Vector shapes can be combined by union, subtract and more (#187)

Building a shape out of simpler ones meant drawing the result point by
point. The Vector Art tab now selects several shapes at once and
combines them with a boolean operation into one ordinary shape.

- Ctrl/Cmd+click in the tab and Ctrl or Shift+click in the shape list
  add a shape to the selection. Shift stays pan in the canvas.
- A COMBINE block in the inspector offers Union, Subtract, Intersect
  and Exclude. Subtract removes every upper shape from the lowest one.
- The result replaces the lowest selected shape and keeps its name,
  colours, stroke and place in the order. It is one undo step.
- Curves are flattened to 0.2 canvas pixels, a circle gets about 48
  points. Open lines and single shapes cannot be combined, the buttons
  are off and a hint gives the reason. An empty result is rejected.
- Delete now removes every selected shape, and the selection drops
  shapes that no longer exist.
- The geometry comes from clipper2-ts 2.0.1, pinned. It has no
  dependencies and speaks even-odd directly. Only the engine module
  is imported, the bundle grows by 55 kB instead of 110 kB.

Findings:

- clipper2-js was tried and dropped. It pulls in Angular 15 with a
  high npm audit finding and no fix.
- Pixi finds holes by containment, not by even-odd depth. An island
  inside a hole would be drawn as a hole again. Result rings are
  emitted outer ring before its islands, and a test mirrors Pixi's
  rule.
- A depth sort turned out to be dead code in a counter-check and was
  removed. A pixel check in 47h stayed green without the subtract
  and now checks both colours.

Tests:

- ./check.sh: 134 files, 1986 tests, exit 0 (before 133 / 1968)
- npm run build: exit 0
- GM_STEPS=47, freshly seeded: ALL PASSED, 44 checks (before 39)
- 18 new unit tests, 5 new e2e checks in block 47h
- Five counter-checks bite, originals restored and verified with cmp
- Check sum 1222 in README and wiki, calculated (1217 + 5)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/commands/vectorArtCommands.ts

@@ -12,7 +12,8 @@ import { newId } from '../ids';
 import type { VectorRing, VectorShape, VectorStroke } from '../model/Asset';
 import type { Project } from '../model/Project';
 import { cloneVectorRing, SHAPE_FILL_DEFAULT } from '../model/vectorart';
-import { moveItem, type Removed, removeById, removeFirst, restoreAt } from '../util/list';
+import { combineBlocker, combinedRings, type ShapeBoolean } from '../model/vectorBoolean';
+import { moveItem, type Removed, removeById, removeFirst, removeWhere, restoreAt } from '../util/list';
 import { type Command, CommandError } from './Command';
 import { nextNumberedName } from './naming';
 import { requireVectorArt, requireVectorShape } from './require';
@@ -71,6 +72,87 @@ export class RemoveVectorShapeCommand implements Command {
   }
 }
 
+/** Takes several shapes out in one step – Delete on a selection of more than one (#187). */
+export class RemoveVectorShapesCommand implements Command {
+  readonly label = 'Remove shapes';
+  private removed: Removed<VectorShape>[] = [];
+
+  constructor(
+    private readonly assetId: string,
+    private readonly shapeIds: readonly string[],
+  ) {}
+
+  execute(project: Project): void {
+    const asset = requireVectorArt(project, this.assetId);
+    this.removed = removeWhere(asset.shapes, (shape) => this.shapeIds.includes(shape.id));
+    if (this.removed.length === 0) throw new CommandError('No such shape');
+  }
+
+  undo(project: Project): void {
+    restoreAt(requireVectorArt(project, this.assetId).shapes, this.removed);
+  }
+}
+
+const COMBINE_LABELS: Record<ShapeBoolean, string> = {
+  union: 'Union shapes',
+  subtract: 'Subtract shapes',
+  intersect: 'Intersect shapes',
+  exclude: 'Exclude shapes',
+};
+
+const NOTHING_LEFT: Record<ShapeBoolean, string> = {
+  union: 'The shapes cover nothing',
+  subtract: 'Nothing is left – the shapes above cover the lowest one completely',
+  intersect: 'The shapes have nothing in common',
+  exclude: 'The shapes cancel each other out completely',
+};
+
+/**
+ * Combines shapes into one (#187). The result takes the place, name and look of the lowest of them –
+ * the one the others are cut out of when subtracting – and the others go. It is an ordinary shape of
+ * corners afterwards; an undo is the way back to the shapes it was made of.
+ */
+export class CombineVectorShapesCommand implements Command {
+  readonly label: string;
+  private removed: Removed<VectorShape>[] = [];
+  private previous: VectorRing[] = [];
+  private resultId = '';
+
+  constructor(
+    private readonly assetId: string,
+    private readonly shapeIds: readonly string[],
+    private readonly op: ShapeBoolean,
+  ) {
+    this.label = COMBINE_LABELS[op];
+  }
+
+  /** The shape that holds the result: the lowest of the combined ones. Empty until executed. */
+  get shapeId(): string {
+    return this.resultId;
+  }
+
+  execute(project: Project): void {
+    const asset = requireVectorArt(project, this.assetId);
+    const shapes = asset.shapes.filter((shape) => this.shapeIds.includes(shape.id));
+    if (shapes.length !== new Set(this.shapeIds).size) throw new CommandError('No such shape');
+    const blocked = combineBlocker(shapes);
+    if (blocked) throw new CommandError(blocked);
+    const rings = combinedRings(shapes, this.op);
+    if (rings.length === 0) throw new CommandError(NOTHING_LEFT[this.op]);
+    const base = shapes[0]!;
+    this.resultId = base.id;
+    this.previous = base.rings;
+    base.rings = rings;
+    this.removed = removeWhere(asset.shapes, (shape) => shape !== base && this.shapeIds.includes(shape.id));
+  }
+
+  undo(project: Project): void {
+    const asset = requireVectorArt(project, this.assetId);
+    restoreAt(asset.shapes, this.removed);
+    requireVectorShape(project, this.assetId, this.resultId).rings = this.previous;
+  }
+}
+
 export class RenameVectorShapeCommand implements Command {
   readonly label = 'Rename shape';
   private previous = '';

