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forgejo/web_src/js/utils/image.js
silverwind b3f2447bc4
Downscale pasted PNG images based on metadata (#29123)
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">

(cherry picked from commit 5e72526da4e915791f03af056890e16821bde052)
2024-02-19 22:58:33 +01:00

47 lines
1.5 KiB
JavaScript

export async function pngChunks(blob) {
const uint8arr = new Uint8Array(await blob.arrayBuffer());
const chunks = [];
if (uint8arr.length < 12) return chunks;
const view = new DataView(uint8arr.buffer);
if (view.getBigUint64(0) !== 9894494448401390090n) return chunks;
const decoder = new TextDecoder();
let index = 8;
while (index < uint8arr.length) {
const len = view.getUint32(index);
chunks.push({
name: decoder.decode(uint8arr.slice(index + 4, index + 8)),
data: uint8arr.slice(index + 8, index + 8 + len),
});
index += len + 12;
}
return chunks;
}
// decode a image and try to obtain width and dppx. If will never throw but instead
// return default values.
export async function imageInfo(blob) {
let width = 0; // 0 means no width could be determined
let dppx = 1; // 1 dot per pixel for non-HiDPI screens
if (blob.type === 'image/png') { // only png is supported currently
try {
for (const {name, data} of await pngChunks(blob)) {
const view = new DataView(data.buffer);
if (name === 'IHDR' && data?.length) {
// extract width from mandatory IHDR chunk
width = view.getUint32(0);
} else if (name === 'pHYs' && data?.length) {
// extract dppx from optional pHYs chunk, assuming pixels are square
const unit = view.getUint8(8);
if (unit === 1) {
dppx = Math.round(view.getUint32(0) / 39.3701) / 72; // meter to inch to dppx
}
}
}
} catch {}
}
return {width, dppx};
}