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Merged
matmen merged 1 commit into from
Oct 16, 2026
Merged

faster PNG decoding and encoding #64

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2 changes: 2 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,8 @@
strings are written without bounds checks unless they run past the image.
- SVG rendering is about 15-20% faster, with identical results: groups with opacity, clip paths, masks and nested
viewports are composited only where they painted.
- PNG decoding is about 25% faster and encoding about 10% faster, with identical results: the paeth filter is
computed without branches, 8-bit RGB images are converted without per-sample function calls.

### Deprecated

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59 changes: 44 additions & 15 deletions src/codecs/png.ts
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Expand Up @@ -125,7 +125,9 @@ function filter(data: Uint8Array, rowBytes: number, height: number, bpp: number)
for (let i = bpp; i < rowBytes; i++) {
const a = data[row + i - bpp], b = data[row + i - rowBytes], c = data[row + i - rowBytes - bpp];
const pa = Math.abs(b - c), pb = Math.abs(a - c), pc = Math.abs(a + b - 2 * c);
out[o + i] = data[row + i] - (pa <= pb && pa <= pc ? a : pb <= pc ? b : c);
// the predictor without branches, see unfilter
const notA = (pb - pa | pc - pa) >> 31, takeC = (pc - pb) >> 31;
out[o + i] = data[row + i] - (a ^ (a ^ b ^ (b ^ c) & takeC) & notA);
}
}

Expand Down Expand Up @@ -191,7 +193,7 @@ function pixelFormat(images: Uint8Array[]): PixelFormat {

// collect up to 257 distinct colors, runs of equal pixels are skipped cheaply
let colors: Map<number, number> | null = new Map();
for (const rgba of images) {
scan: for (const rgba of images) {
const pixels = new Uint32Array(rgba.buffer, rgba.byteOffset, rgba.length >> 2);
let previous = -1;
for (let i = 0; i < pixels.length; i++) {
Expand All @@ -206,6 +208,8 @@ function pixelFormat(images: Uint8Array[]): PixelFormat {
if (colors.size === 256) colors = null;
else colors.set(value, colors.size);
}
// nothing left to find out: RGBA it is
if (!colors && !opaque && !grey) break scan;
}
}

Expand Down Expand Up @@ -253,16 +257,18 @@ function pixelFormat(images: Uint8Array[]): PixelFormat {
if (channels === 4) return rgba;
const count = width * height;
const data = new Uint8Array(count * channels);
for (let i = 0, o = 0; i < count; i++) {
const s = i * 4;
if (grey) data[o++] = rgba[s];
else {
data[o++] = rgba[s];
data[o++] = rgba[s + 1];
data[o++] = rgba[s + 2];
if (channels === 3) {
for (let s = 0, o = 0; o < data.length; s += 4, o += 3) {
data[o] = rgba[s];
data[o + 1] = rgba[s + 1];
data[o + 2] = rgba[s + 2];
}
if (!opaque) data[o++] = rgba[s + 3];
}
} else if (channels === 2) {
for (let s = 0, o = 0; o < data.length; s += 4, o += 2) {
data[o] = rgba[s];
data[o + 1] = rgba[s + 3];
}
} else for (let s = 0, o = 0; o < data.length; s += 4, o++) data[o] = rgba[s];
return data;
}
};
Expand Down Expand Up @@ -747,7 +753,9 @@ function unfilter(data: Uint8Array, offset: number, height: number, rowBytes: nu
for (let i = row + bpp; i < row + rowBytes; i++) {
const a = out[i - bpp], b = out[i - rowBytes], c = out[i - rowBytes - bpp];
const pa = Math.abs(b - c), pb = Math.abs(a - c), pc = Math.abs(a + b - 2 * c);
out[i] = data[src + i] + (pa <= pb && pa <= pc ? a : pb <= pc ? b : c);
// without branches (photos make them unpredictable): all ones when a loses to b or c, when c beats b
const notA = (pb - pa | pc - pa) >> 31, takeC = (pc - pb) >> 31;
out[i] = data[src + i] + (a ^ (a ^ b ^ (b ^ c) & takeC) & notA);
}
}
} else throw new Error(`PNG: invalid filter type ${filter}`);
Expand Down Expand Up @@ -824,6 +832,22 @@ function createConverter(colorType: number, depth: number, palette: Uint8Array |
? [transparency[0] << 8 | transparency[1], transparency[2] << 8 | transparency[3], transparency[4] << 8 | transparency[5]].map(v => v & maxValue)
: null;

// 8-bit samples are bytes that need no scaling, read directly (the most common kind of opaque PNG)
if (depth === 8) {
const [kr, kg, kb] = key || [-1, -1, -1];
return (pixels, rowBytes, w, h, out, place) => {
for (let y = 0; y < h; y++) {
for (let x = 0, s = y * rowBytes, o = place.row(y), step = place.step; x < w; x++, s += 3, o += step) {
const r = pixels[s], g = pixels[s + 1], b = pixels[s + 2];
out[o] = r;
out[o + 1] = g;
out[o + 2] = b;
out[o + 3] = r === kr && g === kg && b === kb ? 0 : 0xff;
}
}
};
}

return (pixels, rowBytes, w, h, out, place) => {
for (let y = 0; y < h; y++) {
const row = pixels.subarray(y * rowBytes, (y + 1) * rowBytes);
Expand Down Expand Up @@ -861,9 +885,14 @@ function createConverter(colorType: number, depth: number, palette: Uint8Array |
// non-interlaced 8-bit RGBA maps 1:1 onto the output
if (place.step === 4 && w * 4 === rowBytes && place.row(0) === 0 && place.row(1) === rowBytes) return out.set(pixels);

for (let y = 0; y < h; y++)
for (let x = 0, o = place.row(y); x < w; x++, o += place.step)
out.set(pixels.subarray(y * rowBytes + x * 4, y * rowBytes + x * 4 + 4), o);
for (let y = 0; y < h; y++) {
for (let x = 0, s = y * rowBytes, o = place.row(y), step = place.step; x < w; x++, s += 4, o += step) {
out[o] = pixels[s];
out[o + 1] = pixels[s + 1];
out[o + 2] = pixels[s + 2];
out[o + 3] = pixels[s + 3];
}
}
};
}

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