<!--
Motion Blur — After Effects-style shutter-based motion blur.
Usage: paste this snippet into your composition, then call
attachMotionBlur() with any element animated by your GSAP timeline.
How it works — the same model After Effects uses for the layer Motion Blur
switch (temporal supersampling), synthesised inside the page because the
renderer captures one instant per frame with no shutter:
1. After every timeline update (each rendered frame), the snippet seeks
the timeline to `samplesPerFrame` sub-frame times spread evenly across
the shutter window, reads each target's RESOLVED `transform` at every
sample, then restores the frame time. Seeks run with events suppressed,
so no user callbacks fire and the DOM ends up exactly where the frame
left it. Reading the resolved matrix rather than a list of named GSAP
properties is what makes every driver work: translation, scale,
rotation, 3D rotation and skew arrive in the same one value, and a
driver nobody thought of needs no change here.
2. One copy of the element per sample is stacked in an isolated group,
each carrying its sample's transform, each at 1/N opacity with
`mix-blend-mode: plus-lighter`. plus-lighter adds premultiplied colour,
so N+1 copies at 1/N sum to the average — the shutter integral. The
isolation is load-bearing: without a transparent backdrop of its own
the first copy would add onto the page and blow a light background out
to white. Result: a smear along the actual trajectory whose extent is
speed × shutter time, with no decaying one-sided trail. The tails —
where fewer copies overlap — step down evenly at 1/N per copy; wherever
the element travels less than its own size the copies pile up past 1
and clamp, so the middle of the smear is solid rather than a plateau.
3. The element itself paints over that group, sharp and at full opacity.
No single copy ever exceeds 1/N, so a moving edge resolves as a
staircase of ghosts behind a solid frame-time instance. The group
carries the element's own opacity, so a beat that moves and fades at
once fades its smear with it.
4. When every sampled transform collapses to the current one (element at
rest), the group is hidden so the element renders sharp.
Shutter window (AE semantics, per frame at time t, frame interval 1/fps):
shutterTime = shutterAngle / 360 / fps
windowStart = t + shutterPhase / 360 / fps
copy k = windowStart + k / N × shutterTime, k = 0..N (N+1 copies)
N counts sub-intervals, so both ends of the window carry a copy.
Defaults (shutterAngle 720, shutterPhase -360) integrate [t - 1/fps, t + 1/fps]:
a solid moving at 2000 px/s at 30 fps smears 133 px, centred on its position.
These defaults are measured, not assumed. On a 1920×1080 / 30 fps After Effects
export of translating text, the outermost duplicate on the trailing side sits
exactly at the previous frame's position and the outermost leading duplicate
exactly at the next frame's, with 8 evenly spaced duplicates in between on each
side (window = 2 frames = 720°, phase -360°, 16 sub-intervals). Reading the ghost
staircase across a stroke gives steps of 0.063 ± 0.002 of the sharp text
intensity — a flat 1/16 per duplicate, with no taper toward the window edges
(triangle weighting scores 1.6 dB worse against the same export). The sharp
instance on top is what keeps total ink above the unblurred frame's.
Requirements:
- Elements must be animated via `transform` (GSAP x/y/scale/rotation and the
3D and skew properties all qualify), not left/top. Transformed ancestors
are still not compensated.
- A 3D beat reads its perspective off the element's parent and applies it per
copy, so the vanishing point follows each copy's transform-origin rather
than the parent's perspective-origin. Those agree when the element is
centred in its perspective parent, which is how a 3D beat is authored.
- A copy drops the element's id but keeps its classes, because a class rule is
the only thing that can still style a copy's pseudo-elements. So a selector
written against a class matches the copies too: after attaching, address the
element by id or by reference, never by a class a copy also carries.
- A target is blurred once: a second attachMotionBlur() call naming the same
element leaves the first call's copies in place rather than stacking a
second set over them.
- Cost is N + 1 copies of the target's whole subtree, restyled on attach and
on resize and re-transformed every frame. Point it at the element that
moves, not at a container holding it.
- The host must render the timeline for the blur to update — HyperFrames
seeks every frame. A fresh paused timeline that is never seeked shows no
blur on its first frame.
- Call attachMotionBlur() AFTER defining all tweens (the timeline's final
duration must be known), before window.__timelines registration.
- GSAP must be loaded before this snippet executes.
API:
attachMotionBlur(selector, timeline, options?)
Options:
shutterAngle — degrees of the frame interval the shutter is open
(default 720 = two frames, as measured off the AE
reference; 360 = one frame, 0 disables)
shutterPhase — degrees offset of the window start from the frame
time (default -360 = centred on the frame, one frame
back, like the AE reference)
samplesPerFrame — sub-intervals of the shutter window, so N+1 duplicates
each at 1/N opacity (default 16, max 64)
fps — composition frame rate (default: the root's data-fps,
else 30). Pass it explicitly when rendering with an
fps override (`hyperframes render --fps`).
