/// <summary>
/// Reads the image descriptor.
/// </summary>
private void ReadImageDescriptor()
{
this.stream.Read(this.buffer, 0, 9);
this.imageDescriptor = GifImageDescriptor.Parse(this.buffer);
if (this.imageDescriptor.Height == 0 || this.imageDescriptor.Width == 0)
{
GifThrowHelper.ThrowInvalidImageContentException("Width or height should not be 0");
}
}
/// <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];
var 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 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 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 <TPixel> previousFrame = null;
ImageFrame <TPixel> currentFrame = null;
ImageFrame <TPixel> image;
if (this.previousFrame == null)
{
// This initializes the image to become fully transparent because the alpha channel is zero.
this.image = new Image <TPixel>(this.configuration, imageWidth, imageHeight, this.metaData);
this.SetFrameMetaData(this.image.Frames.RootFrame.MetaData);
image = this.image.Frames.RootFrame;
}
else
{
if (this.graphicsControlExtension != null &&
this.graphicsControlExtension.DisposalMethod == DisposalMethod.RestoreToPrevious)
{
previousFrame = this.previousFrame;
}
currentFrame = this.image.Frames.AddFrame(this.previousFrame); // this clones the frame and adds it the collection
this.SetFrameMetaData(currentFrame.MetaData);
image = currentFrame;
this.RestoreToBackground(image);
}
int 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;
}
Span <TPixel> rowSpan = image.GetPixelRowSpan(writeY);
Rgba32 rgba = new Rgba32(0, 0, 0, 255);
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;
ref TPixel pixel = ref rowSpan[x];
rgba.Rgb = colorTable.GetRgb24(indexOffset);
pixel.PackFromRgba32(rgba);
}
i++;
}
}
/// <summary>
/// Reads the image descriptor.
/// </summary>
private void ReadImageDescriptor()
{
this.stream.Read(this.buffer, 0, 9);
this.imageDescriptor = GifImageDescriptor.Parse(this.buffer);
}
/// <summary>
/// Reads the image descriptor
/// </summary>
/// <returns><see cref="GifImageDescriptor"/></returns>
private GifImageDescriptor ReadImageDescriptor()
{
this.stream.Read(this.buffer, 0, 9);
return(GifImageDescriptor.Parse(this.buffer));
}
