private int CalculateScanlineStep(PngColorTypeInformation colorTypeInformation)
{
int scanlineStep = 1;
if (_header.BitDepth >= 8)
{
scanlineStep = (colorTypeInformation.ChannelsPerColor * _header.BitDepth) / 8;
}
return(scanlineStep);
}
private int CalculateScanlineLength(PngColorTypeInformation colorTypeInformation)
{
int scanlineLength = (_header.Width * _header.BitDepth * colorTypeInformation.ChannelsPerColor);
int amount = scanlineLength % 8;
if (amount != 0)
{
scanlineLength += 8 - amount;
}
return(scanlineLength / 8);
}
public Image Decode(Stream stream)
{
_stream = stream;
_stream.Seek(8, SeekOrigin.Current);
bool isEndChunckReached = false;
PngChunk currentChunk = null;
byte[] palette = null;
byte[] paletteAlpha = null;
using (MemoryStream dataStream = new MemoryStream())
{
while ((currentChunk = ReadChunk()) != null)
{
if (isEndChunckReached)
{
throw new ImageFormatException("Image does not end with end chunk.");
}
if (currentChunk.Type == PngChunkTypes.Header)
{
ReadHeaderChunk(currentChunk.Data);
ValidateHeader();
}
else if (currentChunk.Type == PngChunkTypes.Data)
{
dataStream.Write(currentChunk.Data, 0, currentChunk.Data.Length);
}
else if (currentChunk.Type == PngChunkTypes.Palette)
{
palette = currentChunk.Data;
}
else if (currentChunk.Type == PngChunkTypes.PaletteAlpha)
{
paletteAlpha = currentChunk.Data;
}
else if (currentChunk.Type == PngChunkTypes.End)
{
isEndChunckReached = true;
}
}
if (_header.Width > Image.MaxWidth || _header.Height > Image.MaxHeight)
{
throw new ArgumentOutOfRangeException(
$"The input png '{ _header.Width }x{ _header.Height }' is bigger then the max allowed size '{ Image.MaxWidth }x{ Image.MaxHeight }'");
}
byte[] pixels = new byte[_header.Width * _header.Height * 4];
PngColorTypeInformation colorTypeInformation = _colorTypes[_header.ColorType];
if (colorTypeInformation != null)
{
IColorReader colorReader = colorTypeInformation.CreateColorReader(palette, paletteAlpha);
ReadScanlines(dataStream, pixels, colorReader, colorTypeInformation);
}
return(new Image(_header.Width, _header.Height, pixels));
}
}
private void ReadScanlines(MemoryStream dataStream, byte[] pixels, IColorReader colorReader, PngColorTypeInformation colorTypeInformation)
{
dataStream.Position = 0;
int scanlineLength = CalculateScanlineLength(colorTypeInformation);
int scanlineStep = CalculateScanlineStep(colorTypeInformation);
byte[] lastScanline = new byte[scanlineLength];
byte[] currScanline = new byte[scanlineLength];
byte a = 0;
byte b = 0;
byte c = 0;
int row = 0, filter = 0, column = -1;
using (var compressedStream = new ZlibInflateStream(dataStream))
{
int readByte = 0;
while ((readByte = compressedStream.ReadByte()) >= 0)
{
if (column == -1)
{
filter = readByte;
column++;
}
else
{
currScanline[column] = (byte)readByte;
if (column >= scanlineStep)
{
a = currScanline[column - scanlineStep];
c = lastScanline[column - scanlineStep];
}
else
{
a = 0;
c = 0;
}
b = lastScanline[column];
if (filter == 1)
{
currScanline[column] = (byte)(currScanline[column] + a);
}
else if (filter == 2)
{
currScanline[column] = (byte)(currScanline[column] + b);
}
else if (filter == 3)
{
currScanline[column] = (byte)(currScanline[column] + (byte)((a + b) / 2));
}
else if (filter == 4)
{
currScanline[column] = (byte)(currScanline[column] + PaethPredicator(a, b, c));
}
column++;
if (column == scanlineLength)
{
colorReader.ReadScanline(currScanline, pixels, _header);
column = -1;
row++;
var tmp = currScanline;
currScanline = lastScanline;
lastScanline = tmp;
}
}
}
}
}
private void ReadScanlines(MemoryStream dataStream, byte[] pixels, IColorReader colorReader, PngColorTypeInformation colorTypeInformation)
{
dataStream.Position = 0;
int scanlineLength = CalculateScanlineLength(colorTypeInformation);
int scanlineStep = CalculateScanlineStep(colorTypeInformation);
byte[] lastScanline = new byte[scanlineLength];
byte[] currScanline = new byte[scanlineLength];
byte a = 0;
byte b = 0;
byte c = 0;
int row = 0, filter = 0, column = -1;
using (var compressedStream = new ZlibInflateStream(dataStream))
{
int readByte = 0;
while ((readByte = compressedStream.ReadByte()) >= 0)
{
if (column == -1)
{
filter = readByte;
column++;
}
else
{
currScanline[column] = (byte)readByte;
if (column >= scanlineStep)
{
a = currScanline[column - scanlineStep];
c = lastScanline[column - scanlineStep];
}
else
{
a = 0;
c = 0;
}
b = lastScanline[column];
if (filter == 1)
{
currScanline[column] = (byte)(currScanline[column] + a);
}
else if (filter == 2)
{
currScanline[column] = (byte)(currScanline[column] + b);
}
else if (filter == 3)
{
currScanline[column] = (byte)(currScanline[column] + (byte)((a + b) / 2));
}
else if (filter == 4)
{
currScanline[column] = (byte)(currScanline[column] + PaethPredicator(a, b, c));
}
column++;
if (column == scanlineLength)
{
colorReader.ReadScanline(currScanline, pixels, _header);
column = -1;
row++;
var tmp = currScanline;
currScanline = lastScanline;
lastScanline = tmp;
}
}
}
}
}
private int CalculateScanlineStep(PngColorTypeInformation colorTypeInformation)
{
int scanlineStep = 1;
if (_header.BitDepth >= 8)
{
scanlineStep = (colorTypeInformation.ChannelsPerColor * _header.BitDepth) / 8;
}
return scanlineStep;
}
private int CalculateScanlineLength(PngColorTypeInformation colorTypeInformation)
{
int scanlineLength = (_header.Width * _header.BitDepth * colorTypeInformation.ChannelsPerColor);
int amount = scanlineLength % 8;
if (amount != 0)
{
scanlineLength += 8 - amount;
}
return scanlineLength / 8;
}