src/core/model/vectorBoolean.ts

@@ -0,0 +1,150 @@
+/**
+ * Boolean operations on vector art shapes (#187): union, subtract, intersect and exclude. The
+ * clipping itself is Clipper2 (`clipper2-ts`) – robust polygon clipping is a field of its own, and
+ * its even-odd fill rule is exactly how a shape's rings fill (#129).
+ *
+ * The work runs on the sampled outline: a curve is cut into straight pieces fine enough that the
+ * sampling stays under `FLATTEN_TOLERANCE`, and the result is corners only. The result is an ordinary
+ * shape again – no compound that remembers where it came from.
+ *
+ * Only the clipping engine is imported, not the package's `Clipper` namespace: that one pulls in
+ * offsetting, triangulation and Minkowski sums as well, twice the bytes in the bundle (measured:
+ * 101 kB against 50 kB minified).
+ */
+import { ClipType, FillRule, type PathD, type PathsD } from 'clipper2-ts/dist/Core.js';
+import { ClipperD, type PolyPathD, PolyTreeD } from 'clipper2-ts/dist/Engine.js';
+import type { VectorRing, VectorShape } from './Asset';
+import type { Vec2 } from './Properties';
+import { handleAt, isDrawableRing } from './vectorart';
+
+export type ShapeBoolean = 'union' | 'subtract' | 'intersect' | 'exclude';
+
+/** How far a sampled curve may stray from the true one, in canvas pixels. */
+export const FLATTEN_TOLERANCE = 0.2;
+const MAX_CURVE_STEPS = 100;
+/** Decimal places Clipper keeps; a hundredth of a canvas pixel is finer than any supersampling. */
+const PRECISION = 2;
+
+/**
+ * Why the shapes cannot be combined, or null when they can. `shapes` is the selection; an open line
+ * has no inside to add or cut away, and a half-drawn one (too short to draw) is simply passed over.
+ */
+export function combineBlocker(shapes: readonly VectorShape[]): string | null {
+  if (shapes.length < 2) return 'Ctrl+click a second shape to combine.';
+  if (shapes.some((shape) => shape.rings.some((ring) => !ring.closed && isDrawableRing(ring))))
+    return 'An open line cannot be combined.';
+  return null;
+}
+
+/**
+ * The rings of the combined shape; `shapes` in drawing order, the first one being the lowest. Subtract
+ * cuts every later shape out of the first. Empty when nothing is left.
+ *
+ * The rings come out in the order the rasteriser needs: Pixi takes a ring for a hole of the outline
+ * drawn last when that one contains it, so every outline is followed by its own holes, and an
+ * island inside a hole comes before the outline around it – after it, it would count as a hole too.
+ */
+export function combinedRings(shapes: readonly VectorShape[], op: ShapeBoolean): VectorRing[] {
+  const regions = shapes.map(shapeRegion);
+  const [first = [], ...rest] = regions;
+  let result: PathsD;
+  if (op === 'union') result = unite(regions.flat(), FillRule.NonZero);
+  else if (op === 'subtract')
+    result = clip(ClipType.Difference, first, unite(rest.flat(), FillRule.NonZero), FillRule.NonZero);
+  else {
+    const type = op === 'intersect' ? ClipType.Intersection : ClipType.Xor;
+    result = rest.reduce((acc, next) => clip(type, acc, next, FillRule.NonZero), first);
+  }
+  const clipper = new ClipperD(PRECISION);
+  clipper.addSubjectPaths(result);
+  const tree = new PolyTreeD();
+  clipper.execute(ClipType.Union, FillRule.NonZero, tree);
+  const rings: VectorRing[] = [];
+  collectOutlines(tree, rings);
+  return rings;
+}
+
+/**
+ * Every outline under `node`, each followed by its holes. An outline goes in only after the islands
+ * inside its holes have gone in – that is the island-first order `combinedRings` promises.
