@ -30,6 +30,12 @@ namespace SixLabors.ImageSharp.Formats.WebP
} }
} }
/// <summary>
/// If there are not many unique pixel values, it may be more efficient to create a color index array and replace the pixel values by the array's indices.
/// This will reverse the color index transform.
/// </summary>
/// <param name="transform">The transform data contains color table size and the entries in the color table.</param>
/// <param name="pixelData">The pixel data to apply the reverse transform on.</param>
public static void ColorIndexInverseTransform ( Vp8LTransform transform , Span < uint > pixelData ) public static void ColorIndexInverseTransform ( Vp8LTransform transform , Span < uint > pixelData )
{ {
int bitsPerPixel = 8 > > transform . Bits ; int bitsPerPixel = 8 > > transform . Bits ;
@ -80,6 +86,12 @@ namespace SixLabors.ImageSharp.Formats.WebP
} }
} }
/// <summary>
/// The goal of the color transform is to de-correlate the R, G and B values of each pixel.
/// Color transform keeps the green (G) value as it is, transforms red (R) based on green and transforms blue (B) based on green and then based on red.
/// </summary>
/// <param name="transform">The transform data.</param>
/// <param name="pixelData">The pixel data to apply the inverse transform on.</param>
public static void ColorSpaceInverseTransform ( Vp8LTransform transform , Span < uint > pixelData ) public static void ColorSpaceInverseTransform ( Vp8LTransform transform , Span < uint > pixelData )
{ {
int width = transform . XSize ; int width = transform . XSize ;
@ -124,6 +136,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
} }
} }
/// <summary>
/// Reverses the color space transform.
/// </summary>
/// <param name="m">The color transform element.</param>
/// <param name="pixelData">The pixel data to apply the inverse transform on.</param>
/// <param name="start">The start index of reverse transform.</param>
/// <param name="numPixels">The number of pixels to apply the transform.</param>
public static void TransformColorInverse ( Vp8LMultipliers m , Span < uint > pixelData , int start , int numPixels ) public static void TransformColorInverse ( Vp8LMultipliers m , Span < uint > pixelData , int start , int numPixels )
{ {
int end = start + numPixels ; int end = start + numPixels ;
@ -144,24 +163,14 @@ namespace SixLabors.ImageSharp.Formats.WebP
} }
} }
public static void ExpandColorMap ( int numColors , Span < uint > transformData , Span < uint > newColorMap ) /// <summary>
{ /// The predictor transform can be used to reduce entropy by exploiting the fact that neighboring pixels are often correlated.
newColorMap [ 0 ] = transformData [ 0 ] ; /// In the predictor transform, the current pixel value is predicted from the pixels already decoded (in scan-line order) and only the residual value (actual - predicted) is encoded.
Span < byte > data = MemoryMarshal . Cast < uint , byte > ( transformData ) ; /// The prediction mode determines the type of prediction to use. We divide the image into squares and all the pixels in a square use same prediction mode.
Span < byte > newData = MemoryMarshal . Cast < uint , byte > ( newColorMap ) ; /// </summary>
int i ; /// <param name="transform">The transform data.</param>
for ( i = 4 ; i < 4 * numColors ; + + i ) /// <param name="pixelData">The pixel data to apply the inverse transform.</param>
{ /// <param name="output">The resulting pixel data with the reversed transformation data.</param>
// Equivalent to AddPixelEq(), on a byte-basis.
newData [ i ] = ( byte ) ( ( data [ i ] + newData [ i - 4 ] ) & 0xff ) ;
}
for ( ; i < 4 * newColorMap . Length ; + + i )
{
newData [ i ] = 0 ; // black tail.
}
}
public static void PredictorInverseTransform ( Vp8LTransform transform , Span < uint > pixelData , Span < uint > output ) public static void PredictorInverseTransform ( Vp8LTransform transform , Span < uint > pixelData , Span < uint > output )
{ {
int processedPixels = 0 ; int processedPixels = 0 ;
@ -198,6 +207,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
xEnd = width ; xEnd = width ;
} }
// There are 14 different prediction modes.
// In each prediction mode, the current pixel value is predicted from one or more neighboring pixels whose values are already known.
int startIdx = processedPixels + x ; int startIdx = processedPixels + x ;
int numberOfPixels = xEnd - x ; int numberOfPixels = xEnd - x ;
switch ( predictorMode ) switch ( predictorMode )
@ -602,9 +613,26 @@ namespace SixLabors.ImageSharp.Formats.WebP
return ( idx > > 8 ) & 0xff ; return ( idx > > 8 ) & 0xff ;
} }
public static void ExpandColorMap ( int numColors , Span < uint > transformData , Span < uint > newColorMap )
{
newColorMap [ 0 ] = transformData [ 0 ] ;
Span < byte > data = MemoryMarshal . Cast < uint , byte > ( transformData ) ;
Span < byte > newData = MemoryMarshal . Cast < uint , byte > ( newColorMap ) ;
int i ;
for ( i = 4 ; i < 4 * numColors ; + + i )
{
// Equivalent to AddPixelEq(), on a byte-basis.
newData [ i ] = ( byte ) ( ( data [ i ] + newData [ i - 4 ] ) & 0xff ) ;
}
for ( ; i < 4 * newColorMap . Length ; + + i )
{
newData [ i ] = 0 ; // black tail.
}
}
private static int ColorTransformDelta ( sbyte colorPred , sbyte color ) private static int ColorTransformDelta ( sbyte colorPred , sbyte color )
{ {
int delta = ( ( sbyte ) colorPred * color ) > > 5 ;
return ( ( int ) colorPred * color ) > > 5 ; return ( ( int ) colorPred * color ) > > 5 ;
} }
Brian Popow
6 years ago