later = place(s, s.mapId, { x: 0, y: 0 });
+    const { collisions, world, goTo, map } = run(s);
+    goTo(60);
+    const collider = world.findByObjectId(s.heroCollider)!;
+    const maps = [map, world.findByObjectId(later)!];
+    const tiles = collisions.enters.filter((pair) => pair.tile);
+    expect(tiles).toHaveLength(4);
+    expect(tiles.every((pair) => pair.a === collider && maps.includes(pair.b))).toBe(true);
+    // The wall under the water is each map's own pair, and it names no tile.
+    expect(collisions.enters.filter((pair) => !pair.tile)).toHaveLength(2);
+    // Nothing presses on a trigger tile, whatever the map's solid cells say of the same collider.
+    for (const pair of tiles) expect(collisions.contactOf(pair)).toBeNull();
+  });
+
+  it('touching says so on a trigger tile, and no more off it', () => {
+    const s = setup();
+    const { collisions, world, hero, map, goTo } = run(s);
+    goTo(0);
+    expect(collisions.touching(world, hero, map)).toBe(false);
+    goTo(100);
+    expect(collisions.touching(world, hero, map)).toBe(true);
+    expect(collisions.touching(world, map, hero)).toBe(true);
+    goTo(100, 100);
+    expect(collisions.touching(world, hero, map)).toBe(false);
+  });
+
+  it('a move goes through it and a ray does not meet it, even with Solid ticked', () => {
+    const s = setup();
+    s.stack.execute(new SetAssetPropertyCommand(s.waterId, TILE_SOLID_KEY, true));
+    const { collisions, world, hero, goTo } = run(s);
+    goTo(0);
+    expect(collisions.moveAndSlide(world, hero, 150, 0)).toMatchObject({ blockedX: false, hit: false });
+    expect(hero.x).toBe(150);
+    const ray = { ignore: hero, triggers: true, layers: ALL_LAYERS };
+    expect(collisions.raycast(world, { x: -200, y: -24 }, { x: 1, y: 0 }, 500, ray)).toBeNull();
+  });
+
+  it('nothing is told for a map switched off, a collider switched off, or layers that do not meet', () => {
+    const s = setup();
+    const { collisions, world, map, goTo } = run(s);
+    const collider = world.findByObjectId(s.heroCollider)!;
+    goTo(100);
+    expect(told(s, collisions.enters)).toHaveLength(3);
+    map.props.enabled = false;
+    goTo(100);
+    expect(told(s, collisions.exits)).toHaveLength(3);
+    map.props.enabled = true;
+    collider.props.enabled = false;
+    goTo(100);
+    expect(told(s, collisions.enters)).toEqual([]);
+    collider.props.enabled = true;
+    map.props[COLLIDES_WITH_KEY] = 0;
+    goTo(100);
+    expect(told(s, collisions.enters)).toEqual([]);
+    expect(collisions.touching(world, collider, map)).toBe(false);
+    map.props[COLLIDES_WITH_KEY] = ALL_LAYERS;
+    goTo(100);
+    expect(told(s, collisions.enters)).toHaveLength(3);
+  });
+
+  it('follows the placed map, and a tile set during the run', () => {
+    const s = setup();
+    const { collisions, world, map, goTo } = run(s);
+    goTo(0);
+    // Cell 4,4 spans -48 … 0: the hero stands on it and on 5,4.
+    expect(world.setTile(map, '', { x: 4, y: 4 }, s.lavaId)).toBe(true);
+    goTo(0);
+    expect(told(s, collisions.enters)).toEqual(['area lava 4,4', 'cell lava 4,4']);
+    world.setTile(map, '', { x: 4, y: 4 }, '');
+    goTo(0);
+    expect(told(s, collisions.exits)).toEqual(['area lava 4,4', 'cell lava 4,4']);
+    // The map moved 48 to the left: the pond now begins at the hero's cell.
