@ -5,6 +5,7 @@ using System;
using System.Buffers ;
using System.Collections.Generic ;
using System.Runtime.CompilerServices ;
using SixLabors.ImageSharp.Memory ;
namespace SixLabors.ImageSharp.Formats.WebP
@ -14,6 +15,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
/// </summary>
internal class AlphaDecoder : IDisposable
{
private readonly MemoryAllocator memoryAllocator ;
/// <summary>
/// Initializes a new instance of the <see cref="AlphaDecoder"/> class.
/// </summary>
@ -28,7 +31,9 @@ namespace SixLabors.ImageSharp.Formats.WebP
this . Width = width ;
this . Height = height ;
this . Data = data ;
this . memoryAllocator = memoryAllocator ;
this . LastRow = 0 ;
int totalPixels = width * height ;
var compression = ( AlphaCompressionMethod ) ( alphaChunkHeader & 0x03 ) ;
if ( compression ! = AlphaCompressionMethod . NoCompression & & compression ! = AlphaCompressionMethod . WebPLosslessCompression )
@ -45,8 +50,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
WebPThrowHelper . ThrowImageFormatException ( $"unexpected alpha filter method {filter} found" ) ;
}
this . Alpha = memoryAllocator . Allocate < byte > ( totalPixels ) ;
this . AlphaFilterType = ( WebPAlphaFilterType ) filter ;
this . Alpha = memoryAllocator . Allocate < byte > ( width * height ) ;
this . Vp8LDec = new Vp8LDecoder ( width , height , memoryAllocator ) ;
if ( this . Compressed )
@ -54,6 +59,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
var bitReader = new Vp8LBitReader ( data ) ;
this . LosslessDecoder = new WebPLosslessDecoder ( bitReader , memoryAllocator , configuration ) ;
this . LosslessDecoder . DecodeImageStream ( this . Vp8LDec , width , height , true ) ;
this . Use8BDecode = Is8BOptimizable ( this . Vp8LDec . Metadata ) ;
}
}
@ -110,11 +116,11 @@ namespace SixLabors.ImageSharp.Formats.WebP
private WebPLosslessDecoder LosslessDecoder { get ; }
/// <summary>
/// Gets or sets a value indicating whether the decoding needs 1 byte per pixel for decoding.
/// Gets a value indicating whether the decoding needs 1 byte per pixel for decoding.
/// Although Alpha Channel requires only 1 byte per pixel, sometimes Vp8LDecoder may need to allocate
/// 4 bytes per pixel internally during decode.
/// </summary>
public bool Use8BDecode { get ; set ; }
public bool Use8BDecode { get ; }
/// <summary>
/// Decodes and filters the maybe compressed alpha data.
@ -162,7 +168,15 @@ namespace SixLabors.ImageSharp.Formats.WebP
}
else
{
this . LosslessDecoder . DecodeAlphaData ( this ) ;
if ( this . Use8BDecode )
{
this . LosslessDecoder . DecodeAlphaData ( this ) ;
}
else
{
this . LosslessDecoder . DecodeImageData ( this . Vp8LDec , this . Vp8LDec . Pixels . Memory . Span ) ;
this . ExtractAlphaRows ( this . Vp8LDec ) ;
}
}
}
@ -204,6 +218,25 @@ namespace SixLabors.ImageSharp.Formats.WebP
this . PrevRow = lastRow - 1 ;
}
/// <summary>
/// Once the image-stream is decoded into ARGB color values, the transparency information will be extracted from the green channel of the ARGB quadruplet.
/// </summary>
/// <param name="dec">The VP8L decoder.</param>
private void ExtractAlphaRows ( Vp8LDecoder dec )
{
int numRowsToProcess = dec . Height ;
int width = dec . Width ;
Span < uint > pixels = dec . Pixels . Memory . Span ;
Span < uint > input = pixels ;
Span < byte > output = this . Alpha . Memory . Span ;
// Extract alpha (which is stored in the green plane).
int pixelCount = width * numRowsToProcess ;
WebPLosslessDecoder . ApplyInverseTransforms ( dec , input , this . memoryAllocator ) ;
this . AlphaApplyFilter ( 0 , numRowsToProcess , output , width ) ;
ExtractGreen ( input , output , pixelCount ) ;
}
private static void HorizontalUnfilter ( Span < byte > prev , Span < byte > input , Span < byte > dst , int width )
{
byte pred = ( byte ) ( prev = = null ? 0 : prev [ 0 ] ) ;
@ -252,7 +285,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
}
}
private static bool Is8bOptimizable ( Vp8LMetadata hdr )
/// <summary>
/// Row-processing for the special case when alpha data contains only one
/// transform (color indexing), and trivial non-green literals.
/// </summary>
/// <param name="hdr">The VP8L meta data.</param>
/// <returns>True, if alpha channel has one byte per pixel, otherwise 4.</returns>
private static bool Is8BOptimizable ( Vp8LMetadata hdr )
{
if ( hdr . ColorCacheSize > 0 )
{
@ -264,17 +303,17 @@ namespace SixLabors.ImageSharp.Formats.WebP
for ( int i = 0 ; i < hdr . NumHTreeGroups ; + + i )
{
List < HuffmanCode [ ] > htrees = hdr . HTreeGroups [ i ] . HTrees ;
if ( htrees [ HuffIndex . Red ] [ 0 ] . Value > 0 )
if ( htrees [ HuffIndex . Red ] [ 0 ] . BitsUsed > 0 )
{
return false ;
}
if ( htrees [ HuffIndex . Blue ] [ 0 ] . Value > 0 )
if ( htrees [ HuffIndex . Blue ] [ 0 ] . BitsUsed > 0 )
{
return false ;
}
if ( htrees [ HuffIndex . Alpha ] [ 0 ] . Value > 0 )
if ( htrees [ HuffIndex . Alpha ] [ 0 ] . BitsUsed > 0 )
{
return false ;
}
@ -290,6 +329,15 @@ namespace SixLabors.ImageSharp.Formats.WebP
return ( ( g & ~ 0xff ) is 0 ) ? g : ( g < 0 ) ? 0 : 2 5 5 ; // clip to 8bit
}
[MethodImpl(InliningOptions.ShortMethod)]
private static void ExtractGreen ( Span < uint > argb , Span < byte > alpha , int size )
{
for ( int i = 0 ; i < size ; + + i )
{
alpha [ i ] = ( byte ) ( argb [ i ] > > 8 ) ;
}
}
/// <inheritdoc/>
public void Dispose ( )
{
Brian Popow
6 years ago