+ */
+function collectOutlines(node: PolyPathD, into: VectorRing[]): void {
+  for (let i = 0; i < node.count; i++) {
+    const outline = node.child(i);
+    const rings = [toRing(outline.poly)];
+    for (let j = 0; j < outline.count; j++) {
+      const hole = outline.child(j);
+      rings.push(toRing(hole.poly));
+      collectOutlines(hole, into);
+    }
+    into.push(...rings.filter((ring) => ring.points.length >= 3));
+  }
+}
+
+function toRing(path: PathD | null): VectorRing {
+  return { closed: true, points: (path ?? []).map((p) => ({ x: round(p.x), y: round(p.y) })) };
+}
+
+function round(value: number): number {
+  return Math.round(value * 100) / 100;
+}
+
+/**
+ * What a shape covers, as Clipper paths: its closed rings under the even-odd rule the rasteriser
+ * uses, turned into outlines that do not overlap – so any number of shapes can then be put together
+ * under the non-zero rule without one shape's hole cancelling another's fill.
+ */
+function shapeRegion(shape: VectorShape): PathsD {
+  const rings = shape.rings.filter((ring) => ring.closed && isDrawableRing(ring)).map(flattenRing);
+  return unite(rings, FillRule.EvenOdd);
+}
+
+function unite(paths: PathsD, fillRule: FillRule): PathsD {
+  return clip(ClipType.Union, paths, [], fillRule);
+}
+
+function clip(type: ClipType, subject: PathsD, clipBy: PathsD, fillRule: FillRule): PathsD {
+  const clipper = new ClipperD(PRECISION);
+  clipper.addSubjectPaths(subject);
+  clipper.addClipPaths(clipBy);
+  const solution: PathsD = [];
+  clipper.execute(type, fillRule, solution);
+  return solution;
+}
+
+/** A closed ring as corners only: every curved segment cut into straight pieces. */
+export function flattenRing(ring: VectorRing): Vec2[] {
+  const points = ring.points;
+  const out: Vec2[] = [];
+  for (const [i, from] of points.entries()) {
+    out.push({ x: from.x, y: from.y });
+    const to = points[(i + 1) % points.length]!;
+    if (!from.out && !to.in) continue;
+    const c1 = handleAt(from, 'out') ?? from;
+    const c2 = handleAt(to, 'in') ?? to;
+    const steps = curveSteps(from, c1, c2, to);
+    for (let step = 1; step < steps; step++) out.push(cubicAt(from, c1, c2, to, step / steps));
+  }
+  return out;
+}
+
+/**
+ * How many straight pieces keep a cubic within the tolerance – Wang's formula: the largest second
+ * difference of the control points bounds how far a chord can stray from the curve.
+ */
+function curveSteps(p0: Vec2, p1: Vec2, p2: Vec2, p3: Vec2): number {
+  const m = Math.max(
+    Math.hypot(p0.x - 2 * p1.x + p2.x, p0.y - 2 * p1.y + p2.y),
+    Math.hypot(p1.x - 2 * p2.x + p3.x, p1.y - 2 * p2.y + p3.y),
+  );
+  return Math.min(MAX_CURVE_STEPS, Math.max(1, Math.ceil(Math.sqrt((0.75 * m) / FLATTEN_TOLERANCE))));
+}
+
+function cubicAt(p0: Vec2, p1: Vec2, p2: Vec2, p3: Vec2, t: number): Vec2 {
+  const u = 1 - t;
+  const a = u * u * u;
+  const b = 3 * u * u * t;
+  const c = 3 * u * t * t;
+  const d = t * t * t;
+  return { x: a * p0.x + b * p1.x + c * p2.x + d * p3.x, y: a * p0.y + b * p1.y + c * p2.y + d * p3.y };
+}

src/editor/debugHook.ts

@@ -518,6 +518,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
         }));
       },
       selected: () => (state.selection?.kind === 'shape' ? state.selection.shapeId : ''),
+      selectedIds: () => state.selectedShapeIds,
       /**
        * The shape list in the inspector (#129, stage 4): the order it shows, top first, and where a
        * row sits. A row carries a name that also stands in the tab, so a check cannot click it by text.