+    map.x = -48;
+    goTo(20);
+    expect(told(s, collisions.enters)).toEqual(['area water 6,4', 'cell water 6,4']);
+  });
+
+  it('a cell grown by its collision scale is met from the cell next to it', () => {
+    const s = setup();
+    s.stack.execute(new SetAssetPropertyCommand(s.waterId, TILE_COLLISION_SCALE_KEY, 2));
+    const { collisions, goTo } = run(s);
+    // Cell 6 begins at 48 and reaches half a cell further out, to 24; the hero ends at 30, in cell 5.
+    goTo(10);
+    expect(told(s, collisions.enters)).toEqual(['area water 6,4', 'cell water 6,4']);
+  });
+
+  it('a system stopped and started again forgets what it told: the pond is entered anew', () => {
+    const s = setup();
+    const { collisions, world, goTo } = run(s);
+    goTo(100);
+    expect(told(s, collisions.enters)).toHaveLength(3);
+    const system = collisions as CollisionSource & System;
+    system.stop?.(world);
+    expect(collisions.enters.length + collisions.stays.length).toBe(0);
+    system.start?.(world);
+    goTo(100);
+    expect(told(s, collisions.enters)).toHaveLength(3);
+    expect(told(s, collisions.stays)).toEqual([]);
+  });
+
+  it('a collider under the map itself is told nothing', () => {
+    const s = setup();
+    const own = new AddColliderCommand(s.sceneId, s.mapObject);
+    s.stack.execute(own);
+    s.stack.execute(new SetOverrideCommand(s.sceneId, own.objectId, 'width', 400));
+    const { collisions, world, goTo } = run(s);
+    goTo(0);
+    const collider = world.findByObjectId(own.objectId)!;
+    expect(collisions.enters.filter((pair) => pair.tile && pair.a === collider)).toEqual([]);
+  });
+});
+
+describe.each(COLLISION_SYSTEMS)('$system: the collision events tell the tile (#160)', ({ physics }) => {
+  beforeAll(async () => {
+    await loadPhysics();
+  });
+
+  /** A behavior on the hero logging five lines per event: the label, the tile, its cell and the other body. */
+  function logging(s: Setup, type: string, label: string, params = {}) {
+    const behavior = makeBehavior(s, `${label} listener`);
+    place(s, behavior.id, { x: 0, y: 0 }, s.hero);
+    const edit = graphEditor(s, behavior);
+    const event = edit.addNode(type, params);
+    const logs = ['tile', 'cellX', 'cellY', 'other'].map(() => edit.addNode('Log'));
+    const mark = edit.addNode('Log', { message: label });
+    edit.wire([
+      [event, 'exec', mark, 'exec'],
+      [mark, 'exec', logs[0]!, 'exec'],
+      [logs[0]!, 'exec', logs[1]!, 'exec'],
+      [logs[1]!, 'exec', logs[2]!, 'exec'],
+      [logs[2]!, 'exec', logs[3]!, 'exec'],
+      [event, 'tile', logs[0]!, 'message'],
+      [event, 'cellX', logs[1]!, 'message'],
+      [event, 'cellY', logs[2]!, 'message'],
+      [event, 'other', logs[3]!, 'message'],
+    ]);
+    return { edit, event };
+  }
+
+  const walk = (s: Setup) => {
+    const started = startWorld(s, { physics, collisions: true, behaviors: true });
+    const hero = started.world.findByObjectId(s.hero)!;
+    const goTo = (x: number) => {
+      hero.x = x;
+      started.world.tick(1 / 60);
+      const said = started.world.log.splice(0);
+      // Five lines per event: the label, the tile, the cell and the other body.
