@@ -0,0 +1,80 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { createCompositor, type MoverFrame } from "./composite";
+import { IDENTITY, translate } from "./pose";
+
+function fakeContext() {
+  return {
+    drawImage: vi.fn(),
+    clearRect: vi.fn(),
+    setTransform: vi.fn(),
+    globalAlpha: 1,
+    globalCompositeOperation: "source-over",
+  };
+}
+let offscreen: Array<ReturnType<typeof fakeContext>> = [];
+
+beforeEach(() => {
+  offscreen = [];
+  vi.stubGlobal(
+    "OffscreenCanvas",
+    class {
+      constructor(
+        public width: number,
+        public height: number,
+      ) {}
+      getContext() {
+        const ctx = fakeContext();
+        offscreen.push(ctx);
+        return ctx;
+      }
+    },
+  );
+});
+afterEach(() => vi.unstubAllGlobals());
+
+const capture = {} as CanvasImageSource;
+const background = {} as CanvasImageSource;
+const slide = (from: number, to: number, n = 16) =>
+  Array.from({ length: n }, (_, k) => translate(IDENTITY, from + ((to - from) * (k + 0.5)) / n, 0));
+const mover = (w: number, h: number, taps: ReturnType<typeof slide>): MoverFrame => ({
+  w,
+  h,
+  now: translate(IDENTITY, 50, 0),
+  taps,
+});
+
+describe("createCompositor", () => {
+  it("keeps no crop for a mover that does not travel a pixel, and draws only the background", () => {
+    const c = createCompositor(100, 100);
+    const movers = [mover(10, 10, slide(0, 0.5))];
+    const kept = c.crop(capture, movers);
+    expect(kept).toEqual([false]);
+    const out = fakeContext();
+    expect(c.draw(background, out as never, movers, kept)).toEqual({ blurred: 0, taps: 0 });
+    expect(out.drawImage).toHaveBeenCalledTimes(1);
+    expect(out.drawImage.mock.calls[0]?.[0]).toBe(background);
+  });
+
+  it("crops a moving mover once and lays one tap per draw over the background", () => {
+    const c = createCompositor(1000, 100);
+    const movers = [mover(10, 10, slide(0, 100))];
+    const kept = c.crop(capture, movers);
+    expect(kept).toEqual([true]);
+    const crop = offscreen.at(-1);
+    expect(crop?.drawImage).toHaveBeenCalledWith(capture, 0, 0);
+    const out = fakeContext();
+    const stats = c.draw(background, out as never, movers, kept);
+    const accum = offscreen[0];
+    expect(stats.blurred).toBe(1);
+    expect(accum?.drawImage).toHaveBeenCalledTimes(stats.taps);
+    expect(accum?.globalAlpha).toBeCloseTo(1 / stats.taps, 9);
+    // Background first, then the trail over it.
+    expect(out.drawImage.mock.calls[0]).toEqual([background, 0, 0]);
+    expect(out.drawImage).toHaveBeenCalledTimes(2);
+  });
+
+  it("leaves a mover sharp when its box is too big to copy every frame", () => {
+    const c = createCompositor(100, 100);
+    expect(c.crop(capture, [mover(1000, 1000, slide(0, 100))])).toEqual([false]);
+  });
+});
@@ -0,0 +1,135 @@
+/**
+ * The per-frame draw: a background, then each mover's own pixels redrawn along its path.
+ *
+ * Each mover is cropped from a capture of the scene and drawn at every tap pose into a float
+ * buffer, additive at weight 1/n: the shutter integral. Its coverage fades at the path ends, so
+ * the trail needs a background without the mover (a second capture, movers hidden); falling back
+ * to the sharp frame leaves thin lines looking unblurred.
+ *
+ * Image-space approximation: a mover's box contents move with it; ink outside it is not redrawn.
