Function encodePng

src/utils/ansiToPng.ts:307–334

* Encode an RGBA pixel buffer as PNG. Minimal encoder: 8-bit depth, * color type 6 (RGBA), filter 0 (none) on every scanline, single IDAT.

(px: Uint8Array, width: number, height: number)

 * color type 6 (RGBA), filter 0 (none) on every scanline, single IDAT.
 */
function encodePng(px: Uint8Array, width: number, height: number): Buffer {
  // IHDR
  const ihdr = Buffer.alloc(13)
  ihdr.writeUInt32BE(width, 0)
  ihdr.writeUInt32BE(height, 4)
  ihdr[8] = 8 // bit depth
  ihdr[9] = 6 // color type: RGBA
  ihdr[10] = 0 // compression: deflate
  ihdr[11] = 0 // filter method
  ihdr[12] = 0 // interlace: none

  // IDAT: each scanline prefixed with filter byte 0.
  const stride = width * 4
  const raw = Buffer.alloc(height * (stride + 1))
  for (let y = 0; y < height; y++) {
    const dst = y * (stride + 1)
    raw[dst] = 0
    raw.set(px.subarray(y * stride, (y + 1) * stride), dst + 1)
  }
  const idat = deflateSync(raw)

  return Buffer.concat([
    PNG_SIG,
    chunk('IHDR', ihdr),
    chunk('IDAT', idat),
    chunk('IEND', new Uint8Array(0)),
  ])
}