/// <summary>
/// Read gif header
/// </summary>
/// <returns>Data of gif header</returns>
/// <exception cref="ArgumentException">If file is not gif file</exception>
public GifHeader ReadHeader()
{
AssertToken(Token.Header);
// Header
headerStartPosition = currentStream.Position;
firstFrameStartPosition = -1;
currentStream.Read(headerBuffer, 0, headerBuffer.Length);
if (BitUtils.CheckString(headerBuffer, "GIF87a"))
{
header.version = GifVersion.Gif87a;
}
else if (BitUtils.CheckString(headerBuffer, "GIF89a"))
{
header.version = GifVersion.Gif89a;
}
else
{
throw new ArgumentException("Invalid or corrupted Gif file");
}
// Screen descriptor
header.width = currentStream.ReadInt16LittleEndian();
header.height = currentStream.ReadInt16LittleEndian();
var flags = currentStream.ReadByte8();
header.globalColorTableSize = BitUtils.GetColorTableSize(flags.GetBitsFromByte(0, 3));
header.sortColors = flags.GetBitFromByte(3);
header.colorResolution = flags.GetBitsFromByte(4, 3);
header.hasGlobalColorTable = flags.GetBitFromByte(7);
header.transparentColorIndex = currentStream.ReadByte8();
header.pixelAspectRatio = currentStream.ReadByte8();
canvas.SetSize(header.width, header.height);
if (header.hasGlobalColorTable)
{
SetCurrentToken(Token.Palette);
nextPaletteIsGlobal = true;
}
else
{
DetermineNextToken();
}
return(header);
}
private Token DetermineNextToken()
{
while (true)
{
var blockType = currentStream.ReadByte8();
switch (blockType)
{
case ExtensionBlock:
var extensionType = currentStream.ReadByte8();
switch (extensionType)
{
case commentLabel: return(SetCurrentToken(Token.Comment));
case PlainTextLabel: return(SetCurrentToken(Token.PlainText));
case GraphicControlLabel: return(SetCurrentToken(Token.GraphicsControl));
case applicationExtensionLabel:
{
currentStream.AssertByte(11);
currentStream.Read(extensionApplicationBuffer, 0, 11);
var token = BitUtils.CheckString(extensionApplicationBuffer, "NETSCAPE2.0")
? Token.NetscapeExtension
: Token.ApplicationExtension;
return(SetCurrentToken(token));
}
default: BitUtils.SkipGifBlocks(currentStream); break;
}
break;
case ImageDescriptorBlock: return(SetCurrentToken(Token.ImageDescriptor));
case EndOfFile: return(SetCurrentToken(Token.EndOfFile));
default: throw new ArgumentException(String.Format("Unknown block type {0}", blockType));
}
}
}
