async Task ParseImageDescriptorAsync(GIFDecoderStreamReader stream, GIFBitmapDecoder decoder, GIFBitmap previousBitmap, bool ignoreImageData)
{
await ParseGIFBitmapHeaderAsync(stream).ConfigureAwait(false);
if (IsTransparent)
{
ColorTable.SetTransparency(TransparencyIndex);
}
DataPosition = stream.CurrentPosition;
if (!ignoreImageData)
{
// Decode LZW data stream.
await decoder.DecodeAsync(stream, _header.Width, _header.Height).ConfigureAwait(false);
// Compose bitmap from decoded data stream.
decoder.Compose(_header, this, previousBitmap);
// Consume block terminator.
await stream.SkipBlockAsync().ConfigureAwait(false);
}
else
{
// Read pass variable length LZW data stream.
// First byte is LZW code size followed by data blocks repeated until block terminator.
stream.Read();
await stream.SkipBlockAsync().ConfigureAwait(false);
}
if (IsTransparent)
{
ColorTable.ResetTransparency();
}
}
async Task ParseApplicationExtensionAsync(GIFDecoderStreamReader stream)
{
var blockSize = await stream.ReadBlockAsync().ConfigureAwait(false);
if (blockSize >= NetscapeApplicationExtensionID.Length)
{
var buffer = stream.CurrentBlockBuffer;
string identifier = System.Text.Encoding.UTF8.GetString(buffer, 0, NetscapeApplicationExtensionID.Length);
if (identifier.Equals(NetscapeApplicationExtensionID, StringComparison.OrdinalIgnoreCase))
{
await ParseNetscapeApplicationExtensionAsync(stream).ConfigureAwait(false);
return;
}
}
await stream.SkipBlockAsync().ConfigureAwait(false);
return;
}
async Task ParseExtensionAsync(GIFDecoderStreamReader stream)
{
int blockCode = stream.Read();
switch (blockCode)
{
case GIFBlockCodes.GraphicsControlExtension:
ParseGraphicControlExtension(stream);
break;
case GIFBlockCodes.ApplicationExtensionBlock:
await ParseApplicationExtensionAsync(stream).ConfigureAwait(false);
break;
default:
await stream.SkipBlockAsync().ConfigureAwait(false);
break;
}
}