+      const events: string[] = [];
+      for (let i = 0; i < said.length; i += 5) events.push(said.slice(i, i + 5).join(' '));
+      return events;
+    };
+    return { ...started, hero, goTo };
+  };
+
+  it('enter and exit run once for the pond by default, and once per cell with "per cell"', () => {
+    const s = setup();
+    logging(s, 'OnCollisionEnter', 'in');
+    logging(s, 'OnCollisionExit', 'out');
+    logging(s, 'OnCollisionEnter', 'in-cell', { perCell: true });
+    logging(s, 'OnCollisionExit', 'out-cell', { perCell: true });
+    const { goTo } = walk(s);
+    const w = s.waterId;
+    expect(goTo(0)).toEqual([]);
+    expect(goTo(60)).toEqual([`in ${w} 6 4 Ground`, `in-cell ${w} 6 4 Ground`]);
+    expect(goTo(100)).toEqual([`in-cell ${w} 7 4 Ground`]);
+    expect(goTo(140)).toEqual([`in-cell ${w} 8 4 Ground`, `out-cell ${w} 6 4 Ground`]);
+    // At 140 the hero was nearer to cell 7 than to 8, and that is the area's last word.
+    expect(goTo(0)).toEqual([`out ${w} 7 4 Ground`, `out-cell ${w} 7 4 Ground`, `out-cell ${w} 8 4 Ground`]);
+  });
+
+  it('stay tells the nearest cell of the area, or every cell lain on', () => {
+    const s = setup();
+    logging(s, 'OnCollisionStay', 'on');
+    logging(s, 'OnCollisionStay', 'on-cell', { perCell: true });
+    const { goTo } = walk(s);
+    const w = s.waterId;
+    expect(goTo(100)).toEqual([]);
+    expect(goTo(90)).toEqual([`on ${w} 6 4 Ground`, `on-cell ${w} 6 4 Ground`, `on-cell ${w} 7 4 Ground`]);
+    expect(goTo(110)).toEqual([`on ${w} 7 4 Ground`, `on-cell ${w} 6 4 Ground`, `on-cell ${w} 7 4 Ground`]);
+  });
+
+  it('"per cell" is read from what feeds it, each time the event comes', () => {
+    const s = setup();
+    const { edit, event } = logging(s, 'OnCollisionEnter', 'in');
+    const held = edit.addNode('KeyIsDown', { key: 'c' });
+    edit.connect(held, 'down', event, 'perCell');
+    const { goTo, input } = walk(s);
+    const w = s.waterId;
+    expect(goTo(60)).toEqual([`in ${w} 6 4 Ground`]);
+    input.press('c');
+    // With the key held the node hears cells: the area it stands in is not news to it, cell 7 is.
+    expect(goTo(100)).toEqual([`in ${w} 7 4 Ground`]);
+  });
+
+  it('a stay with an interval keeps one clock per cell', () => {
+    const s = setup();
+    logging(s, 'OnCollisionStay', 'on-cell', { perCell: true, interval: 0.04 });
+    const { goTo } = walk(s);
+    const w = s.waterId;
+    // Cell 6 is entered two frames before cell 7, so its 0.04 s are up while cell 7 still waits.
+    expect(goTo(60)).toEqual([]);
+    expect(goTo(60)).toEqual([]);
+    expect(goTo(100)).toEqual([]);
+    expect(goTo(100)).toEqual([`on-cell ${w} 6 4 Ground`]);
+    expect(goTo(100)).toEqual([]);
+    // Three frames on, cell 7 is due for the first time and cell 6, with 0.01 s left over, again.
+    expect(goTo(100)).toEqual([`on-cell ${w} 6 4 Ground`, `on-cell ${w} 7 4 Ground`]);
+  });
+
+  it('another collider and the solid cells of a map tell no tile: empty and -1, -1', () => {
+    const s = setup();
+    logging(s, 'OnCollisionEnter', 'in');
+    s.paint([[5, 4]], s.wallId);
+    const crate = place(s, s.spriteId, { x: -200, y: -24 });
+    s.stack.execute(new AddColliderCommand(s.sceneId, crate));
+    const { goTo } = walk(s);
+    // 0 … 40: over the wall in cell 5 (0 … 48), short of the water.
