decodeFrame method

@override Image decodeFrame(int frame)

Decode a single frame from the data stat was set with startDecode. If frame is out of the range of available frames, null is returned. Non animated image files will only have frame 0. An AnimationFrame is returned, which provides the image, and top-left coordinates of the image, as animated frames may only occupy a subset of the canvas.

Implementation

@override
Image decodeFrame(int frame) {
  if (info == null) {
    return Image.empty();
  }

  final inf = info!;

  _input.offset = inf.header.imageOffset;

  final bpp = inf.bitsPerPixel;
  final rowStride = ((inf.width * bpp + 31) ~/ 32) * 4;
  final nc = forceRgba
      ? 4
      : bpp == 1 || bpp == 4 || bpp == 8
          ? 1
          : bpp == 32
              ? 4
              : 3;
  final format = forceRgba
      ? Format.uint8
      : bpp == 1
          ? Format.uint1
          : bpp == 2
              ? Format.uint2
              : bpp == 4
                  ? Format.uint4
                  : bpp == 8
                      ? Format.uint8
                      // BMP allows > 4 bit per channel for 16bpp, so we have
                      // to scale it up to 8-bit
                      : bpp == 16
                          ? Format.uint8
                          : bpp == 24
                              ? Format.uint8
                              : bpp == 32
                                  ? Format.uint8
                                  : Format.uint8;
  final palette = forceRgba ? null : inf.palette;

  final image = Image(
      width: inf.width,
      height: inf.height,
      format: format,
      numChannels: nc,
      palette: palette);

  for (var y = image.height - 1; y >= 0; --y) {
    final line = inf.readBottomUp ? y : image.height - 1 - y;
    final row = _input.readBytes(rowStride);
    final w = image.width;
    var x = 0;
    final p = image.getPixel(0, line);
    while (x < w) {
      inf.decodePixel(row, (r, g, b, a) {
        if (x < w) {
          if (forceRgba && inf.palette != null) {
            final pi = r as int;
            final pr = inf.palette!.getRed(pi);
            final pg = inf.palette!.getGreen(pi);
            final pb = inf.palette!.getBlue(pi);
            final pa = inf.palette!.getAlpha(pi);
            p.setRgba(pr, pg, pb, pa);
          } else {
            p.setRgba(r, g, b, a);
          }
          p.moveNext();
          x++;
        }
      });
    }
  }

  return image;
}