Sprite sheet export writes premultiplied RGB, so semi-transparent glows come out dark #4

Hi — thanks for the library, the effects are genuinely nice. I ran the full check.mjs suite (244/244 pass) and then hit something on the export path that looks like a real bug.

What happens

tools/lib/fxlib.mjs → spriteSheet() creates the sheet on a fully transparent canvas (canvas(cols*w, rows*h, [0,0,0,0])) and pastes each frame with canvas.paste():

const a = src[s + 3] / 255;
for (let c = 0; c < 3; c++) data[d + c] = src[s + c] * a + data[d + c] * (1 - a);
data[d + 3] = Math.max(data[d + 3], src[s + 3]);

That is an alpha-over blend, but the result is stored without dividing by the output alpha. With a transparent destination (dst = 0), the RGB written to the PNG is src.rgb * src.a — i.e. premultiplied — while the alpha channel is correct.

A consumer reading the PNG as straight (non-premultiplied) RGBA — Aseprite, Godot, Unity, Phaser's default — multiplies by alpha a second time when compositing, so soft edges come out darker than the source.

Repro

node tools/render.mjs fire/torch --sheet

Then crop frame 0 out of records/fire/torch/torch.png using the rects in torch.json, and compare it with the same frame rendered standalone (renderFrames(...).frames[0].clear).

Measured (fire/torch, frame 0, 72×120)

So it is a clean premultiply, not compression noise. Visually: the torch's glow is gone in the sheet frame and the smoke gets darker. In this effect 6,740 of 8,640 pixels (78%) are semi-transparent, so it affects most of the fire / lightning / holy / shadow categories.

Suggested fix

Normalize by the output alpha inside paste(). It leaves opaque destinations (the contact sheet, fill = [12,11,16,255]) exactly as they are today:

const a = src[s + 3] / 255, da = data[d + 3] / 255, oa = a + da * (1 - a);
if (oa > 0) {
  for (let c = 0; c < 3; c++) data[d + c] = (src[s + c] * a + data[d + c] * da * (1 - a)) / oa;
  data[d + 3] = oa * 255;
}

A narrower fix is to copy RGB straight when data[d + 3] === 0, but the version above also stays correct if anything is ever pasted onto a partly transparent canvas.

Environment

Windows 11, node v24.14.0, pixelfx 0.3.0 (main, 2026-11-03).

Happy to attach the side-by-side PNG (source frame vs. cropped sheet frame) if that helps.