+ */
+
+import { type Affine, invert, sweep, thinTaps } from "./pose";
+
+export interface MoverFrame {
+  w: number;
+  h: number;
+  /** The pose the capture shows (frame time). */
+  now: Affine;
+  /** Poses across the shutter. */
+  taps: Affine[];
+}
+
+interface CompositeStats {
+  blurred: number;
+  taps: number;
+}
+
+type Context2D = CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
+interface Layer {
+  canvas: OffscreenCanvas;
+  ctx: OffscreenCanvasRenderingContext2D;
+}
+
+// Float16 keeps n additive 1/n-weighted taps exact; 8-bit loses up to ~n/2 levels per channel.
+function makeLayer(w: number, h: number): Layer {
+  const canvas = new OffscreenCanvas(w, h);
+  let ctx: OffscreenCanvasRenderingContext2D | null = null;
+  try {
+    ctx = canvas.getContext("2d", { colorType: "float16" } as CanvasRenderingContext2DSettings);
+  } catch {
+    ctx = null;
+  }
+  ctx = ctx ?? canvas.getContext("2d");
+  if (!ctx) throw new Error("capture motion blur: no 2D context for the accumulation buffer");
+  return { canvas, ctx };
+}
+
+/** Whether a mover travels far enough within this shutter to draw a trail at all. */
+function moves(mover: MoverFrame, frameW: number, frameH: number): boolean {
+  const { travel, rect } = sweep(mover.taps, mover.w, mover.h, frameW, frameH);
+  return rect !== null && travel >= 1;
+}
+
+export function createCompositor(width: number, height: number) {
+  const accum = makeLayer(width, height);
+  const crops: Layer[] = [];
+  // A mover box this many times the frame is too big to copy every frame; it is left sharp.
+  const maxCropArea = width * height * 4;
+
+  const cropLayer = (i: number, w: number, h: number): Layer => {
+    let layer = crops[i];
+    if (!layer || layer.canvas.width < w || layer.canvas.height < h) {
+      layer = makeLayer(
+        Math.max(w, layer?.canvas.width ?? 1),
+        Math.max(h, layer?.canvas.height ?? 1),
+      );
+      crops[i] = layer;
+    }
+    return layer;
+  };
+
+  /** Copy each mover's frame-time pixels out of `src`; false marks a mover to leave sharp. */
+  function crop(src: CanvasImageSource, movers: MoverFrame[]): boolean[] {
+    return movers.map((mover, i) => {
+      const { w, h } = mover;
+      const back = invert(mover.now);
+      if (!back || w * h > maxCropArea || !moves(mover, width, height)) return false;
+      const c = cropLayer(i, w, h).ctx;
+      c.setTransform(1, 0, 0, 1, 0, 0);
+      c.globalCompositeOperation = "source-over";
+      c.globalAlpha = 1;
+      c.clearRect(0, 0, w, h);
+      c.setTransform(back.a, back.b, back.c, back.d, back.e, back.f);
+      c.drawImage(src, 0, 0);
+      return true;
+    });
+  }
+
+  function trail(out: Context2D, mover: MoverFrame, layer: Layer): number {
+    const { w, h } = mover;
+    const { travel, rect } = sweep(mover.taps, w, h, width, height);
+    if (!rect) return 0;
+    const taps = thinTaps(mover.taps, travel);
+    const a = accum.ctx;
+    a.setTransform(1, 0, 0, 1, 0, 0);
+    a.globalCompositeOperation = "source-over";
+    a.globalAlpha = 1;
+    a.clearRect(rect.x, rect.y, rect.w, rect.h);
+    a.globalCompositeOperation = "lighter";
+    a.globalAlpha = 1 / taps.length;
+    for (const m of taps) {
+      a.setTransform(m.a, m.b, m.c, m.d, m.e, m.f);
+      a.drawImage(layer.canvas, 0, 0, w, h, 0, 0, w, h);
+    }
+    out.drawImage(accum.canvas, rect.x, rect.y, rect.w, rect.h, rect.x, rect.y, rect.w, rect.h);
+    return taps.length;
+  }
+
+  /** `background` into `out`, then the trail of every mover `crop` kept. */
+  function draw(
+    background: CanvasImageSource,
+    out: Context2D,
+    movers: MoverFrame[],
+    kept: boolean[],
+  ): CompositeStats {
+    out.setTransform(1, 0, 0, 1, 0, 0);
+    out.globalAlpha = 1;
+    out.globalCompositeOperation = "copy";
+    out.drawImage(background, 0, 0);
+    out.globalCompositeOperation = "source-over";
+    const stats: CompositeStats = { blurred: 0, taps: 0 };
+    movers.forEach((mover, i) => {
+      const layer = crops[i];
+      if (!kept[i] || !layer) return;
+      const n = trail(out, mover, layer);
+      if (n > 0) {
+        stats.blurred++;
+        stats.taps += n;
+      }
+    });
+    return stats;
+  }
+
+  return { crop, draw };
+}
@@ -0,0 +1,67 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { discoverMovers } from "./discover";
+
+// jsdom lays nothing out: each element's rect follows the x its fake timeline writes.
