/// <inheritdoc />
public void Compress(TiffCompressionContext context, ReadOnlyMemory <byte> input, IBufferWriter <byte> outputWriter)
{
if (context is null)
{
throw new ArgumentNullException(nameof(context));
}
if (context.PhotometricInterpretation != TiffPhotometricInterpretation.WhiteIsZero && context.PhotometricInterpretation != TiffPhotometricInterpretation.BlackIsZero)
{
throw new NotSupportedException("Modified Huffman compression does not support this photometric interpretation.");
}
if (context.BitsPerSample.Count != 1 || context.BitsPerSample[0] != 8)
{
throw new NotSupportedException("Unsupported bits per sample.");
}
context.BitsPerSample = TiffValueCollection.Single <ushort>(1);
ReadOnlySpan <byte> inputSpan = input.Span;
int width = context.ImageSize.Width;
int height = context.ImageSize.Height;
var bitWriter = new BitWriter2(outputWriter, 4096);
// Process every scanline
for (int row = 0; row < height; row++)
{
ReadOnlySpan <byte> rowSpan = inputSpan.Slice(0, width);
inputSpan = inputSpan.Slice(width);
CcittEncodingTable currentTable = CcittEncodingTable.WhiteInstance;
CcittEncodingTable otherTable = CcittEncodingTable.BlackInstance;
// ModifiedHuffman compression assumes WhiteIsZero photometric interpretation is used.
// Since the first run is white run, we look for black pixel in the first iteration.
byte nextRunPixel = 255;
while (!rowSpan.IsEmpty)
{
// Get the length of the current run
int runLength = rowSpan.IndexOf(nextRunPixel);
if (runLength < 0)
{
runLength = rowSpan.Length;
}
currentTable.EncodeRun(ref bitWriter, runLength);
rowSpan = rowSpan.Slice(runLength);
// Switch to the other color
CcittHelper.SwapTable(ref currentTable, ref otherTable);
nextRunPixel = (byte)~nextRunPixel;
}
bitWriter.AdvanceAlignByte();
}
bitWriter.Flush();
}
private static void Encode2DScanline(ref BitWriter2 bitWriter, ReferenceScanline referenceScanline, ReadOnlySpan <byte> scanline)
{
int width = scanline.Length;
CcittEncodingTable currentTable = CcittEncodingTable.WhiteInstance;
CcittEncodingTable otherTable = CcittEncodingTable.BlackInstance;
CodingScanline codingScanline = new CodingScanline(whiteIsZero: true, scanline);
byte a0Byte = 0;
int a0 = -1;
while (true)
{
int a1 = codingScanline.FindA1(a0, a0Byte);
int b1 = referenceScanline.FindB1(a0, a0Byte);
int b2 = referenceScanline.FindB2(b1);
if (b2 < a1)
{
EncodePassCode(ref bitWriter);
a0 = b2;
}
else
{
int a1b1 = b1 - a1;
if (a1b1 >= -3 && a1b1 <= 3)
{
// Vertical mode coding
switch (a1b1)
{
case 3:
EncodeVerticalL3Code(ref bitWriter);
break;
case 2:
EncodeVerticalL2Code(ref bitWriter);
break;
case 1:
EncodeVerticalL1Code(ref bitWriter);
break;
case 0:
EncodeVertical0Code(ref bitWriter);
break;
case -1:
EncodeVerticalR1Code(ref bitWriter);
break;
case -2:
EncodeVerticalR2Code(ref bitWriter);
break;
case -3:
EncodeVerticalR3Code(ref bitWriter);
break;
default:
break;
}
CcittHelper.SwapTable(ref currentTable, ref otherTable);
a0 = a1;
}
else
{
int a2 = codingScanline.FindA2(a1);
EncodeHorizontalCode(ref bitWriter);
currentTable.EncodeRun(ref bitWriter, a1 - a0);
otherTable.EncodeRun(ref bitWriter, a2 - a1);
a0 = a2;
}
}
if (a0 >= width)
{
break;
}
a0Byte = scanline[a0];
}
}
