public override SKQuantizedFrame Quantize(ReadOnlySpan <SKColor> colors)
{
var pixels = new byte[3 * colors.Length];
for (int i = 0, k = 0; i < colors.Length; i++)
{
var c = colors[i];
pixels[k++] = c.Red;
pixels[k++] = c.Green;
pixels[k++] = c.Blue;
}
int len = pixels.Length;
int nPix = len / 3;
neuQuant = new NeuQuant(len, SamplingFactor);
var palette = neuQuant.Process(pixels);
var frame = new SKQuantizedFrame();
frame.ColorDepth = 8;
frame.PaletteSize = 7;
frame.IndexedPixels = new byte[nPix];
frame.Palette = palette;
for (int i = 0, k = 0; i < nPix; i++)
{
int index = neuQuant.Map(
pixels[k++] & 0xff,
pixels[k++] & 0xff,
pixels[k++] & 0xff);
frame.IndexedPixels[i] = (byte)index;
}
return(frame);
}
private void WriteImageDescriptor(SKQuantizedFrame quantizedFrame)
{
// image separator
stream.WriteByte(0x2c);
// x, y, w, h
WriteShort(0);
WriteShort(0);
WriteShort(frameSize.Width);
WriteShort(frameSize.Height);
// packed fields
if (firstFrame)
{
// no LCT - GCT is used for first (or only) frame
stream.WriteByte(0);
}
else
{
// specify normal LCT
stream.WriteByte(Convert.ToByte(
0x80 | // 1 local color table 1=yes
0 | // 2 interlace - 0=no
0 | // 3 sorted - 0=no
0 | // 4-5 reserved
quantizedFrame.PaletteSize)); // 6-8 size of color table
}
}
private void WritePalette(SKQuantizedFrame quantizedFrame)
{
stream.Write(quantizedFrame.Palette, 0, quantizedFrame.Palette.Length);
var n = (3 * 256) - quantizedFrame.Palette.Length;
for (var i = 0; i < n; i++)
{
stream.WriteByte(0);
}
}
private void WriteLogicalScreenDescriptor(SKQuantizedFrame quantizedFrame)
{
// logical screen size
WriteShort(frameSize.Width);
WriteShort(frameSize.Height);
// packed fields
stream.WriteByte(Convert.ToByte(
0x80 | // 1 : global color table flag = 1 (gct used)
0x70 | // 2-4 : color resolution = 7
0x00 | // 5 : gct sort flag = 0
quantizedFrame.PaletteSize)); // 6-8 : gct size
stream.WriteByte(0); // background color index
stream.WriteByte(0); // pixel aspect ratio - assume 1:1
}
// Write*
private void WriteGraphicControlExtension(SKQuantizedFrame quantizedFrame, SKGifEncoderFrameInfo frameInfo)
{
stream.WriteByte(0x21); // extension introducer
stream.WriteByte(0xf9); // GCE label
stream.WriteByte(4); // data block size
int transp, disp;
if (frameInfo.TransparentColor != SKColor.Empty)
{
transp = 1;
disp = 2;
}
else
{
transp = 0;
disp = 0;
}
if (frameInfo.DisposalMethod >= 0)
{
disp = ((int)frameInfo.DisposalMethod) & 7; // user override
}
disp <<= 2;
// packed fields
stream.WriteByte(Convert.ToByte(
0 | // 1:3 reserved
disp | // 4:6 disposal
0 | // 7 user input - 0 = none
transp)); // 8 transparency flag
WriteShort(frameInfo.Duration / 10); // delay x 1/100 sec
// transparent color index
if (frameInfo.TransparentColor != SKColor.Empty)
{
stream.WriteByte(quantizer.GetColorIndex(frameInfo.TransparentColor));
}
else
{
stream.WriteByte(0);
}
stream.WriteByte(0); // block terminator
}
private void WritePixels(SKQuantizedFrame quantizedFrame)
{
encoder.Encode(stream, quantizedFrame);
}