src/editor/inspector/Inspector.ts

@@ -98,7 +98,7 @@ export class Inspector extends Panel {
     else if (s.kind === 'asset') buildAssetSection(this.host, s.assetId);
     else if (s.kind === 'node') buildNodeSection(this.host, s.assetId, s.nodeId);
     else if (s.kind === 'frame') buildFrameSection(this.host, s.assetId, s.frameId);
-    else if (s.kind === 'shape') buildShapeSection(this.host, s.assetId, s.shapeId);
+    else if (s.kind === 'shape') buildShapeSection(this.host, s.assetId, s.shapeId, s.shapeIds);
     else if (s.kind === 'annotation') buildAnnotationSection(this.host, s.assetId, s.annotationId);
     else if (s.kind === 'track') buildTrackSection(this.host, s.assetId, s.trackId, s.keys);
     else if (s.objectIds.length > 1) buildObjectsSection(this.host, s.holderId, s.objectIds);
@@ -112,7 +112,7 @@ export class Inspector extends Panel {
     if (s.kind === 'asset') return assetSignature(this.state.project, s.assetId);
     if (s.kind === 'node') return nodeSignature(this.state.project, s.assetId, s.nodeId);
     if (s.kind === 'frame') return frameSignature(this.state.project, s.assetId, s.frameId);
-    if (s.kind === 'shape') return shapeSignature(this.state.project, s.assetId, s.shapeId);
+    if (s.kind === 'shape') return shapeSignature(this.state.project, s.assetId, s.shapeId, s.shapeIds);
     if (s.kind === 'annotation') return annotationSignature(this.state.project, s.assetId, s.annotationId);
     if (s.kind === 'track') return trackSignature(this.state.project, s.assetId, s.trackId, s.keys);
     return objectSignature(this.state, s.holderId, s.objectIds, s.objectId);

src/editor/inspector/shapeSection.ts

@@ -10,8 +10,10 @@
  */
 import {
   AddVectorShapeCommand,
+  CombineVectorShapesCommand,
   MoveVectorShapeCommand,
   RemoveVectorShapeCommand,
+  RemoveVectorShapesCommand,
   RenameVectorShapeCommand,
   SetVectorShapeStyleCommand,
 } from '../../core/commands/vectorArtCommands';
@@ -24,8 +26,10 @@ import {
   type ShapeOrderMove,
   shapeOrderTarget,
 } from '../../core/model/vectorart';
+import { combineBlocker, type ShapeBoolean } from '../../core/model/vectorBoolean';
 import type { PropertyDef } from '../../core/properties/PropertySchema';
 import { Button, HStack, Theme } from '../../ui';
+import { shapeSelection } from '../state';
 import { ShapeList } from '../vectorart/ShapeList';
 import { createFieldRow, type FieldHandlers } from './fieldFactory';
 import { addGap, addNote, addTitle, type InspectorHost } from './host';
@@ -48,6 +52,13 @@ const ORDER_BUTTONS: { move: ShapeOrderMove; label: string }[] = [
   { move: 'back', label: 'To back' },
 ];
 
+const COMBINE_BUTTONS: { op: ShapeBoolean; label: string }[] = [
+  { op: 'union', label: 'Union' },
+  { op: 'subtract', label: 'Subtract' },
+  { op: 'intersect', label: 'Intersect' },
+  { op: 'exclude', label: 'Exclude' },
+];
+
 const FILL_DEF: PropertyDef = { key: 'fill', label: 'Fill', kind: 'color', default: SHAPE_FILL_DEFAULT };
 const OPACITY_DEF: PropertyDef = {
   key: 'opacity',
@@ -90,33 +101,53 @@ export function shapeListSignature(asset: VectorArtAsset): string {
 }
 
 /**
- * Structure-defining facts of a shape selection: the list above it, and whether the shape has an
- * outline – its colour and width are rows of their own and appear with it.
+ * Structure-defining facts of a shape selection: the list above it, whether the shape has an
+ * outline – its colour and width are rows of their own and appear with it – and whether the
+ * selected shapes can be combined, which switches the four buttons and the note under them.
  */
-export function shapeSignature(project: Project, assetId: string, shapeId: string): string {
+export function shapeSignature(
+  project: Project,
+  assetId: string,
+  shapeId: string,
+  shapeIds: readonly string[],
+): string {
   const asset = findAssetOf(project, assetId, 'vectorart');
   const shape = asset && findShape(asset, shapeId);
   if (!asset || !shape) return 'missing';
-  return `${shapeListSignature(asset)}|${shape.stroke ? 'stroke' : 'none'}`;
+  const blocked = combineBlocker(selectedShapes(asset, shapeIds)) ?? '';
+  return `${shapeListSignature(asset)}|${shape.stroke ? 'stroke' : 'none'}|${shapeIds.join(',')}|${blocked}`;
+}
+
+/** The selected shapes in drawing order, the lowest first – the order they are combined in. */
+function selectedShapes(asset: VectorArtAsset, shapeIds: readonly string[]): VectorShape[] {
+  return asset.shapes.filter((shape) => shapeIds.includes(shape.id));
 }
 