+    expect(goTo(20)).toEqual(['in  -1 -1 Ground']);
+    expect(goTo(-170)).toEqual(['in  -1 -1 Hero']);
+  });
+
+  it('a behavior on the map hears who walks onto its tiles', () => {
+    const s = setup();
+    const behavior = makeBehavior(s, 'Pond');
+    place(s, behavior.id, { x: 0, y: 0 }, s.mapObject);
+    const edit = graphEditor(s, behavior);
+    const enter = edit.addNode('OnCollisionEnter');
+    const who = edit.addNode('Log');
+    const where = edit.addNode('Log');
+    edit.wire([
+      [enter, 'exec', who, 'exec'],
+      [who, 'exec', where, 'exec'],
+      [enter, 'other', who, 'message'],
+      [enter, 'cellX', where, 'message'],
+    ]);
+    const { world, hero } = walk(s);
+    hero.x = 180;
+    world.tick(1 / 60);
+    // Water and lava are two areas, so two events.
+    expect(world.log).toEqual(['Hero', '8', 'Hero', '9']);
+  });
+});
+
+describe('a dynamic body falls through trigger tiles and is told (#160)', () => {
+  beforeAll(async () => {
+    await loadPhysics();
+  });
+
+  it('keeps falling, and enters and leaves the pond on the way', () => {
+    const s = setup();
+    s.stack.execute(new AddRigidbodyCommand(s.sceneId, s.hero));
+    const { world, collisions } = startWorld(s, { physics: true, collisions: true });
+    const hero = world.findByObjectId(s.hero)!;
+    // Above the pond's middle cell (96 … 144), which lies at -48 … 0.
+    hero.x = 120;
+    hero.y = -150;
+    const seen: string[] = [];
+    for (let i = 0; i < 90; i++) {
+      world.tick(1 / 60);
+      for (const pair of collisions.enters) if (pair.tile && !pair.tile.perCell) seen.push('enter');
+      for (const pair of collisions.exits) if (pair.tile && !pair.tile.perCell) seen.push('exit');
+    }
+    expect(seen).toEqual(['enter', 'exit']);
+    expect(hero.y).toBeGreaterThan(100);
+  });
+});

tools/e2e/40-tilemap.mjs

@@ -96,6 +96,27 @@ const tileGraph = {
   ],
 };
 
+/** A behavior that logs the tile and the cell of every `On collision enter` (#160), in three lines. */
+function triggerGraph(prefix) {
+  const id = (name) => `${prefix}-${name}`;
+  return {
+    nodes: [
+      node(id('enter'), 'OnCollisionEnter', 0, 0),
+      node(id('tile'), 'Log', 400, 0),
+      node(id('x'), 'Log', 700, 0),
+      node(id('y'), 'Log', 1000, 0),
+    ],
+    edges: [
+      edge(id('e1'), id('enter'), 'exec', id('tile'), 'exec'),
+      edge(id('e2'), id('enter'), 'tile', id('tile'), 'message'),
+      edge(id('e3'), id('tile'), 'exec', id('x'), 'exec'),
+      edge(id('e4'), id('enter'), 'cellX', id('x'), 'message'),
+      edge(id('e5'), id('x'), 'exec', id('y'), 'exec'),
+      edge(id('e6'), id('enter'), 'cellY', id('y'), 'message'),
+    ],
+  };
+}
+
 /** Whether a box lies inside another, give or take a pixel of rounding. */
 function inside(inner, outer) {
   return (
@@ -1229,6 +1250,88 @@ export async function tilemap(t) {
     passed && passed.x > -170 + 20,
     `world x=${passed?.x}`,
   );
+  // 40l-iv. Trigger tiles (#160). The switch sits on the tile next to Solid; the map then outlines
+  //          the cells in the trigger colour and blocks nothing there. Two behaviors under the crate
+  //          listen as it walks across the block: one hears the area, one – "per cell" ticked on its
+  //          node – every cell.
