Function makePngWithFilter

packages/engine/src/utils/alphaBlit.test.ts:186–257

* Build a PNG with a specific filter type applied to every row. Encodes a * 3×2 RGBA image with unique per-channel values so any cross-channel mistake * in the defilter loop shows up as an assertion failure. * * @param filterType 0=None, 1=Sub, 2=Up, 3=Average, 4=Paeth

(filterType: 0 | 1 | 2 | 3 | 4)

 * @param filterType  0=None, 1=Sub, 2=Up, 3=Average, 4=Paeth
 */
function makePngWithFilter(filterType: 0 | 1 | 2 | 3 | 4): {
  png: Buffer;
  expectedPixels: number[];
} {
  const width = 3;
  const height = 2;
  const bpp = 4; // RGBA, 8-bit
  const stride = width * bpp;

  // Unique pixels so any defilter bug is observable
  const expectedPixels = [
    10, 20, 30, 255, 50, 60, 70, 255, 90, 100, 110, 255, 130, 140, 150, 255, 170, 180, 190, 255,
    210, 220, 230, 255,
  ];

  const filtered: number[] = [];
  const prev = new Uint8Array(stride);
  for (let y = 0; y < height; y++) {
    filtered.push(filterType);
    const rowStart = y * stride;
    const curr = new Uint8Array(stride);
    for (let x = 0; x < stride; x++) curr[x] = expectedPixels[rowStart + x] ?? 0;

    const out = new Uint8Array(stride);
    for (let x = 0; x < stride; x++) {
      const a = x >= bpp ? (curr[x - bpp] ?? 0) : 0;
      const b = prev[x] ?? 0;
      const c = x >= bpp ? (prev[x - bpp] ?? 0) : 0;
      const cv = curr[x] ?? 0;
      switch (filterType) {
        case 0:
          out[x] = cv;
          break;
        case 1:
          out[x] = (cv - a) & 0xff;
          break;
        case 2:
          out[x] = (cv - b) & 0xff;
          break;
        case 3:
          out[x] = (cv - Math.floor((a + b) / 2)) & 0xff;
          break;
        case 4:
          out[x] = (cv - paethRef(a, b, c)) & 0xff;
          break;
      }
    }
    for (let x = 0; x < stride; x++) filtered.push(out[x] ?? 0);
    prev.set(curr);
  }

  const ihdr = Buffer.allocUnsafe(13);
  ihdr.writeUInt32BE(width, 0);
  ihdr.writeUInt32BE(height, 4);
  ihdr[8] = 8;
  ihdr[9] = 6;
  ihdr[10] = 0;
  ihdr[11] = 0;