 /** The shapes of the asset, with nothing chosen: what the asset's own section shows underneath. */
 export function buildShapeListSection(host: InspectorHost, asset: VectorArtAsset): void {
   addTitle(host, 'SHAPES');
-  addShapeList(host, asset, '');
+  addShapeList(host, asset, []);
   addNote(host, 'The top row is drawn over the ones below it.', 'small');
   addGap(host);
   host.body.add(addShapeButton(host, asset.id));
 }
 
-/** Name, place in the drawing order and style of one shape, under the list it is picked from. */
-export function buildShapeSection(host: InspectorHost, assetId: string, shapeId: string): void {
+/**
+ * Name, place in the drawing order and style of the primary shape, under the list it is picked
+ * from, and the buttons that combine all selected shapes into one (#187).
+ */
+export function buildShapeSection(
+  host: InspectorHost,
+  assetId: string,
+  shapeId: string,
+  shapeIds: readonly string[],
+): void {
   const asset = findAssetOf(host.state.project, assetId, 'vectorart');
   const shape = asset && findShape(asset, shapeId);
   if (!asset || !shape) return;
 
   addTitle(host, 'SHAPES');
-  addShapeList(host, asset, shapeId);
+  addShapeList(host, asset, shapeIds);
   addNote(host, 'Drag a row to draw it earlier or later.', 'small');
 
   // Two rows of two: four of them side by side cut "Backward" down to "Backw…" at the panel's width.
@@ -134,6 +165,8 @@ export function buildShapeSection(host: InspectorHost, assetId: string, shapeId:
     host.body.add(order);
   }
 
+  addCombineButtons(host, asset, shapeIds);
+
   addTitle(host, 'VECTOR SHAPE');
   addShapeName(host, assetId, shape);
   styleRow(
@@ -177,11 +210,16 @@ export function buildShapeSection(host: InspectorHost, assetId: string, shapeId:
   } else addNote(host, 'Without an outline only the fill is drawn.', 'small');
 
   addGap(host);
+  const several = shapeIds.length > 1;
   host.body.add(
-    new Button('Delete shape', {
+    new Button(several ? 'Delete shapes' : 'Delete shape', {
       variant: 'danger',
       onClick: () => {
-        host.state.execute(new RemoveVectorShapeCommand(assetId, shapeId));
+        host.state.execute(
+          several
+            ? new RemoveVectorShapesCommand(assetId, shapeIds)
+            : new RemoveVectorShapeCommand(assetId, shapeId),
+        );
         host.state.select({ kind: 'asset', assetId });
       },
     }),
@@ -189,6 +227,32 @@ export function buildShapeSection(host: InspectorHost, assetId: string, shapeId:
   host.body.add(addShapeButton(host, assetId));
 }
 
+/**
+ * Union, subtract, intersect, exclude – two rows of two, like the order buttons. They are there with
+ * one shape selected too, switched off, so the note under them can say how to get a second one.
+ */
+function addCombineButtons(host: InspectorHost, asset: VectorArtAsset, shapeIds: readonly string[]): void {
+  const shapes = selectedShapes(asset, shapeIds);
+  const blocked = combineBlocker(shapes);
+  addTitle(host, 'COMBINE');
+  for (let start = 0; start < COMBINE_BUTTONS.length; start += 2) {
+    const row = new HStack({ gap: Theme.gap });
+    for (const { op, label } of COMBINE_BUTTONS.slice(start, start + 2)) {
+      const button = new Button(label, {
+        variant: 'ghost',
+        onClick: () => {
+          const command = new CombineVectorShapesCommand(asset.id, shapeIds, op);
+          if (host.state.execute(command)) host.state.select(shapeSelection(asset.id, [command.shapeId]));
+        },
+      });
+      button.setEnabled(blocked === null);
+      row.add(button, 1);
+    }
+    host.body.add(row);
+  }
+  addNote(host, blocked ?? `The result keeps "${shapes[0]?.name ?? ''}", the lowest.`, 'small');
+}
+
 /** Pushes the shape's current name and style into the rows. False when it is gone (rebuild). */
 export function refreshShapeSection(host: InspectorHost, assetId: string, shapeId: string): boolean {
   const shape = currentShape(host, assetId, shapeId);
@@ -202,7 +266,7 @@ export function refreshShapeSection(host: InspectorHost, assetId: string, shapeI
   return true;
 }
 
