/// <summary>
/// Applies all PNG filters to the given scanline and returns the filtered scanline that is deemed
/// to be most compressible, using lowest total variation as proxy for compressibility.
/// </summary>
/// <param name="rawScanline">The raw scanline</param>
/// <param name="previousScanline">The previous scanline</param>
/// <param name="byteCount">The number of bytes per pixel</param>
/// <returns></returns>
private byte[] GetOptimalFilteredScanline(byte[] rawScanline, byte[] previousScanline, int byteCount)
{
List <Tuple <byte[], int> > candidates = new List <Tuple <byte[], int> >();
byte[] sub = SubFilter.Encode(rawScanline, byteCount);
candidates.Add(new Tuple <byte[], int>(sub, CalculateTotalVariation(sub)));
byte[] up = UpFilter.Encode(rawScanline, previousScanline);
candidates.Add(new Tuple <byte[], int>(up, CalculateTotalVariation(up)));
byte[] average = AverageFilter.Encode(rawScanline, previousScanline, byteCount);
candidates.Add(new Tuple <byte[], int>(average, CalculateTotalVariation(average)));
byte[] paeth = PaethFilter.Encode(rawScanline, previousScanline, byteCount);
candidates.Add(new Tuple <byte[], int>(paeth, CalculateTotalVariation(paeth)));
int lowestTotalVariation = int.MaxValue;
int lowestTotalVariationIndex = 0;
for (int i = 0; i < candidates.Count; i++)
{
if (candidates[i].Item2 < lowestTotalVariation)
{
lowestTotalVariationIndex = i;
lowestTotalVariation = candidates[i].Item2;
}
}
return(candidates[lowestTotalVariationIndex].Item1);
}
/// <summary>
/// Decodes the raw pixel data row by row
/// </summary>
/// <typeparam name="T">The pixel format.</typeparam>
/// <typeparam name="TP">The packed format. <example>long, float.</example></typeparam>
/// <param name="pixelData">The pixel data.</param>
/// <param name="pixels">The image pixels.</param>
private void DecodePixelData <T, TP>(byte[] pixelData, T[] pixels)
where T : IPackedVector <TP>
where TP : struct
{
byte[] previousScanline = new byte[this.bytesPerScanline];
for (int y = 0; y < this.header.Height; y++)
{
byte[] scanline = new byte[this.bytesPerScanline];
Array.Copy(pixelData, y * this.bytesPerScanline, scanline, 0, this.bytesPerScanline);
FilterType filterType = (FilterType)scanline[0];
byte[] defilteredScanline;
switch (filterType)
{
case FilterType.None:
defilteredScanline = NoneFilter.Decode(scanline);
break;
case FilterType.Sub:
defilteredScanline = SubFilter.Decode(scanline, bytesPerPixel);
break;
case FilterType.Up:
defilteredScanline = UpFilter.Decode(scanline, previousScanline);
break;
case FilterType.Average:
defilteredScanline = AverageFilter.Decode(scanline, previousScanline, bytesPerPixel);
break;
case FilterType.Paeth:
defilteredScanline = PaethFilter.Decode(scanline, previousScanline, bytesPerPixel);
break;
default:
throw new ImageFormatException("Unknown filter type.");
}
previousScanline = defilteredScanline;
ProcessDefilteredScanline <T, TP>(defilteredScanline, y, pixels);
}
}