+function element(parent: Element, id: string): HTMLElement {
+  const el = document.createElement("div");
+  el.id = id;
+  parent.append(el);
+  el.getBoundingClientRect = () => {
+    const x = parseFloat(el.style.transform.replace("translateX(", "")) || 0;
+    return { left: x, top: 0, right: x + 10, bottom: 10, width: 10, height: 10 } as DOMRect;
+  };
+  return el;
+}
+
+function setup() {
+  const scene = document.createElement("div");
+  const fast = element(scene, "fast");
+  const fastChild = element(fast, "fast-child");
+  const slow = element(scene, "slow");
+  const still = element(scene, "still");
+  const off = element(scene, "off");
+  off.setAttribute("data-motion-blur", "off");
+  // 1 s at 30 fps: `fast` and `off` cross 300 px (10 px/frame), `slow` 15 px (0.5 px/frame).
+  const seek = (t: number): void => {
+    fast.style.transform = `translateX(${300 * t}px)`;
+    fastChild.style.transform = `translateX(${200 * t}px)`;
+    slow.style.transform = `translateX(${15 * t}px)`;
+    off.style.transform = `translateX(${300 * t}px)`;
+  };
+  seek(0);
+  return { scene, fast, fastChild, slow, still, off, seek };
+}
+
+describe("discoverMovers", () => {
+  afterEach(() => vi.restoreAllMocks());
+
+  it("keeps fast, outermost movers that are not opted out", () => {
+    const { scene, fast, seek } = setup();
+    const { movers } = discoverMovers({ scene, durationSeconds: 1, fps: 30, seek });
+    expect(movers).toEqual([fast]);
+  });
+
+  it("marks every changing element dynamic, slow and opted-out ones included", () => {
+    const { scene, fast, fastChild, slow, still, off, seek } = setup();
+    const { dynamic } = discoverMovers({ scene, durationSeconds: 1, fps: 30, seek });
+    expect(
+      dynamic.has(fast) && dynamic.has(fastChild) && dynamic.has(slow) && dynamic.has(off),
+    ).toBe(true);
+    expect(dynamic.has(still)).toBe(false);
+  });
+
+  it("drops a mover in a 3D context", () => {
+    const { scene, fast, seek } = setup();
+    const real = window.getComputedStyle;
+    vi.spyOn(window, "getComputedStyle").mockImplementation((el: Element) => {
+      if (el === fast)
+        return {
+          transform: "matrix3d(1,0,0,0,0,1,0,0,0,0,1,0,0,0,0,1)",
+          transformStyle: "flat",
+          perspective: "none",
+        } as CSSStyleDeclaration;
+      return real(el);
+    });
+    expect(discoverMovers({ scene, durationSeconds: 1, fps: 30, seek }).movers).toEqual([]);
+  });
+});