-function addShapeList(host: InspectorHost, asset: VectorArtAsset, selectedId: string): void {
+function addShapeList(host: InspectorHost, asset: VectorArtAsset, selectedIds: readonly string[]): void {
   if (asset.shapes.length === 0) {
     built = null;
     addNote(host, 'Nothing drawn yet – take the Pen in the Vector art tab.');
@@ -212,8 +276,11 @@ function addShapeList(host: InspectorHost, asset: VectorArtAsset, selectedId: st
     host.state,
     asset.id,
     asset.shapes.map((shape) => shape.id),
-    selectedId,
-    (shapeId) => host.state.select({ kind: 'shape', assetId: asset.id, shapeId }),
+    selectedIds,
+    (shapeId, toggle) =>
+      toggle
+        ? host.state.toggleShape(asset.id, shapeId)
+        : host.state.select(shapeSelection(asset.id, [shapeId])),
     (from, to) => void host.state.execute(new MoveVectorShapeCommand(asset.id, from, to)),
   );
   built = list;
@@ -224,8 +291,7 @@ function addShapeButton(host: InspectorHost, assetId: string): Button {
   return new Button('Add shape', {
     onClick: () => {
       const command = new AddVectorShapeCommand(assetId);
-      if (host.state.execute(command))
-        host.state.select({ kind: 'shape', assetId, shapeId: command.shapeId });
+      if (host.state.execute(command)) host.state.select(shapeSelection(assetId, [command.shapeId]));
     },
   });
 }

src/editor/state.ts

@@ -37,8 +37,11 @@ export type Selection =
   | { kind: 'node'; assetId: string; nodeId: string; nodeIds: string[]; annotationIds: string[] }
   /** A frame rectangle inside a texture asset (the Texture tab). */
   | { kind: 'frame'; assetId: string; frameId: string }
-  /** One shape of a vector art asset (the Vector art tab, #129). */
-  | { kind: 'shape'; assetId: string; shapeId: string }
+  /**
+   * One or more shapes of a vector art asset (the Vector art tab, #129). `shapeId` is the primary
+   * one, whose points and style are edited; `shapeIds` holds all of them for combining (#187).
+   */
+  | { kind: 'shape'; assetId: string; shapeId: string; shapeIds: string[] }
   /** One or more groups or comments of a graph, `annotationId` being the primary one. */
   | { kind: 'annotation'; assetId: string; annotationId: string; annotationIds: string[] }
   /**
@@ -99,6 +102,12 @@ export function nodeSelection(
   );
 }
 
+/** Builds a shape selection; the last id is the primary one unless given explicitly. */
+export function shapeSelection(assetId: string, shapeIds: readonly string[], primary?: string): Selection {
+  const picked = pickIds(shapeIds, primary);
+  return picked && { kind: 'shape', assetId, shapeId: picked.primary, shapeIds: picked.ids };
+}
+
 /** Builds an annotation selection; the last id is the primary one unless given explicitly. */
 export function annotationSelection(
   assetId: string,
@@ -410,6 +419,9 @@ export class EditorState {
     // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     if (selection?.kind === 'object' && !selection.objectIds)
       selection = objectSelection(selection.holderId, [selection.objectId]);
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
+    if (selection?.kind === 'shape' && !selection.shapeIds)
+      selection = shapeSelection(selection.assetId, [selection.shapeId]);
     // A graph selection carries two lists, and a short form may leave out either – `{ nodeId }`
     // alone, or `{ nodeIds }` without the boxes. Filling both here keeps every reader of them
     // from having to ask whether they are there.
@@ -447,6 +459,19 @@ export class EditorState {
     this.select(objectSelection(holderId, next, current.includes(objectId) ? undefined : objectId));
   }
 
+  /** Ctrl+click on a shape: in or out of the selection; emptied, the drawing itself stays selected. */
+  toggleShape(assetId: string, shapeId: string): void {
+    const s = this.selection;
+    const current = s?.kind === 'shape' && s.assetId === assetId ? s.shapeIds : [];
+    const held = current.includes(shapeId);
+    const next = held ? current.filter((id) => id !== shapeId) : [...current, shapeId];
+    this.select(shapeSelection(assetId, next, held ? undefined : shapeId) ?? { kind: 'asset', assetId });
+  }
+
+  get selectedShapeIds(): string[] {
+    return this.selection?.kind === 'shape' ? this.selection.shapeIds : [];
+  }
+
   toggleNode(assetId: string, nodeId: string): void {
     const s = this.selection;
     const current = s?.kind === 'node' && s.assetId === assetId ? s.nodeIds : [];
@@ -520,6 +545,16 @@ export class EditorState {
       else if (!asset.frames.some((f) => f.id === s.frameId))
         this.select({ kind: 'asset', assetId: s.assetId });
     }
+    if (s.kind === 'shape') {
+      const asset = findAssetOf(this.project, s.assetId, 'vectorart');
+      // As with a frame: the drawing stays open when its shapes vanished (a combine, an undo).
+      if (!asset) this.select(null);
+      else {
+        const alive = s.shapeIds.filter((id) => asset.shapes.some((shape) => shape.id === id));
+        if (alive.length !== s.shapeIds.length)
+          this.select(shapeSelection(s.assetId, alive, s.shapeId) ?? { kind: 'asset', assetId: s.assetId });
+      }
+    }
     if (s.kind === 'track') {
       const asset = findAssetOf(this.project, s.assetId, 'animation');
       if (!asset) this.select(null);
@@ -675,7 +710,8 @@ export function sameSelection(a: Selection, b: Selection): boolean {
     );
   }
   if (a.kind === 'frame' && b.kind === 'frame') return a.assetId === b.assetId && a.frameId === b.frameId;
-  if (a.kind === 'shape' && b.kind === 'shape') return a.assetId === b.assetId && a.shapeId === b.shapeId;
+  if (a.kind === 'shape' && b.kind === 'shape')
+    return a.assetId === b.assetId && a.shapeId === b.shapeId && sameIds(a.shapeIds, b.shapeIds);
   if (a.kind === 'track' && b.kind === 'track')
     return (
       a.assetId === b.assetId &&

