forgejo/web_src/js/utils/image.js

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};
}