ImageSharp/src/ImageSharp.Formats.Gif/GifDecoderCore.cs

// <copyright file="GifDecoderCore.cs" company="James Jackson-South">
// Copyright (c) James Jackson-South and contributors.
// Licensed under the Apache License, Version 2.0.
// </copyright>

namespace ImageSharp.Formats
{
    using System;
    using System.Buffers;
    using System.IO;

    /// <summary>
    /// Performs the gif decoding operation.
    /// </summary>
    /// <typeparam name="TColor">The pixel format.</typeparam>
    internal class GifDecoderCore<TColor>
        where TColor : struct, IPackedPixel, IEquatable<TColor>
    {
        /// <summary>
        /// The temp buffer used to reduce allocations.
        /// </summary>
        private readonly byte[] buffer = new byte[16];

        /// <summary>
        /// The image to decode the information to.
        /// </summary>
        private Image<TColor> decodedImage;

        /// <summary>
        /// The currently loaded stream.
        /// </summary>
        private Stream currentStream;

        /// <summary>
        /// The global color table.
        /// </summary>
        private byte[] globalColorTable;

        /// <summary>
        /// The global color table length
        /// </summary>
        private int globalColorTableLength;

        /// <summary>
        /// The previous frame.
        /// </summary>
        private ImageFrame<TColor> previousFrame;

        /// <summary>
        /// The area to restore.
        /// </summary>
        private Rectangle? restoreArea;

        /// <summary>
        /// The logical screen descriptor.
        /// </summary>
        private GifLogicalScreenDescriptor logicalScreenDescriptor;

        /// <summary>
        /// The graphics control extension.
        /// </summary>
        private GifGraphicsControlExtension graphicsControlExtension;

        /// <summary>
        /// Decodes the stream to the image.
        /// </summary>
        /// <param name="image">The image to decode to.</param>
        /// <param name="stream">The stream containing image data. </param>
        public void Decode(Image<TColor> image, Stream stream)
        {
            try
            {
                this.decodedImage = image;

                this.currentStream = stream;

                // Skip the identifier
                this.currentStream.Skip(6);
                this.ReadLogicalScreenDescriptor();

                if (this.logicalScreenDescriptor.GlobalColorTableFlag)
                {
                    this.globalColorTableLength = this.logicalScreenDescriptor.GlobalColorTableSize * 3;
                    this.globalColorTable = ArrayPool<byte>.Shared.Rent(this.globalColorTableLength);

                    // Read the global color table from the stream
                    stream.Read(this.globalColorTable, 0, this.globalColorTableLength);
                }

                // Loop though the respective gif parts and read the data.
                int nextFlag = stream.ReadByte();
                while (nextFlag != GifConstants.Terminator)
                {
                    if (nextFlag == GifConstants.ImageLabel)
                    {
                        this.ReadFrame();
                    }
                    else if (nextFlag == GifConstants.ExtensionIntroducer)
                    {
                        int label = stream.ReadByte();
                        switch (label)
                        {
                            case GifConstants.GraphicControlLabel:
                                this.ReadGraphicalControlExtension();
                                break;
                            case GifConstants.CommentLabel:
                                this.ReadComments();
                                break;
                            case GifConstants.ApplicationExtensionLabel:
                                this.Skip(12); // No need to read.
                                break;
                            case GifConstants.PlainTextLabel:
                                this.Skip(13); // Not supported by any known decoder.
                                break;
                        }
                    }
                    else if (nextFlag == GifConstants.EndIntroducer)
                    {
                        break;
                    }

                    nextFlag = stream.ReadByte();
                }
            }
            finally
            {
                if (this.globalColorTable != null)
                {
                    ArrayPool<byte>.Shared.Return(this.globalColorTable);
                }
            }
        }

        /// <summary>
        /// Reads the graphic control extension.
        /// </summary>
        private void ReadGraphicalControlExtension()
        {
            this.currentStream.Read(this.buffer, 0, 6);

            byte packed = this.buffer[1];

            this.graphicsControlExtension = new GifGraphicsControlExtension
            {
                DelayTime = BitConverter.ToInt16(this.buffer, 2),
                TransparencyIndex = this.buffer[4],
                TransparencyFlag = (packed & 0x01) == 1,
                DisposalMethod = (DisposalMethod)((packed & 0x1C) >> 2)
            };
        }

        /// <summary>
        /// Reads the image descriptor
        /// </summary>
        /// <returns><see cref="GifImageDescriptor"/></returns>
        private GifImageDescriptor ReadImageDescriptor()
        {
            this.currentStream.Read(this.buffer, 0, 9);

            byte packed = this.buffer[8];

