/// <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"></param>
/// <param name="previousScanline"></param>
/// <param name="bytesPerPixel"></param>
/// <returns></returns>
private byte[] GetOptimalFilteredScanline(byte[] rawScanline, byte[] previousScanline, int bytesPerPixel)
{
var candidates = new List <Tuple <byte[], int> >();
var sub = SubFilter.Encode(rawScanline, bytesPerPixel);
candidates.Add(new Tuple <byte[], int>(sub, CalculateTotalVariation(sub)));
var up = UpFilter.Encode(rawScanline, previousScanline);
candidates.Add(new Tuple <byte[], int>(up, CalculateTotalVariation(up)));
var average = AverageFilter.Encode(rawScanline, previousScanline, bytesPerPixel);
candidates.Add(new Tuple <byte[], int>(average, CalculateTotalVariation(average)));
var paeth = PaethFilter.Encode(rawScanline, previousScanline, bytesPerPixel);
candidates.Add(new Tuple <byte[], int>(paeth, CalculateTotalVariation(paeth)));
int lowestTotalVariation = Int32.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);
}
private void DecodePixelData(byte[][] pixelData)
{
data = new Color[width * height];
byte[] previousScanline = new byte[bytesPerScanline];
for (int y = 0; y < height; y++)
{
var scanline = pixelData[y];
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 Exception("Unknown filter type.");
}
previousScanline = defilteredScanline;
ProcessDefilteredScanline(defilteredScanline, y);
}
}
