48 lines
1.5 KiB
JavaScript
48 lines
1.5 KiB
JavaScript
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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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