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>

tests/core/vector-boolean.test.ts

@@ -0,0 +1,207 @@
+/**
+ * Boolean operations on vector art shapes (#187). The areas are measured by sampling the result
+ * under the even-odd rule the rasteriser fills with, so a test sees what would be painted – not
+ * merely how many rings came out.
+ */
+import { Polygon } from 'pixi.js';
+import { describe, expect, it } from 'vitest';
+import { CreateAssetCommand } from '../../src/core/commands/assetCommands';
+import {
+  AddVectorShapeCommand,
+  CombineVectorShapesCommand,
+  RemoveVectorShapesCommand,
+  SetVectorShapeStyleCommand,
+} from '../../src/core/commands/vectorArtCommands';
+import type { VectorArtAsset, VectorRing, VectorShape } from '../../src/core/model/Asset';
+import { findAsset } from '../../src/core/model/Project';
+import {
+  combineBlocker,
+  combinedRings,
+  FLATTEN_TOLERANCE,
+  type ShapeBoolean,
+} from '../../src/core/model/vectorBoolean';
+import { makeProject } from '../helpers';
+
+function square(x: number, y: number, side: number): VectorRing {
+  return {
+    closed: true,
+    points: [
+      { x, y },
+      { x: x + side, y },
+      { x: x + side, y: y + side },
+      { x, y: y + side },
+    ],
+  };
+}
+
+/** A circle of four symmetric points, the handles a third of a quarter arc long (kappa). */
+function circle(cx: number, cy: number, r: number): VectorRing {
+  const k = 0.5523 * r;
+  return {
+    closed: true,
+    points: [
+      { x: cx + r, y: cy, in: { x: 0, y: -k }, out: { x: 0, y: k } },
+      { x: cx, y: cy + r, in: { x: k, y: 0 }, out: { x: -k, y: 0 } },
+      { x: cx - r, y: cy, in: { x: 0, y: k }, out: { x: 0, y: -k } },
+      { x: cx, y: cy - r, in: { x: -k, y: 0 }, out: { x: k, y: 0 } },
+    ],
+  };
+}
+
+function shape(id: string, ...rings: VectorRing[]): VectorShape {
+  return { id, name: id, rings, fill: 0x808080, opacity: 1 };
+}
+
+function inside(ring: VectorRing, x: number, y: number): boolean {
+  let hit = false;
+  const p = ring.points;
+  for (let i = 0, j = p.length - 1; i < p.length; j = i++) {
+    const a = p[i]!;
+    const b = p[j]!;
+    if (a.y > y !== b.y > y && x < ((b.x - a.x) * (y - a.y)) / (b.y - a.y) + a.x) hit = !hit;
+  }
+  return hit;
+}
+
+/** The area painted under even-odd, counted at the centres of half-pixel cells. */
+function paintedArea(rings: readonly VectorRing[], from = -5, to = 60): number {
+  let cells = 0;
+  for (let x = from + 0.25; x < to; x += 0.5)
+    for (let y = from + 0.25; y < to; y += 0.5)
+      if (rings.filter((ring) => inside(ring, x, y)).length % 2 === 1) cells += 1;
+  return cells * 0.25;
+}
+
+const A = () => shape('a', square(0, 0, 20));
+const B = () => shape('b', square(10, 10, 20));
+
+describe('combining shapes', () => {
+  it.each<[ShapeBoolean, number]>([
+    ['union', 700],
+    ['subtract', 300],
+    ['intersect', 100],
+    ['exclude', 600],
+  ])('%s of two overlapping squares paints %i square pixels', (op, area) => {
+    expect(paintedArea(combinedRings([A(), B()], op))).toBe(area);
+  });
+
+  it('subtracts every later shape from the first, whatever order they overlap in', () => {
+    const c = shape('c', square(0, 10, 5));
+    // 400 − the 100 B takes − the 25 C takes: C and B do not overlap inside A.
+    expect(paintedArea(combinedRings([A(), B(), c], 'subtract'))).toBe(275);
+  });
+
+  it('reads each shape by the even-odd rule: an overlap of its own rings is a hole', () => {
+    const crossed = shape('x', square(0, 0, 20), square(10, 0, 20));
+    // The two rings overlap in 10×20 – even-odd leaves that empty: 800 − 2·200 = 400.
+    const far = shape('f', square(40, 40, 10));
+    expect(paintedArea(combinedRings([crossed, far], 'union'))).toBe(500);
+  });
+
+  it('puts an island in a hole before its outline, so Pixi does not take it for a hole', () => {
+    const donut = shape('d', square(0, 0, 50), square(10, 10, 30));
+    const island = shape('i', square(20, 20, 10));
+    const rings = combinedRings([donut, island], 'union');
+    expect(paintedArea(rings)).toBe(2500 - 900 + 100);
+    // Pixi's own rule (ShapePath.addPath with checkForHoles): a polygon is a hole of the last
+    // outline when that one contains it. The island has to come out as an outline of its own.
