private void ReadLogicalScreenDescriptor()
{
byte[] buffer = new byte[7];
_stream.Read(buffer, 0, buffer.Length);
byte packed = buffer[4];
_logicalScreenDescriptor = new GifLogicalScreenDescriptor();
_logicalScreenDescriptor.Width = BitConverter.ToInt16(buffer, 0);
_logicalScreenDescriptor.Height = BitConverter.ToInt16(buffer, 2);
_logicalScreenDescriptor.Background = buffer[5];
_logicalScreenDescriptor.GlobalColorTableFlag = ((packed & 0x80) >> 7) == 1;
_logicalScreenDescriptor.GlobalColorTableSize = 2 << (packed & 0x07);
}
/// <summary>
/// Decodes the image from the specified stream and sets
/// the data to image.
/// </summary>
/// <param name="image">The image, where the data should be set to.
/// Cannot be null (Nothing in Visual Basic).</param>
/// <param name="stream">The stream, where the image should be
/// decoded from. Cannot be null (Nothing in Visual Basic).</param>
/// <exception cref="ArgumentNullException">
/// <para><paramref name="image"/> is null (Nothing in Visual Basic).</para>
/// <para>- or -</para>
/// <para><paramref name="stream"/> is null (Nothing in Visual Basic).</para>
/// </exception>
public void Decode(ExtendedImage image, Stream stream)
{
try
{
_image = image;
_stream = stream;
_stream.Seek(6, SeekOrigin.Current);
ReadLogicalScreenDescriptor();
if (_logicalScreenDescriptor.GlobalColorTableFlag == true)
{
_globalColorTable = new byte[_logicalScreenDescriptor.GlobalColorTableSize * 3];
// Read the global color table from the stream
stream.Read(_globalColorTable, 0, _globalColorTable.Length);
}
int nextFlag = stream.ReadByte();
while (nextFlag != -1)
{
if (nextFlag == ImageLabel)
{
ReadFrame();
}
else if (nextFlag == ExtensionIntroducer)
{
int gcl = stream.ReadByte();
switch (gcl)
{
case GraphicControlLabel:
ReadGraphicalControlExtension();
break;
case CommentLabel:
ReadComments();
break;
case ApplicationExtensionLabel:
Skip(12);
break;
case PlainTextLabel:
Skip(13);
break;
}
}
else if (nextFlag == EndIntroducer)
{
break;
}
nextFlag = stream.ReadByte();
}
}
catch (Exception e)
{
this._currentFrame = null;
this._globalColorTable = null;
this._graphicsControl = null;
if (this._image != null)
{
this._image.UriSource = null;
}
this._image = null;
this._logicalScreenDescriptor = null;
this._stream = null;
throw new Exception("Gif failed to decode");
}
}
