/// <summary>
/// Writes the logical screen descriptor to the stream.
/// </summary>
/// <typeparam name="T">The pixel format.</typeparam>
/// <typeparam name="TP">The packed format. <example>long, float.</example></typeparam>
/// <param name="image">The image to encode.</param>
/// <param name="writer">The writer to write to the stream with.</param>
/// <param name="tranparencyIndex">The transparency index to set the default backgound index to.</param>
private void WriteLogicalScreenDescriptor <T, TP>(Image <T, TP> image, EndianBinaryWriter writer, int tranparencyIndex)
where T : IPackedVector <TP>
where TP : struct
{
GifLogicalScreenDescriptor descriptor = new GifLogicalScreenDescriptor
{
Width = (short)image.Width,
Height = (short)image.Height,
GlobalColorTableFlag = false, // Always false for now.
GlobalColorTableSize = this.bitDepth - 1,
BackgroundColorIndex = (byte)(tranparencyIndex > -1 ? tranparencyIndex : 255)
};
writer.Write((ushort)descriptor.Width);
writer.Write((ushort)descriptor.Height);
PackedField field = new PackedField();
field.SetBit(0, descriptor.GlobalColorTableFlag); // 1 : Global color table flag = 1 || 0 (GCT used/ not used)
field.SetBits(1, 3, descriptor.GlobalColorTableSize); // 2-4 : color resolution
field.SetBit(4, false); // 5 : GCT sort flag = 0
field.SetBits(5, 3, descriptor.GlobalColorTableSize); // 6-8 : GCT size. 2^(N+1)
// Reduce the number of writes
byte[] arr =
{
field.Byte,
descriptor.BackgroundColorIndex, // Background Color Index
descriptor.PixelAspectRatio // Pixel aspect ratio. Assume 1:1
};
writer.Write(arr);
}
/// <summary>
/// Writes the image descriptor to the stream.
/// </summary>
/// <typeparam name="T">The pixel format.</typeparam>
/// <typeparam name="TP">The packed format. <example>long, float.</example></typeparam>
/// <param name="image">The <see cref="ImageBase{T,TP}"/> to be encoded.</param>
/// <param name="writer">The stream to write to.</param>
private void WriteImageDescriptor <T, TP>(ImageBase <T, TP> image, EndianBinaryWriter writer)
where T : IPackedVector <TP>
where TP : struct
{
writer.Write(GifConstants.ImageDescriptorLabel); // 2c
// TODO: Can we capture this?
writer.Write((ushort)0); // Left position
writer.Write((ushort)0); // Top position
writer.Write((ushort)image.Width);
writer.Write((ushort)image.Height);
PackedField field = new PackedField();
field.SetBit(0, true); // 1: Local color table flag = 1 (LCT used)
field.SetBit(1, false); // 2: Interlace flag 0
field.SetBit(2, false); // 3: Sort flag 0
field.SetBits(5, 3, this.bitDepth - 1); // 4-5: Reserved, 6-8 : LCT size. 2^(N+1)
writer.Write(field.Byte);
}
/// <summary>
/// Writes the graphics control extension to the stream.
/// </summary>
/// <typeparam name="T">The pixel format.</typeparam>
/// <typeparam name="TP">The packed format. <example>long, float.</example></typeparam>
/// <param name="image">The <see cref="ImageBase{T,TP}"/> to encode.</param>
/// <param name="writer">The stream to write to.</param>
/// <param name="transparencyIndex">The index of the color in the color palette to make transparent.</param>
private void WriteGraphicalControlExtension <T, TP>(ImageBase <T, TP> image, EndianBinaryWriter writer, int transparencyIndex)
where T : IPackedVector <TP>
where TP : struct
{
// TODO: Check transparency logic.
bool hasTransparent = transparencyIndex > -1;
DisposalMethod disposalMethod = hasTransparent
? DisposalMethod.RestoreToBackground
: DisposalMethod.Unspecified;
GifGraphicsControlExtension extension = new GifGraphicsControlExtension()
{
DisposalMethod = disposalMethod,
TransparencyFlag = hasTransparent,
TransparencyIndex = transparencyIndex,
DelayTime = image.FrameDelay
};
// Reduce the number of writes.
byte[] intro =
{
GifConstants.ExtensionIntroducer,
GifConstants.GraphicControlLabel,
4 // Size
};
writer.Write(intro);
PackedField field = new PackedField();
field.SetBits(3, 3, (int)extension.DisposalMethod); // 1-3 : Reserved, 4-6 : Disposal
// TODO: Allow this as an option.
field.SetBit(6, false); // 7 : User input - 0 = none
field.SetBit(7, extension.TransparencyFlag); // 8: Has transparent.
writer.Write(field.Byte);
writer.Write((ushort)extension.DelayTime);
writer.Write((byte)(extension.TransparencyIndex == -1 ? 255 : extension.TransparencyIndex));
writer.Write(GifConstants.Terminator);
}