public void ExtractPalettedAlphaRows(int lastRow)
{
// For vertical and gradient filtering, we need to decode the part above the
// cropTop row, in order to have the correct spatial predictors.
int topRow = this.AlphaFilterType is WebpAlphaFilterType.None or WebpAlphaFilterType.Horizontal ? 0 : this.LastRow;
int firstRow = this.LastRow < topRow ? topRow : this.LastRow;
if (lastRow > firstRow)
{
// Special method for paletted alpha data.
Span <byte> output = this.Alpha.Memory.Span;
Span <uint> pixelData = this.Vp8LDec.Pixels.Memory.Span;
Span <byte> pixelDataAsBytes = MemoryMarshal.Cast <uint, byte>(pixelData);
Span <byte> dst = output.Slice(this.Width * firstRow);
Span <byte> input = pixelDataAsBytes.Slice(this.Vp8LDec.Width * firstRow);
if (this.Vp8LDec.Transforms.Count == 0 || this.Vp8LDec.Transforms[0].TransformType != Vp8LTransformType.ColorIndexingTransform)
{
WebpThrowHelper.ThrowImageFormatException("error while decoding alpha channel, expected color index transform data is missing");
}
Vp8LTransform transform = this.Vp8LDec.Transforms[0];
ColorIndexInverseTransformAlpha(transform, firstRow, lastRow, input, dst);
this.AlphaApplyFilter(firstRow, lastRow, dst, this.Width);
}
this.LastRow = lastRow;
}
private static void ColorIndexInverseTransformAlpha(
Vp8LTransform transform,
int yStart,
int yEnd,
Span <byte> src,
Span <byte> dst)
{
int bitsPerPixel = 8 >> transform.Bits;
int width = transform.XSize;
Span <uint> colorMap = transform.Data.Memory.Span;
if (bitsPerPixel < 8)
{
int srcOffset = 0;
int dstOffset = 0;
int pixelsPerByte = 1 << transform.Bits;
int countMask = pixelsPerByte - 1;
int bitMask = (1 << bitsPerPixel) - 1;
for (int y = yStart; y < yEnd; y++)
{
int packedPixels = 0;
for (int x = 0; x < width; x++)
{
if ((x & countMask) == 0)
{
packedPixels = src[srcOffset];
srcOffset++;
}
dst[dstOffset] = GetAlphaValue((int)colorMap[packedPixels & bitMask]);
dstOffset++;
packedPixels >>= bitsPerPixel;
}
}
}
else
{
MapAlpha(src, colorMap, dst, yStart, yEnd, width);
}
}