-->
<script>
(function () {
if (!window._hfMbUid) window._hfMbUid = 0;
if (!window._hfMbAttached) window._hfMbAttached = new WeakSet();
// Shutter length is measured in frames, so the snippet must know the
// composition frame rate: explicit option, else the root's data-fps, else 30.
function resolveFps(optionFps) {
var explicit = Number(optionFps);
if (explicit > 0) return explicit;
var root = document.querySelector("[data-composition-id][data-fps]");
var rootFps = root ? Number(root.getAttribute("data-fps")) : 0;
if (rootFps > 0) return rootFps;
return 30;
}
function numOption(value, fallback) {
return value !== undefined ? Number(value) : fallback;
}
// A resolved `transform` is either "none", a 2D matrix(a,b,c,d,e,f) or a 3D
// matrix3d of 16. Splitting the numbers into the linear part and the
// translation is what lets the deadband below use a pixel tolerance on the
// one and an angle/ratio tolerance on the other.
function parseTransform(value) {
if (!value || value === "none") return { linear: [1, 0, 0, 1], translate: [0, 0, 0] };
var nums = value
.slice(value.indexOf("(") + 1, -1)
.split(",")
.map(parseFloat);
if (nums.length === 16) {
return {
linear: [
nums[0],
nums[1],
nums[2],
nums[4],
nums[5],
nums[6],
nums[8],
nums[9],
nums[10],
nums[3],
nums[7],
nums[11],
nums[15],
],
translate: [nums[12], nums[13], nums[14]],
};
}
return {
linear: [nums[0], nums[1], nums[2], nums[3]],
translate: [nums[4], nums[5], 0],
};
}
// Deadband: half a pixel of travel, a thousandth of a unit of linear change,
// about 0.06 degrees or 0.1% of scale. Below it the average is the source.
// The 2D and 3D forms have different arities, so a change of form counts as
// movement rather than being compared component by component.
function differs(a, b) {
if (a.linear.length !== b.linear.length) return true;
for (var i = 0; i < a.linear.length; i++) {
if (Math.abs(a.linear[i] - b.linear[i]) > 0.001) return true;
}
for (var j = 0; j < 3; j++) {
if (Math.abs(a.translate[j] - b.translate[j]) > 0.5) return true;
}
return false;
}
window.attachMotionBlur = function (selector, tl, opts) {
opts = opts || {};
var shutterAngle = numOption(opts.shutterAngle, 720);
var shutterPhase = numOption(opts.shutterPhase, -360);
var requestedSamples = numOption(opts.samplesPerFrame, 16);
var samples = Number.isFinite(requestedSamples)
? Math.max(2, Math.min(64, Math.round(requestedSamples)))
: 16;
var fps = resolveFps(opts.fps);
var items = Array.isArray(selector) ? selector : [selector];
var targets = items.reduce(function (acc, s) {
if (typeof s === "string") {
document.querySelectorAll(s).forEach(function (el) {
acc.push(el);
});
} else if (s instanceof Element) {
acc.push(s);
}
return acc;
}, []);
// `samples` counts sub-intervals of the shutter window, so there is one copy at
// every interval boundary — both ends of the window included — and each carries
// 1/samples of the source. That is what the reference export shows: the outermost
// copy sits exactly one frame away from the frame time, not half a step short.
var copies = samples + 1;
var alpha = String(1 / samples);
// A copy is styled by nothing that selected the original: dropping the id also
// drops every `#id` rule that gave it size, colour and font. So each copy carries
// its own resolved style inline. Chrome returns "" for a computed style's cssText,
// so the declarations are enumerated rather than taken wholesale.
function resolvedStyles(source, out) {
var cs = getComputedStyle(source);
var text = "";
for (var i = 0; i < cs.length; i++) {
text += cs[i] + ":" + cs.getPropertyValue(cs[i]) + ";";
}
// A copy is a still of one instant. Left live, an inherited transition or
// keyframe animation would carry it somewhere the timeline never sampled.
out.push(text + "transition:none;animation:none;");
for (var c = 0; c < source.children.length; c++) {
resolvedStyles(source.children[c], out);
}
return out;
}
function paintResolvedStyles(copy, styles, cursor) {
copy.style.cssText = styles[cursor.i++];
for (var c = 0; c < copy.children.length; c++) {
paintResolvedStyles(copy.children[c], styles, cursor);
}
}
// Perspective applies to a parent's children only, and a copy is a grandchild.
