/// <summary>
/// Apply predictor decode
/// </summary>
/// <param name="InputBuffer">Input buffer</param>
/// <returns>Output buffer</returns>
internal byte[] PredictorDecode
(
byte[] InputBuffer
)
{
// test for /DecodeParams
PdfDictionary DecodeParms = Dictionary.FindValue("/DecodeParms").ToDictionary;
// none found
if (DecodeParms == null)
{
return(InputBuffer);
}
// look for predictor code. if default (none or 1) do nothing
if (!DecodeParms.FindValue("/Predictor").GetInteger(out int Predictor) || Predictor == 1)
{
return(InputBuffer);
}
// we only support predictor code 12
if (Predictor != 12)
{
return(null);
}
// get width
DecodeParms.FindValue("/Columns").GetInteger(out int Width);
if (Width < 0)
{
throw new ApplicationException("/DecodeParms /Columns is negative");
}
if (Width == 0)
{
Width = 1;
}
// calculate rows
int Rows = InputBuffer.Length / (Width + 1);
if (Rows < 1)
{
throw new ApplicationException("/DecodeParms /Columns is greater than stream length");
}
// create output buffer
byte[] OutputBuffer = new byte[Rows * Width];
// reset pointers
int InPtr = 1;
int OutPtr = 0;
int OutPrevPtr = 0;
// first row (ignore filter)
while (OutPtr < Width)
{
OutputBuffer[OutPtr++] = InputBuffer[InPtr++];
}
// decode loop
for (int Row = 1; Row < Rows; Row++)
{
// first byte is filter
int Filter = InputBuffer[InPtr++];
// we support PNG filter up only
if (Filter != 2)
{
throw new ApplicationException("/DecodeParms Only supported filter is 2");
}
// convert input to output
for (int Index = 0; Index < Width; Index++)
{
OutputBuffer[OutPtr++] = (byte)(OutputBuffer[OutPrevPtr++] + InputBuffer[InPtr++]);
}
}
return(OutputBuffer);
}