Function decodePng

packages/engine/src/utils/alphaBlit.ts:159–182

(buf: Buffer)

 * Returns a Uint8Array of width*height*4 bytes in RGBA order.
 */
export function decodePng(buf: Buffer): { width: number; height: number; data: Uint8Array } {
  const { width, height, bitDepth, colorType, rawPixels } = decodePngRaw(buf, "decodePng");

  if (bitDepth !== 8) {
    throw new Error(`decodePng: unsupported bit depth ${bitDepth} (expected 8)`);
  }

  const output = new Uint8Array(width * height * 4);

  if (colorType === 6) {
    // RGBA — copy directly
    output.set(rawPixels);
  } else {
    // RGB → RGBA: set alpha to 255
    for (let i = 0; i < width * height; i++) {
      output[i * 4 + 0] = rawPixels[i * 3 + 0] ?? 0;
      output[i * 4 + 1] = rawPixels[i * 3 + 1] ?? 0;
      output[i * 4 + 2] = rawPixels[i * 3 + 2] ?? 0;
      output[i * 4 + 3] = 255;
    }
  }

  return { width, height, data: output };
}

// ── 16-bit PNG decoder ────────────────────────────────────────────────────────