/// <summary>
/// Reads the local color table from the current frame.
/// </summary>
/// <param name="imageDescriptor">The <see cref="GifImageDescriptor"/>.</param>
/// <returns>The <see cref="T:byte[]"/></returns>
private byte[] ReadFrameLocalColorTable(GifImageDescriptor imageDescriptor)
{
byte[] localColorTable = null;
if (imageDescriptor.LocalColorTableFlag)
{
localColorTable = new byte[imageDescriptor.LocalColorTableSize * 3];
this.currentStream.Read(localColorTable, 0, localColorTable.Length);
}
return(localColorTable);
}
/// <summary>
/// Reads an individual gif frame.
/// </summary>
private void ReadFrame()
{
GifImageDescriptor imageDescriptor = this.ReadImageDescriptor();
byte[] localColorTable = this.ReadFrameLocalColorTable(imageDescriptor);
byte[] indices = this.ReadFrameIndices(imageDescriptor);
// Determine the color table for this frame. If there is a local one, use it
// otherwise use the global color table.
byte[] colorTable = localColorTable ?? this.globalColorTable;
this.ReadFrameColors(indices, colorTable, imageDescriptor);
// Skip any remaining blocks
this.Skip(0);
}
/// <summary>
/// Reads the image descriptor
/// </summary>
/// <returns><see cref="GifImageDescriptor"/></returns>
private GifImageDescriptor ReadImageDescriptor()
{
byte[] buffer = new byte[9];
this.currentStream.Read(buffer, 0, buffer.Length);
byte packed = buffer[8];
GifImageDescriptor imageDescriptor = new GifImageDescriptor
{
Left = BitConverter.ToInt16(buffer, 0),
Top = BitConverter.ToInt16(buffer, 2),
Width = BitConverter.ToInt16(buffer, 4),
Height = BitConverter.ToInt16(buffer, 6),
LocalColorTableFlag = ((packed & 0x80) >> 7) == 1,
LocalColorTableSize = 2 << (packed & 0x07),
InterlaceFlag = ((packed & 0x40) >> 6) == 1
};
return(imageDescriptor);
}
/// <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="descriptor">The <see cref="GifImageDescriptor"/></param>
private void ReadFrameColors(byte[] indices, byte[] colorTable, GifImageDescriptor descriptor)
{
int imageWidth = this.logicalScreenDescriptor.Width;
int imageHeight = this.logicalScreenDescriptor.Height;
if (this.currentFrame == null)
{
this.currentFrame = new T[imageWidth * imageHeight];
}
T[] lastFrame = null;
if (this.graphicsControlExtension != null &&
this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
{
lastFrame = new T[imageWidth * imageHeight];
Array.Copy(this.currentFrame, lastFrame, lastFrame.Length);
}
int offset, i = 0;
int interlacePass = 0; // The interlace pass
int interlaceIncrement = 8; // The interlacing line increment
int interlaceY = 0; // The current interlaced line
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++)
{
offset = (writeY * imageWidth) + x;
int index = indices[i];
if (this.graphicsControlExtension == null ||
this.graphicsControlExtension.TransparencyFlag == false ||
this.graphicsControlExtension.TransparencyIndex != index)
{
// Stored in r-> g-> b-> a order.
int indexOffset = index * 3;
T pixel = default(T);
pixel.PackFromBytes(colorTable[indexOffset], colorTable[indexOffset + 1], colorTable[indexOffset + 2], 255);
this.currentFrame[offset] = pixel;
}
i++;
}
}
T[] pixels = new T[imageWidth * imageHeight];
Array.Copy(this.currentFrame, pixels, pixels.Length);
ImageBase <T, TP> currentImage;
if (this.decodedImage.Pixels == null)
{
currentImage = this.decodedImage;
currentImage.SetPixels(imageWidth, imageHeight, pixels);
currentImage.Quality = colorTable.Length / 3;
if (this.graphicsControlExtension != null && this.graphicsControlExtension.DelayTime > 0)
{
this.decodedImage.FrameDelay = this.graphicsControlExtension.DelayTime;
}
}
else
{
ImageFrame <T, TP> frame = new ImageFrame <T, TP>();
currentImage = frame;
currentImage.SetPixels(imageWidth, imageHeight, pixels);
currentImage.Quality = colorTable.Length / 3;
if (this.graphicsControlExtension != null && this.graphicsControlExtension.DelayTime > 0)
{
currentImage.FrameDelay = this.graphicsControlExtension.DelayTime;
}
this.decodedImage.Frames.Add(frame);
}
if (this.graphicsControlExtension != null)
{
if (this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToBackground)
{
for (int y = descriptor.Top; y < descriptor.Top + descriptor.Height; y++)
{
for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width; x++)
{
offset = (y * imageWidth) + x;
// Stored in r-> g-> b-> a order.
this.currentFrame[offset] = default(T);
}
}
}
else if (this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
{
this.currentFrame = lastFrame;
}
}
}