+    let main: { poly: Polygon; holes: number } | null = null;
+    const outlines: { poly: Polygon; holes: number }[] = [];
+    for (const ring of rings) {
+      const poly = new Polygon(ring.points.flatMap((p) => [p.x, p.y]));
+      if (main && main.poly.containsPolygon(poly)) main.holes += 1;
+      else outlines.push((main = { poly, holes: 0 }));
+    }
+    expect(outlines.map((o) => o.holes).sort()).toEqual([0, 1]);
+  });
+
+  it('samples a curve within the tolerance and keeps its points few', () => {
+    const round = shape('r', circle(30, 30, 20));
+    const far = shape('f', square(100, 100, 5));
+    const rings = combinedRings([round, far], 'union');
+    const disc = rings.find((ring) => ring.points.every((p) => p.x < 60))!;
+    const radii = disc.points.map((p) => Math.hypot(p.x - 30, p.y - 30));
+    // Kappa bulges the true curve out by 0.03 % of r; rounding to a hundredth adds 0.01.
+    expect(Math.max(...radii)).toBeLessThan(20 + 0.02);
+    expect(Math.min(...radii)).toBeGreaterThan(20 - FLATTEN_TOLERANCE - 0.02);
+    expect(disc.points.length).toBeGreaterThan(16);
+    expect(disc.points.length).toBeLessThan(64);
+    expect(disc.points.every((p) => !p.in && !p.out)).toBe(true);
+  });
+
+  it('refuses one shape alone and an open line, and says why', () => {
+    expect(combineBlocker([A()])).toMatch(/second shape/);
+    const line = shape('l', {
+      closed: false,
+      points: [
+        { x: 0, y: 0 },
+        { x: 5, y: 5 },
+      ],
+    });
+    expect(combineBlocker([A(), line])).toMatch(/open line/);
+    // A line of one point is still being drawn and draws nothing – it is passed over.
+    const stub = shape('s', square(40, 0, 5), { closed: false, points: [{ x: 0, y: 0 }] });
+    expect(combineBlocker([A(), stub])).toBeNull();
+  });
+});
+
+function drawingWith(...rings: VectorRing[]) {
+  const fixture = makeProject();
+  const create = new CreateAssetCommand('vectorart', 'Logo');
+  fixture.stack.execute(create);
+  const assetId = create.asset.id;
+  const ids = rings.map((ring) => {
+    const add = new AddVectorShapeCommand(assetId, [ring]);
+    fixture.stack.execute(add);
+    return add.shapeId;
+  });
+  const art = () => findAsset(fixture.project, assetId) as VectorArtAsset;
+  return { ...fixture, assetId, ids, art };
+}
+
+describe('the combine command', () => {
+  it('writes the result into the lowest shape, keeps its look and place, and removes the rest', () => {
+    const f = drawingWith(square(40, 40, 10), square(0, 0, 20), square(10, 10, 20));
+    const [first, low, high] = f.ids as [string, string, string];
+    f.stack.execute(
+      new SetVectorShapeStyleCommand(f.assetId, low, { fill: 0xff0000, stroke: { color: 0, width: 2 } }),
+    );
+    // Picked top first: the order of the selection does not decide, the drawing order does.
+    const combine = new CombineVectorShapesCommand(f.assetId, [high, low], 'subtract');
+    f.stack.execute(combine);
+    expect(combine.shapeId).toBe(low);
+    expect(f.art().shapes.map((s) => s.id)).toEqual([first, low]);
+    const result = f.art().shapes[1]!;
+    expect(result).toMatchObject({ name: 'Shape 2', fill: 0xff0000, stroke: { color: 0, width: 2 } });
+    expect(paintedArea(result.rings)).toBe(300);
+  });
+
+  it('is one undo step that puts every shape back as it was', () => {
+    const f = drawingWith(square(0, 0, 20), square(10, 10, 20), square(40, 40, 10));
+    const before = JSON.stringify(f.art().shapes);
+    f.stack.execute(new CombineVectorShapesCommand(f.assetId, f.ids, 'union'));
+    expect(f.art().shapes).toHaveLength(1);
+    f.stack.undo();
+    expect(JSON.stringify(f.art().shapes)).toBe(before);
+    f.stack.redo();
+    expect(paintedArea(f.art().shapes[0]!.rings)).toBe(800);
+  });
+
+  it('refuses a result of nothing and leaves the drawing alone', () => {
+    const f = drawingWith(square(0, 0, 10), square(30, 30, 10));
+    const before = JSON.stringify(f.art().shapes);
+    expect(() => f.stack.execute(new CombineVectorShapesCommand(f.assetId, f.ids, 'intersect'))).toThrow(
+      /nothing in common/,
+    );
+    expect(JSON.stringify(f.art().shapes)).toBe(before);
+  });
+
+  it('removes several shapes in one step and brings them back in their places', () => {
+    const f = drawingWith(square(0, 0, 5), square(10, 0, 5), square(20, 0, 5));
+    const before = f.art().shapes.map((s) => s.id);
+    f.stack.execute(new RemoveVectorShapesCommand(f.assetId, [f.ids[0]!, f.ids[2]!]));
+    expect(f.art().shapes.map((s) => s.id)).toEqual([f.ids[1]]);
+    f.stack.undo();
+    expect(f.art().shapes.map((s) => s.id)).toEqual(before);
+  });
+});