/// <summary>
/// Initializes a new instance of the <see cref="AlphaDecoder"/> class.
/// </summary>
/// <param name="width">The width of the image.</param>
/// <param name="height">The height of the image.</param>
/// <param name="data">The (maybe compressed) alpha data.</param>
/// <param name="alphaChunkHeader">The first byte of the alpha image stream contains information on how to decode the stream.</param>
/// <param name="memoryAllocator">Used for allocating memory during decoding.</param>
/// <param name="configuration">The configuration.</param>
public AlphaDecoder(int width, int height, IMemoryOwner <byte> data, byte alphaChunkHeader, MemoryAllocator memoryAllocator, Configuration configuration)
{
this.Width = width;
this.Height = height;
this.Data = data;
this.memoryAllocator = memoryAllocator;
this.LastRow = 0;
int totalPixels = width * height;
var compression = (WebpAlphaCompressionMethod)(alphaChunkHeader & 0x03);
if (compression is not WebpAlphaCompressionMethod.NoCompression and not WebpAlphaCompressionMethod.WebpLosslessCompression)
{
WebpThrowHelper.ThrowImageFormatException($"unexpected alpha compression method {compression} found");
}
this.Compressed = compression == WebpAlphaCompressionMethod.WebpLosslessCompression;
// The filtering method used. Only values between 0 and 3 are valid.
int filter = (alphaChunkHeader >> 2) & 0x03;
if (filter is < (int)WebpAlphaFilterType.None or > (int)WebpAlphaFilterType.Gradient)
{
WebpThrowHelper.ThrowImageFormatException($"unexpected alpha filter method {filter} found");
}
this.Alpha = memoryAllocator.Allocate <byte>(totalPixels);
this.AlphaFilterType = (WebpAlphaFilterType)filter;
this.Vp8LDec = new Vp8LDecoder(width, height, memoryAllocator);
if (this.Compressed)
{
var bitReader = new Vp8LBitReader(data);
this.LosslessDecoder = new WebpLosslessDecoder(bitReader, memoryAllocator, configuration);
this.LosslessDecoder.DecodeImageStream(this.Vp8LDec, width, height, true);
this.Use8BDecode = this.Vp8LDec.Transforms.Count > 0 && Is8BOptimizable(this.Vp8LDec.Metadata);
}
}
/// <summary>
/// Initializes a new instance of the <see cref="WebpLosslessDecoder"/> class.
/// </summary>
/// <param name="bitReader">Bitreader to read from the stream.</param>
/// <param name="memoryAllocator">Used for allocating memory during processing operations.</param>
/// <param name="configuration">The configuration.</param>
public WebpLosslessDecoder(Vp8LBitReader bitReader, MemoryAllocator memoryAllocator, Configuration configuration)
{
this.bitReader = bitReader;
this.memoryAllocator = memoryAllocator;
this.configuration = configuration;
}