            GifImageDescriptor imageDescriptor = new GifImageDescriptor
            {
                Left = BitConverter.ToInt16(this.buffer, 0),
                Top = BitConverter.ToInt16(this.buffer, 2),
                Width = BitConverter.ToInt16(this.buffer, 4),
                Height = BitConverter.ToInt16(this.buffer, 6),
                LocalColorTableFlag = ((packed & 0x80) >> 7) == 1,
                LocalColorTableSize = 2 << (packed & 0x07),
                InterlaceFlag = ((packed & 0x40) >> 6) == 1
            };

            return imageDescriptor;
        }

        /// <summary>
        /// Reads the logical screen descriptor.
        /// </summary>
        private void ReadLogicalScreenDescriptor()
        {
            this.currentStream.Read(this.buffer, 0, 7);

            byte packed = this.buffer[4];

            this.logicalScreenDescriptor = new GifLogicalScreenDescriptor
            {
                Width = BitConverter.ToInt16(this.buffer, 0),
                Height = BitConverter.ToInt16(this.buffer, 2),
                BackgroundColorIndex = this.buffer[5],
                PixelAspectRatio = this.buffer[6],
                GlobalColorTableFlag = ((packed & 0x80) >> 7) == 1,
                GlobalColorTableSize = 2 << (packed & 0x07)
            };

            if (this.logicalScreenDescriptor.GlobalColorTableSize > 255 * 4)
            {
                throw new ImageFormatException($"Invalid gif colormap size '{this.logicalScreenDescriptor.GlobalColorTableSize}'");
            }

            if (this.logicalScreenDescriptor.Width > this.decodedImage.MaxWidth || this.logicalScreenDescriptor.Height > this.decodedImage.MaxHeight)
            {
                throw new ArgumentOutOfRangeException(
                    $"The input gif '{this.logicalScreenDescriptor.Width}x{this.logicalScreenDescriptor.Height}' is bigger then the max allowed size '{this.decodedImage.MaxWidth}x{this.decodedImage.MaxHeight}'");
            }
        }

        /// <summary>
        /// Skips the designated number of bytes in the stream.
        /// </summary>
        /// <param name="length">The number of bytes to skip.</param>
        private void Skip(int length)
        {
            this.currentStream.Skip(length);

            int flag;

            while ((flag = this.currentStream.ReadByte()) != 0)
            {
                this.currentStream.Skip(flag);
            }
        }

        /// <summary>
        /// Reads the gif comments.
        /// </summary>
        private void ReadComments()
        {
            int flag;

            while ((flag = this.currentStream.ReadByte()) != 0)
            {
                if (flag > GifConstants.MaxCommentLength)
                {
                    throw new ImageFormatException($"Gif comment length '{flag}' exceeds max '{GifConstants.MaxCommentLength}'");
                }

                byte[] flagBuffer = ArrayPool<byte>.Shared.Rent(flag);

                try
                {
                    this.currentStream.Read(flagBuffer, 0, flag);
                    this.decodedImage.Properties.Add(new ImageProperty("Comments", BitConverter.ToString(flagBuffer, 0, flag)));
                }
                finally
                {
                    ArrayPool<byte>.Shared.Return(flagBuffer);
                }
            }
        }

        /// <summary>
        /// Reads an individual gif frame.
        /// </summary>
        private void ReadFrame()
        {
            GifImageDescriptor imageDescriptor = this.ReadImageDescriptor();

            byte[] localColorTable = null;
            byte[] indices = null;
            try
            {
                // Determine the color table for this frame. If there is a local one, use it otherwise use the global color table.
                int length = this.globalColorTableLength;
                if (imageDescriptor.LocalColorTableFlag)
                {
                    length = imageDescriptor.LocalColorTableSize * 3;
                    localColorTable = ArrayPool<byte>.Shared.Rent(length);
                    this.currentStream.Read(localColorTable, 0, length);
                }

                indices = ArrayPool<byte>.Shared.Rent(imageDescriptor.Width * imageDescriptor.Height);

                this.ReadFrameIndices(imageDescriptor, indices);
                this.ReadFrameColors(indices, localColorTable ?? this.globalColorTable, length, imageDescriptor);

                // Skip any remaining blocks
                this.Skip(0);
            }
            finally
            {
                if (localColorTable != null)
                {
                    ArrayPool<byte>.Shared.Return(localColorTable);
                }

                ArrayPool<byte>.Shared.Return(indices);
            }
        }

        /// <summary>
        /// Reads the frame indices marking the color to use for each pixel.
        /// </summary>
        /// <param name="imageDescriptor">The <see cref="GifImageDescriptor"/>.</param>
        /// <param name="indices">The pixel array to write to.</param>
        private void ReadFrameIndices(GifImageDescriptor imageDescriptor, byte[] indices)
        {
            int dataSize = this.currentStream.ReadByte();
            using (LzwDecoder lzwDecoder = new LzwDecoder(this.currentStream))
            {
                lzwDecoder.DecodePixels(imageDescriptor.Width, imageDescriptor.Height, dataSize, indices);
            }
        }