// Sharing the parent's 3D context would need preserve-3d on the group, which
// mix-blend-mode flattens, so each copy carries the perspective itself. See the
// header for what that does to the vanishing point.
function parentPerspective(el) {
var value = el.parentNode ? getComputedStyle(el.parentNode).perspective : "none";
return value && value !== "none" ? "perspective(" + value + ") " : "";
}
var state = targets
.filter(function (el) {
// One group per element. A second call on the same element would stack a
// second set of copies over the first and double the ink, so the first
// call owns it.
if (window._hfMbAttached.has(el)) return false;
window._hfMbAttached.add(el);
return true;
})
.map(function (el) {
var group = document.createElement("div");
group.setAttribute("data-hf-motion-blur", "hf-mb-" + window._hfMbUid++);
group.style.cssText =
"position:absolute;left:0;top:0;width:0;height:0;isolation:isolate;pointer-events:none;display:none;";
var clones = [];
for (var k = 0; k < copies; k++) {
var clone = el.cloneNode(true);
clone.removeAttribute("id");
clones.push(clone);
group.appendChild(clone);
}
el.parentNode.insertBefore(group, el);
var s = { el: el, group: group, clones: clones, samples: [], perspective: "" };
// Resolved styles and the parent's perspective are px once read, so a
// container-relative element needs them again when its box changes. Per frame
// that would walk the whole subtree once per copy; on resize it costs nothing
// on a fixed-size render and keeps a live preview honest.
function snapshot() {
var styles = resolvedStyles(el, []);
s.perspective = parentPerspective(el);
for (var c = 0; c < clones.length; c++) {
paintResolvedStyles(clones[c], styles, { i: 0 });
clones[c].style.position = "absolute";
clones[c].style.margin = "0";
clones[c].style.opacity = alpha;
clones[c].style.mixBlendMode = "plus-lighter";
}
}
snapshot();
if (typeof ResizeObserver === "function") new ResizeObserver(snapshot).observe(el);
return s;
});
function readTransform(el) {
var cs = getComputedStyle(el);
return {
css: cs.transform,
origin: cs.transformOrigin,
opacity: cs.opacity,
parsed: parseTransform(cs.transform),
};
}
var scheduled = false;
function applyShutter() {
var shutterTime = shutterAngle / 360 / fps;
if (!(shutterTime > 0)) {
state.forEach(function (s) {
s.group.style.display = "none";
});
return;
}
var t0 = tl.time();
var duration = tl.duration();
var windowStart = t0 + shutterPhase / 360 / fps;
// Sample the real trajectory: seek to each sub-frame time with events
// suppressed (so no tween callbacks, including this tracker's, fire),
// read the resolved transform, then restore the frame time.
try {
for (var k = 0; k < copies; k++) {
var tk = windowStart + (k / samples) * shutterTime;
tl.time(Math.min(duration, Math.max(0, tk)), true);
state.forEach(function (s) {
s.samples[k] = readTransform(s.el);
});
}
} finally {
tl.time(t0, true);
}
state.forEach(function (s) {
var current = readTransform(s.el);
var moved = false;
for (var i = 0; i < copies; i++) {
if (differs(s.samples[i].parsed, current.parsed)) {
moved = true;
break;
}
}
if (!moved) {
s.group.style.display = "none";
return;
}
// The copies are absolutely positioned inside the group, and the group sits at
// the origin of the element's own containing block, so the element's offset box
// places them. Re-read every frame: a beat is free to move the box itself.
var left = s.el.offsetLeft + "px";
var top = s.el.offsetTop + "px";
var width = s.el.offsetWidth + "px";
var height = s.el.offsetHeight + "px";
for (var j = 0; j < copies; j++) {
var clone = s.clones[j];
clone.style.left = left;
clone.style.top = top;
clone.style.width = width;
clone.style.height = height;
clone.style.transformOrigin = s.samples[j].origin;
clone.style.transform =
s.perspective + (s.samples[j].css === "none" ? "" : s.samples[j].css);
}
// The copies are the element's own ink, so they have to carry its opacity too:
// a beat that moves and fades at once would otherwise leave a full-strength
// smear behind a vanishing element. It rides the group, not the copies, whose
// own opacity is the 1/N shutter weight.
s.group.style.opacity = current.opacity;
s.group.style.display = "";
});
}
// tl.eventCallback("onUpdate") is unavailable under the HyperFrames runtime proxy, so a
// tracker tween's onUpdate fires on every seek. Sampling is deferred to a microtask:
// seeking GSAP from inside its own render reads stale tween state, while the microtask
// runs after the seek returns and before the frame is captured or painted.
var _proxy = { t: 0 };
tl.to(
_proxy,
{
t: 1,
duration: Math.max(tl.duration(), 0.1),
ease: "none",
onUpdate: function () {
if (scheduled) return;
scheduled = true;
Promise.resolve().then(function () {
scheduled = false;
applyShutter();
});
},
},
0,
);
};
})();
</script>
<!--
Timeline integration example:
const tl = gsap.timeline({ paused: true });
tl.fromTo("#my-box", { x: -100 }, { x: 1700, duration: 1.2, ease: "power3.inOut" }, 0.5);
// Extend to data-duration so seeks past the last tween reach the blur callback.
tl.set(document.body, {}, DATA_DURATION);
// Call AFTER tweens, BEFORE window.__timelines registration.
// attachMotionBlur adds a tracking tween with onUpdate — must be called after
// tl.set()/tl.to() have established the final timeline duration.
attachMotionBlur("#my-box", tl, { shutterAngle: 720, samplesPerFrame: 16 });
window.__timelines = window.__timelines || {};
window.__timelines["my-composition"] = tl;
-->
