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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};
}
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