Function makePng16

packages/engine/src/utils/alphaBlit.test.ts:362–394

* Build a 16-bit RGB PNG (colorType 2, bitDepth 16). PNG stores each 16-bit * sample as two big-endian bytes; the decoder must swap them to LE. * * @param pixels Flat array of [r16, g16, b16, r16, g16, b16, ...] values * (one entry per channel sample, 0–65535).

(width: number, height: number, pixels: number[])

 *                (one entry per channel sample, 0–65535).
 */
function makePng16(width: number, height: number, pixels: number[]): Buffer {
  const ihdr = Buffer.allocUnsafe(13);
  ihdr.writeUInt32BE(width, 0);
  ihdr.writeUInt32BE(height, 4);
  ihdr[8] = 16; // bit depth
  ihdr[9] = 2; // color type RGB
  ihdr[10] = 0;
  ihdr[11] = 0;
  ihdr[12] = 0;

  const stride = width * 6; // 3 channels × 2 bytes
  const filtered: number[] = [];
  for (let y = 0; y < height; y++) {
    filtered.push(0); // filter type None
    for (let x = 0; x < width; x++) {
      const baseSample = (y * width + x) * 3;
      for (let ch = 0; ch < 3; ch++) {
        const v = pixels[baseSample + ch] ?? 0;
        filtered.push((v >> 8) & 0xff); // high byte (BE on wire)
        filtered.push(v & 0xff); // low byte
      }
    }
    void stride;
  }

  const idat = deflateSync(Buffer.from(filtered));
  return Buffer.concat([
    PNG_SIG,
    makeChunk("IHDR", ihdr),
    makeChunk("IDAT", idat),
    makeChunk("IEND", Buffer.alloc(0)),
  ]);
}

describe("decodePngToRgb48le", () => {
  it("swaps PNG big-endian samples to little-endian rgb48le", () => {