src/editor/vectorart/ShapeList.ts

@@ -54,12 +54,16 @@ export class ShapeList extends Widget {
   private drag: RowDrag | null = null;
   private readonly stop: () => void;
 
+  /**
+   * `selectedIds` are the rows marked. `onPick` is told the row pressed and whether it was pressed with
+   * Ctrl or Shift, which adds the shape to the selection or takes it out (#187).
+   */
   constructor(
     private readonly state: EditorState,
     private readonly assetId: string,
     shapeIds: readonly string[],
-    private selectedId: string,
-    private readonly onPick: (shapeId: string) => void,
+    private readonly selectedIds: readonly string[],
+    private readonly onPick: (shapeId: string, toggle: boolean) => void,
     private readonly onReorder: (from: number, to: number) => void,
   ) {
     super();
@@ -144,10 +148,11 @@ export class ShapeList extends Widget {
     e.stopPropagation();
     // Picked right away rather than on release: the rows below show what the selected shape is.
     const shapeId = this.shown[index]!;
-    if (shapeId !== this.selectedId) {
-      this.selectedId = shapeId;
-      this.onPick(shapeId);
+    if (e.ctrlKey || e.metaKey || e.shiftKey) {
+      this.onPick(shapeId, true);
+      return;
     }
+    if (this.selectedIds.length !== 1 || this.selectedIds[0] !== shapeId) this.onPick(shapeId, false);
     this.drag = { from: index, startY: y, y, active: false };
   }
 
@@ -181,7 +186,7 @@ export class ShapeList extends Widget {
       row.box.position.set(0, y);
       row.box.visible = Boolean(shape);
       if (!shape) continue;
-      const selected = shapeId === this.selectedId;
+      const selected = this.selectedIds.includes(shapeId);
       if (selected || index === this.hoveredRow)
         this.background
           .rect(0, y, this.w, SHAPE_ROW_HEIGHT)

src/editor/vectorart/VectorArtCanvas.ts

@@ -14,6 +14,7 @@ import { Container, type FederatedPointerEvent, type FederatedWheelEvent, Graphi
 import {
   AddVectorShapeCommand,
   RemoveVectorShapeCommand,
+  RemoveVectorShapesCommand,
   SetVectorShapeRingsCommand,
 } from '../../core/commands/vectorArtCommands';
 import type { VectorArtAsset, VectorPoint, VectorRing, VectorShape } from '../../core/model/Asset';
@@ -48,7 +49,7 @@ import {
   withoutPoint,
   withSmoothToggled,
 } from '../polygonEdit';
-import type { EditorState } from '../state';
+import { type EditorState, shapeSelection } from '../state';
 import { drawChecker } from '../texture/checker';
 import { fitView, imageToLocal, localToImage, type SheetView, zoomAbout } from '../texture/sheetView';
 
@@ -242,12 +243,17 @@ export class VectorArtCanvas extends Widget {
     return s?.kind === 'shape' && s.assetId === this.assetId ? s.shapeId : '';
   }
 
+  private selectedShapeIds(): string[] {
+    const s = this.state.selection;
+    return s?.kind === 'shape' && s.assetId === this.assetId ? s.shapeIds : [];
+  }
+
   private shape(shapeId: string): VectorShape | undefined {
     const asset = this.asset();
     return asset ? findShape(asset, shapeId) : undefined;
   }
 
