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@ -64,17 +64,22 @@ namespace SixLabors.ImageSharp.Formats.Gif |
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/// <summary>
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/// <summary>
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/// Decodes and decompresses all pixel indices from the stream.
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/// Decodes and decompresses all pixel indices from the stream.
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/// </summary>
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/// </summary>
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/// <param name="dataSize">Size of the data.</param>
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/// <param name="minCodeSize">Minimum code size of the data.</param>
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/// <param name="pixels">The pixel array to decode to.</param>
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/// <param name="pixels">The pixel array to decode to.</param>
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public void DecodePixels(int dataSize, Buffer2D<byte> pixels) |
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public void DecodePixels(int minCodeSize, Buffer2D<byte> pixels) |
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{ |
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{ |
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// Calculate the clear code. The value of the clear code is 2 ^ dataSize
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// Calculate the clear code. The value of the clear code is 2 ^ minCodeSize
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int clearCode = 1 << dataSize; |
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int clearCode = 1 << minCodeSize; |
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if (clearCode > MaxStackSize) |
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// It is possible to specify a larger LZW minimum code size than the palette length in bits
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// which may leave a gap in the codes where no colors are assigned.
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// http://www.matthewflickinger.com/lab/whatsinagif/lzw_image_data.asp#lzw_compression
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if (minCodeSize < 2 || clearCode > MaxStackSize) |
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{ |
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{ |
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// Don't attempt to decode the frame indices.
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// Don't attempt to decode the frame indices.
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// The image is most likely corrupted.
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// Theoretically we could determine a min code size from the length of the provided
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return; |
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// color palette but we won't bother since the image is most likely corrupted.
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ThrowBadMinimumCode(); |
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} |
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} |
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// The resulting index table length.
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// The resulting index table length.
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@ -82,7 +87,7 @@ namespace SixLabors.ImageSharp.Formats.Gif |
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int height = pixels.Height; |
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int height = pixels.Height; |
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int length = width * height; |
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int length = width * height; |
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int codeSize = dataSize + 1; |
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int codeSize = minCodeSize + 1; |
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// Calculate the end code
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// Calculate the end code
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int endCode = clearCode + 1; |
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int endCode = clearCode + 1; |
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@ -169,7 +174,7 @@ namespace SixLabors.ImageSharp.Formats.Gif |
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if (code == clearCode) |
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if (code == clearCode) |
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{ |
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{ |
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// Reset the decoder
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// Reset the decoder
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codeSize = dataSize + 1; |
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codeSize = minCodeSize + 1; |
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codeMask = (1 << codeSize) - 1; |
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codeMask = (1 << codeSize) - 1; |
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availableCode = clearCode + 2; |
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availableCode = clearCode + 2; |
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oldCode = NullCode; |
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oldCode = NullCode; |
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@ -258,5 +263,8 @@ namespace SixLabors.ImageSharp.Formats.Gif |
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this.suffix.Dispose(); |
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this.suffix.Dispose(); |
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this.pixelStack.Dispose(); |
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this.pixelStack.Dispose(); |
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} |
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} |
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[MethodImpl(InliningOptions.ColdPath)] |
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public static void ThrowBadMinimumCode() => throw new InvalidImageContentException("Gif Image does not contain a valid LZW minimum code."); |
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} |
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} |
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} |
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} |
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James Jackson-South
4 years ago