        /// <summary>
        /// Reads the frames colors, mapping indices to colors.
        /// </summary>
        /// <param name="indices">The indexed pixels.</param>
        /// <param name="colorTable">The color table containing the available colors.</param>
        /// <param name="colorTableLength">The color table length.</param>
        /// <param name="descriptor">The <see cref="GifImageDescriptor"/></param>
        private unsafe void ReadFrameColors(byte[] indices, byte[] colorTable, int colorTableLength, GifImageDescriptor descriptor)
        {
            int imageWidth = this.logicalScreenDescriptor.Width;
            int imageHeight = this.logicalScreenDescriptor.Height;

            ImageFrame<TColor> previousFrame = null;

            ImageFrame<TColor> currentFrame = null;

            ImageBase<TColor> image;

            if (this.previousFrame == null)
            {
                image = this.decodedImage;

                image.Quality = colorTableLength / 3;

                // This initializes the image to become fully transparent because the alpha channel is zero.
                image.InitPixels(imageWidth, imageHeight);
            }
            else
            {
                if (this.graphicsControlExtension != null &&
                    this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
                {
                    previousFrame = this.previousFrame;
                }

                currentFrame = this.previousFrame.Clone();

                image = currentFrame;

                this.RestoreToBackground(image);

                this.decodedImage.Frames.Add(currentFrame);
            }

            if (this.graphicsControlExtension != null && this.graphicsControlExtension.DelayTime > 0)
            {
                image.FrameDelay = this.graphicsControlExtension.DelayTime;
            }

            int i = 0;
            int interlacePass = 0; // The interlace pass
            int interlaceIncrement = 8; // The interlacing line increment
            int interlaceY = 0; // The current interlaced line

            using (PixelAccessor<TColor> pixelAccessor = image.Lock())
            {
                for (int y = descriptor.Top; y < descriptor.Top + descriptor.Height; y++)
                {
                    // Check if this image is interlaced.
                    int writeY; // the target y offset to write to
                    if (descriptor.InterlaceFlag)
                    {
                        // If so then we read lines at predetermined offsets.
                        // When an entire image height worth of offset lines has been read we consider this a pass.
                        // With each pass the number of offset lines changes and the starting line changes.
                        if (interlaceY >= descriptor.Height)
                        {
                            interlacePass++;
                            switch (interlacePass)
                            {
                                case 1:
                                    interlaceY = 4;
                                    break;
                                case 2:
                                    interlaceY = 2;
                                    interlaceIncrement = 4;
                                    break;
                                case 3:
                                    interlaceY = 1;
                                    interlaceIncrement = 2;
                                    break;
                            }
                        }

                        writeY = interlaceY + descriptor.Top;

                        interlaceY += interlaceIncrement;
                    }
                    else
                    {
                        writeY = y;
                    }

                    for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width; x++)
                    {
                        int index = indices[i];

                        if (this.graphicsControlExtension == null ||
                            this.graphicsControlExtension.TransparencyFlag == false ||
                            this.graphicsControlExtension.TransparencyIndex != index)
                        {
                            int indexOffset = index * 3;

                            TColor pixel = default(TColor);
                            pixel.PackFromBytes(colorTable[indexOffset], colorTable[indexOffset + 1], colorTable[indexOffset + 2], 255);
                            pixelAccessor[x, writeY] = pixel;
                        }

                        i++;
                    }
                }
            }

            if (previousFrame != null)
            {
                this.previousFrame = previousFrame;
                return;
            }

            this.previousFrame = currentFrame == null ? this.decodedImage.ToFrame() : currentFrame;

            if (this.graphicsControlExtension != null &&
                this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToBackground)
            {
                this.restoreArea = new Rectangle(descriptor.Left, descriptor.Top, descriptor.Width, descriptor.Height);
            }
        }

        /// <summary>
        /// Restores the current frame area to the background.
        /// </summary>
        /// <param name="frame">The frame.</param>
        private void RestoreToBackground(ImageBase<TColor> frame)
        {
            if (this.restoreArea == null)
            {
                return;
            }

            // Optimization for when the size of the frame is the same as the image size.
            if (this.restoreArea.Value.Width == this.decodedImage.Width &&
                this.restoreArea.Value.Height == this.decodedImage.Height)
            {
                using (PixelAccessor<TColor> pixelAccessor = frame.Lock())
                {
                    pixelAccessor.Reset();
                }
            }
            else
            {
                using (PixelArea<TColor> emptyRow = new PixelArea<TColor>(this.restoreArea.Value.Width, ComponentOrder.Xyzw))
                {
                    using (PixelAccessor<TColor> pixelAccessor = frame.Lock())
                    {
                        for (int y = this.restoreArea.Value.Top; y < this.restoreArea.Value.Top + this.restoreArea.Value.Height; y++)
                        {
                            pixelAccessor.CopyFrom(emptyRow, y, this.restoreArea.Value.Left);
                        }
                    }
                }
            }

            this.restoreArea = null;
        }
    }
}