/// <inheritdoc/>
protected override void Decompress(BufferedReadStream stream, int byteCount, int stripHeight, Span <byte> buffer)
{
int height = stripHeight;
using System.Buffers.IMemoryOwner <byte> scanLineBuffer = this.Allocator.Allocate <byte>(this.width * 2);
Span <byte> scanLine = scanLineBuffer.GetSpan().Slice(0, this.width);
Span <byte> referenceScanLineSpan = scanLineBuffer.GetSpan().Slice(this.width, this.width);
using var bitReader = new T6BitReader(stream, this.FillOrder, byteCount, this.Allocator);
var referenceScanLine = new CcittReferenceScanline(this.isWhiteZero, this.width);
uint bitsWritten = 0;
for (int y = 0; y < height; y++)
{
scanLine.Fill(0);
Decode2DScanline(bitReader, this.isWhiteZero, referenceScanLine, scanLine);
bitsWritten = this.WriteScanLine(buffer, scanLine, bitsWritten);
scanLine.CopyTo(referenceScanLineSpan);
referenceScanLine = new CcittReferenceScanline(this.isWhiteZero, referenceScanLineSpan);
}
}
private static void Decode2DScanline(T6BitReader bitReader, bool whiteIsZero, CcittReferenceScanline referenceScanline, Span <byte> scanline)
{
int width = scanline.Length;
bitReader.StartNewRow();
// 2D Encoding variables.
int a0 = -1;
byte fillByte = whiteIsZero ? (byte)0 : (byte)255;
// Process every code word in this scanline.
int unpacked = 0;
while (true)
{
// Read next code word and advance pass it.
bool isEol = bitReader.ReadNextCodeWord();
// Special case handling for EOL.
if (isEol)
{
// If a TIFF reader encounters EOFB before the expected number of lines has been extracted,
// it is appropriate to assume that the missing rows consist entirely of white pixels.
scanline.Fill(whiteIsZero ? (byte)0 : (byte)255);
break;
}
// Update 2D Encoding variables.
int b1 = referenceScanline.FindB1(a0, fillByte);
// Switch on the code word.
int a1;
switch (bitReader.Code.Type)
{
case CcittTwoDimensionalCodeType.None:
TiffThrowHelper.ThrowImageFormatException("ccitt compression parsing error, could not read a valid code word.");
break;
case CcittTwoDimensionalCodeType.Pass:
int b2 = referenceScanline.FindB2(b1);
scanline.Slice(unpacked, b2 - unpacked).Fill(fillByte);
unpacked = b2;
a0 = b2;
break;
case CcittTwoDimensionalCodeType.Horizontal:
// Decode M(a0a1)
bitReader.ReadNextRun();
int runLength = (int)bitReader.RunLength;
if (runLength > (uint)(scanline.Length - unpacked))
{
TiffThrowHelper.ThrowImageFormatException("ccitt compression parsing error");
}
scanline.Slice(unpacked, runLength).Fill(fillByte);
unpacked += runLength;
fillByte = (byte)~fillByte;
// Decode M(a1a2)
bitReader.ReadNextRun();
runLength = (int)bitReader.RunLength;
if (runLength > (uint)(scanline.Length - unpacked))
{
TiffThrowHelper.ThrowImageFormatException("ccitt compression parsing error");
}
scanline.Slice(unpacked, runLength).Fill(fillByte);
unpacked += runLength;
fillByte = (byte)~fillByte;
// Prepare next a0
a0 = unpacked;
break;
case CcittTwoDimensionalCodeType.Vertical0:
a1 = b1;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalR1:
a1 = b1 + 1;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalR2:
a1 = b1 + 2;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalR3:
a1 = b1 + 3;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalL1:
a1 = b1 - 1;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalL2:
a1 = b1 - 2;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
case CcittTwoDimensionalCodeType.VerticalL3:
a1 = b1 - 3;
scanline.Slice(unpacked, a1 - unpacked).Fill(fillByte);
unpacked = a1;
a0 = a1;
fillByte = (byte)~fillByte;
bitReader.SwapColor();
break;
default:
throw new NotSupportedException("ccitt extensions are not supported.");
}
// This line is fully unpacked. Should exit and process next line.
if (unpacked == width)
{
break;
}
if (unpacked > width)
{
TiffThrowHelper.ThrowImageFormatException("ccitt compression parsing error, unpacked data > width");
}
}
}