Function decodePngRaw

packages/engine/src/utils/alphaBlit.ts:31–149

* Shared PNG chunk parsing + filter reconstruction. * * Verifies the PNG signature, iterates chunks to collect IHDR metadata and IDAT * payloads, decompresses with zlib, and reconstructs all 5 PNG filter types. * * Returns the defiltered pixel bytes (no filter-type prefix bytes) along with * I

(
  buf: Buffer,
  caller: string,
)

 * IHDR fields so callers can convert to their target pixel format.
 */
function decodePngRaw(
  buf: Buffer,
  caller: string,
): { width: number; height: number; bitDepth: number; colorType: number; rawPixels: Buffer } {
  // Verify PNG signature
  if (
    buf[0] !== 137 ||
    buf[1] !== 80 ||
    buf[2] !== 78 ||
    buf[3] !== 71 ||
    buf[4] !== 13 ||
    buf[5] !== 10 ||
    buf[6] !== 26 ||
    buf[7] !== 10
  ) {
    throw new Error(`${caller}: not a PNG file`);
  }

  let pos = 8;
  let width = 0;
  let height = 0;
  let bitDepth = 0;
  let colorType = 0;
  let interlace = 0;
  let sawIhdr = false;
  const idatChunks: Buffer[] = [];

  while (pos + 12 <= buf.length) {
    const chunkLen = buf.readUInt32BE(pos);
    const chunkType = buf.toString("ascii", pos + 4, pos + 8);
    const chunkData = buf.subarray(pos + 8, pos + 8 + chunkLen);

    if (chunkType === "IHDR") {
      width = chunkData.readUInt32BE(0);
      height = chunkData.readUInt32BE(4);
      bitDepth = chunkData[8] ?? 0;
      colorType = chunkData[9] ?? 0;
      interlace = chunkData[12] ?? 0;
      sawIhdr = true;
    } else if (chunkType === "IDAT") {
      idatChunks.push(Buffer.from(chunkData));
    } else if (chunkType === "IEND") {
      break;
    }

    pos += 12 + chunkLen; // length(4) + type(4) + data(chunkLen) + crc(4)
  }

  if (!sawIhdr) {
    throw new Error(`${caller}: PNG missing IHDR chunk`);
  }
  if (colorType !== 2 && colorType !== 6) {
    throw new Error(`${caller}: unsupported color type ${colorType} (expected 2=RGB or 6=RGBA)`);
  }
  if (interlace !== 0) {
    throw new Error(
      `${caller}: Adam7-interlaced PNGs are not supported (interlace method ${interlace})`,
    );