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Some images like MacOS screenshots contain [pHYs](http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.8) data which we can use to downscale uploaded images so they render in the same dppx ratio in which they were taken. Before: <img width="584" alt="image" src="https://github.com/go-gitea/gitea/assets/115237/50979e3a-5d5a-40dc-a0a4-36eb6e28f14a"> After: <img width="329" alt="image" src="https://github.com/go-gitea/gitea/assets/115237/0690902a-f2fe-4c6b-97b3-6fdd67c21bad">
47 lines
1.5 KiB
JavaScript
47 lines
1.5 KiB
JavaScript
export async function pngChunks(blob) {
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const uint8arr = new Uint8Array(await blob.arrayBuffer());
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const chunks = [];
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if (uint8arr.length < 12) return chunks;
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const view = new DataView(uint8arr.buffer);
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if (view.getBigUint64(0) !== 9894494448401390090n) return chunks;
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const decoder = new TextDecoder();
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let index = 8;
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while (index < uint8arr.length) {
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const len = view.getUint32(index);
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chunks.push({
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name: decoder.decode(uint8arr.slice(index + 4, index + 8)),
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data: uint8arr.slice(index + 8, index + 8 + len),
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});
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index += len + 12;
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}
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return chunks;
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}
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// decode a image and try to obtain width and dppx. If will never throw but instead
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// return default values.
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export async function imageInfo(blob) {
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let width = 0; // 0 means no width could be determined
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let dppx = 1; // 1 dot per pixel for non-HiDPI screens
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if (blob.type === 'image/png') { // only png is supported currently
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try {
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for (const {name, data} of await pngChunks(blob)) {
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const view = new DataView(data.buffer);
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if (name === 'IHDR' && data?.length) {
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// extract width from mandatory IHDR chunk
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width = view.getUint32(0);
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} else if (name === 'pHYs' && data?.length) {
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// extract dppx from optional pHYs chunk, assuming pixels are square
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const unit = view.getUint8(8);
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if (unit === 1) {
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dppx = Math.round(view.getUint32(0) / 39.3701) / 72; // meter to inch to dppx
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}
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}
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}
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} catch {}
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}
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return {width, dppx};
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}
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