-  /** Delete takes the selected shape away – the canvas edits shapes, so that is what it removes. */
+  /** Delete takes the selected shapes away – the canvas edits shapes, so that is what it removes. */
   override onKeyDown(event: KeyboardEvent): boolean {
     if (event.key === 'Escape' && this.drawing) {
       this.endPen();
@@ -255,10 +261,14 @@ export class VectorArtCanvas extends Widget {
       return true;
     }
     if (event.key !== 'Delete' && event.key !== 'Backspace') return super.onKeyDown(event);
-    const shapeId = this.selectedShapeId();
-    if (!shapeId) return super.onKeyDown(event);
+    const shapeIds = this.selectedShapeIds();
+    if (shapeIds.length === 0) return super.onKeyDown(event);
     this.endPen();
-    this.state.execute(new RemoveVectorShapeCommand(this.assetId, shapeId));
+    this.state.execute(
+      shapeIds.length === 1
+        ? new RemoveVectorShapeCommand(this.assetId, shapeIds[0]!)
+        : new RemoveVectorShapesCommand(this.assetId, shapeIds),
+    );
     this.state.select({ kind: 'asset', assetId: this.assetId });
     return true;
   }
@@ -292,7 +302,7 @@ export class VectorArtCanvas extends Widget {
     if (!open) {
       const command = new AddVectorShapeCommand(asset.id, [{ points: [point], closed: false }]);
       if (!this.state.execute(command)) return;
-      this.state.select({ kind: 'shape', assetId: asset.id, shapeId: command.shapeId });
+      this.state.select(shapeSelection(asset.id, [command.shapeId]));
       this.pen = { shapeId: command.shapeId, ring: 0 };
       this.startPenDrag(command.shapeId, 0, 0, local, point);
       return;
@@ -342,7 +352,8 @@ export class VectorArtCanvas extends Widget {
    * A press with the arrow. On the selected shape first, so its points stay reachable under a shape
    * drawn later: a double-click on a point takes it away, Alt makes it round or square again, a
    * press on a point or a handle drags it, one on a line puts a point there. Inside a shape's fill
-   * the whole shape is dragged; on nothing the selection is cleared.
+   * the whole shape is dragged; on nothing the selection is cleared. Ctrl adds a shape to the
+   * selection or takes it out again, which is how shapes are picked for combining (#187).
    */
   private pressSelect(
     asset: VectorArtAsset,
@@ -351,6 +362,14 @@ export class VectorArtCanvas extends Widget {
     tolerance: number,
     e: FederatedPointerEvent,
   ): void {
+    if (e.ctrlKey || e.metaKey) {
+      const hit = this.shapeAt(asset, at);
+      if (hit) {
+        this.pick(null);
+        this.state.toggleShape(asset.id, hit.id);
+      }
+      return;
+    }
     const selected = this.shape(this.selectedShapeId());
     if (selected) {
       for (const [ringIndex, ring] of selected.rings.entries()) {
@@ -360,13 +379,10 @@ export class VectorArtCanvas extends Widget {
         return;
       }
     }
-    // Last first: the later shape is drawn on top, so that is what a click on an overlap means.
-    for (let i = asset.shapes.length - 1; i >= 0; i--) {
-      const shape = asset.shapes[i]!;
-      if (!shape.rings.some((ring) => insideRing(ring, at))) continue;
+    const shape = this.shapeAt(asset, at);
+    if (shape) {
       this.pick(null);
-      if (shape.id !== selected?.id)
-        this.state.select({ kind: 'shape', assetId: asset.id, shapeId: shape.id });
+      this.state.select(shapeSelection(asset.id, [shape.id]));
       this.drag = {
         kind: 'shape',
         shapeId: shape.id,
@@ -387,6 +403,15 @@ export class VectorArtCanvas extends Widget {
     this.redraw();
   }
 
+  /** The shape under a canvas pixel. Last first: a later shape is drawn on top of an earlier one. */
+  private shapeAt(asset: VectorArtAsset, at: Vec2): VectorShape | undefined {
+    for (let i = asset.shapes.length - 1; i >= 0; i--) {
+      const shape = asset.shapes[i]!;
+      if (shape.rings.some((ring) => insideRing(ring, at))) return shape;
+    }
+    return undefined;
+  }
+
   private pressGrip(
     asset: VectorArtAsset,
     shape: VectorShape,
@@ -597,6 +622,8 @@ export class VectorArtCanvas extends Widget {
       this.art.scale.set(this.view.zoom);
       for (const drawing of vectorArtDrawings(asset)) paintDrawing(this.art, drawing);
       this.drawFrame(origin, size);
+      for (const shapeId of this.selectedShapeIds())
+        if (shapeId !== this.selectedShapeId()) this.drawOutline(this.shape(shapeId));
       this.drawShape(this.shape(this.selectedShapeId()));
     }
     this.syncHint(asset);
@@ -609,6 +636,15 @@ export class VectorArtCanvas extends Widget {
       .stroke({ width: 1, color: Theme.colors.marquee });
   }
 
+  /** A shape selected alongside the primary one: its outline only, its points are not editable. */
+  private drawOutline(shape: VectorShape | undefined): void {
+    for (const ring of shape?.rings ?? []) {
+      const screen = ringOutline(ring).map((p) => imageToLocal(this.view, p.x, p.y));
+      if (screen.length >= 2)
+        this.overlay.poly(screen, ring.closed).stroke({ width: 1, color: Theme.colors.accent });
+    }
+  }
+
   /** The selected shape's points and handles, in widget pixels so they keep their size. */
   private drawShape(shape: VectorShape | undefined): void {
     if (!shape) return;