+  await select({ kind: 'asset', assetId: 'at-grass' });
+  await settle();
+  const triggerRow = await json("__gm.inspectorField('Trigger (reports, never blocks)')");
+  if (triggerRow) await click(triggerRow.x, triggerRow.y);
+  await settle();
+  const reporting = await json(`__gm.tilemap.solids(${JSON.stringify(MAP_OBJECT)})`);
+  check(
+    'a tile has a Trigger switch; with it the map outlines its cells as triggers and none as solid',
+    Boolean(triggerRow) &&
+      (await t.asset('at-grass'))?.props.trigger === true &&
+      reporting?.visible === true &&
+      reporting.rects.length === 0 &&
+      JSON.stringify(reporting.triggers) ===
+        JSON.stringify([
+          { x: 0, y: 0, w: 3, h: 3 },
+          { x: 4, y: 0, w: 1, h: 1 },
+        ]),
+    JSON.stringify({ triggerRow, props: (await t.asset('at-grass'))?.props, reporting }),
+  );
+  await shot('40-tilemap-trigger');
+  await mutate(`
+    const assets = __gm.state.project.assets;
+    const ear = (id, name, graph) => {
+      const made = structuredClone(assets.find((a) => a.id === 'mover'));
+      Object.assign(made, { id, name, props: { enabled: true, runInEditor: false }, variables: [], graph });
+      assets.push(made);
+    };
+    ear('tt-area', 'Area ear', ${JSON.stringify(triggerGraph('ta'))});
+    ear('tt-cells', 'Cell ear', ${JSON.stringify(triggerGraph('tc'))});
+    const object = (id, assetId, name) => ({ id, assetId, name, position: { x: 0, y: 0, z: 0 },
+      editorOffset: { x: 0, y: 0 }, overrides: {}, parentId: 'tm-crate', instanceEdits: {} });
+    __gm.state.activeScene.objects.push(
+      object('tm-crate-area', 'tt-area', 'Area ear'), object('tm-crate-cells', 'tt-cells', 'Cell ear'));`);
+  await select({ kind: 'node', assetId: 'tt-cells', nodeId: 'tc-enter' });
+  await settle();
+  const perCellField = await json("__gm.inspectorField('per cell', true)");
+  if (perCellField) await click(perCellField.x, perCellField.y);
+  await settle();
+  const enterParams = await json(
+    "__gm.state.project.assets.find((a) => a.id === 'tt-cells').graph.nodes.find((n) => n.id === 'tc-enter').params",
+  );
+  check(
+    'On collision enter has a "per cell" switch, ticked on the node with the mouse',
+    Boolean(perCellField) && enterParams.perCell === true,
+    JSON.stringify({ perCellField, enterParams }),
+  );
+  await shot('40-tilemap-trigger-node');
+  await evaluate('__gm.state.select(null)');
+  await openTab(t, 'scene');
+  const triggerLogFrom = t.consoleLog.length;
+  const crossed = await playAgainstMap();
+  const heardBy = (name) => {
+    const lines = t.consoleLog
+      .slice(triggerLogFrom)
+      .filter((line) => line.startsWith(`[behavior ${name}]`))
+      .map((line) => line.split('] ').pop());
+    const events = [];
+    for (let i = 0; i + 2 < lines.length; i += 3) events.push(lines.slice(i, i + 3).join(' '));
+    return events;
+  };
+  const areaHeard = heardBy('Area ear');
+  const cellsHeard = heardBy('Cell ear');
+  const names = (events) => events.every((event) => /^at-grass [0-4] [0-2]$/.test(event));
+  check(
+    'the crate walks across the trigger tiles: one enter for the block, one per cell with "per cell"',
+    crossed &&
+      crossed.x > -170 + 20 &&
+      areaHeard.length === 1 &&
+      cellsHeard.length > 1 &&
+      new Set(cellsHeard).size === cellsHeard.length &&
+      names(areaHeard) &&
+      names(cellsHeard),
+    JSON.stringify({ x: crossed?.x, areaHeard, cellsHeard }),
+  );
+  await mutate(`
+    const p = __gm.state.project;
+    p.assets = p.assets.filter((a) => !a.id.startsWith('tt-'));
+    delete p.assets.find((a) => a.id === 'at-grass').props.trigger;`);
   await mutate(`
     const scene = __gm.state.activeScene;
     scene.objects = scene.objects.filter((o) => !o.id.startsWith('tm-crate'));