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perf(highway): draw the held-sustain glow without ctx.shadowBlur (#729)

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shadowBlur cost scales with the blurred device-pixel area. The lit-sustain
trail can span half the canvas, the canvas is DPR-scaled (4x pixels on a
2x Mac), and the blur ran on every frame exactly while a sustain is HELD
— i.e. at the moment the player most notices a hitch. Sustain-heavy songs
(e.g. fingerpicked acoustic charts) hit this constantly.

Replace the blur with three inflated low-alpha fills of the same trail
quad: reads as the same soft shimmering glow (the shimmer LUT still
drives per-frame flicker, feedBack#254 intent preserved) at a flat,
area-independent cost. Also drops the now-dead shadowBlur reset in the
crackle pass; no shadowBlur uses remain in highway.js.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Byron Gamatos <xasiklas@gmail.com>
This commit is contained in:
Jorge Fritis
2026-07-06 09:49:06 +02:00
committed by GitHub
co-authored by Claude Fable 5 Byron Gamatos
parent 612b1f2e0d
commit 1f621e5fe5
1 changed files with 24 additions and 10 deletions
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static/highway.js
+24 -10
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@@ -2004,18 +2004,32 @@ function createHighway() {
const seedBase = (_frameIdx + n.s + ((n.t * 60) | 0)) | 0;
ctx.save();
ctx.fillStyle = col;
ctx.shadowColor = col;
ctx.shadowBlur = (8 + 6 * _shimmerNoise(seedBase)) * a; // shimmering glow
ctx.globalAlpha = (0.45 + 0.45 * a) * (0.78 + 0.22 * _shimmerNoise(seedBase + 17));
ctx.beginPath();
ctx.moveTo(x0 - sw0, y0);
ctx.lineTo(x0 + sw0, y0);
ctx.lineTo(x1 + sw1, y1);
ctx.lineTo(x1 - sw1, y1);
ctx.fill();
// Shimmering glow WITHOUT ctx.shadowBlur: blur cost scales with
// the blurred DEVICE-pixel area, and a held sustain's trail can
// span half the (DPR-scaled) canvas — profiling the "stutters
// while playing" report put this per-frame blur pass at the top
// exactly while a sustain is held. Three inflated low-alpha
// fills of the same quad read as the same soft glow at a flat,
// area-independent cost. The shimmer LUT still drives the
// per-frame size/brightness flicker (feedBack#254 intent).
const glowPx = (8 + 6 * _shimmerNoise(seedBase)) * a;
const baseA = (0.45 + 0.45 * a) * (0.78 + 0.22 * _shimmerNoise(seedBase + 17));
const fillTrail = (inflate) => {
ctx.beginPath();
ctx.moveTo(x0 - sw0 - inflate, y0);
ctx.lineTo(x0 + sw0 + inflate, y0);
ctx.lineTo(x1 + sw1 + inflate, y1);
ctx.lineTo(x1 - sw1 - inflate, y1);
ctx.fill();
};
ctx.globalAlpha = baseA * 0.22;
fillTrail(glowPx);
ctx.globalAlpha = baseA * 0.4;
fillTrail(glowPx * 0.45);
ctx.globalAlpha = baseA;
fillTrail(0);
// Crackling "current" — a jittery white core line down
// the trail, re-randomised each frame.
ctx.shadowBlur = 0;
ctx.globalCompositeOperation = 'lighter';
ctx.globalAlpha = a * (0.55 + 0.45 * _shimmerNoise(seedBase + 31));
ctx.strokeStyle = '#ffffff';