private void ReadHeaderFromFile(BinaryReader reader)
{
byte[] header = reader.ReadBytes(_headerMagic.Length);
if (!ByteUtilities.CompareBytes(header, _headerMagic))
{
throw new FormatException(
"The expected header was not found; the file is not a valid GIF file.");
}
_width = reader.ReadUInt16();
_height = reader.ReadUInt16();
byte flags = reader.ReadByte();
_backgroundColourIndex = reader.ReadByte();
_pixelAspectRatio = reader.ReadByte();
bool hasGlobalColourTable = (flags & 0x80) != 0;
_colourResolutionBits = 1 + (flags & 0x70) >> 4;
_colourTableIsSorted = (flags & 0x08) != 0;
int sizeOfGlobalColourTable = 1 << (1 + (flags & 0x07));
if (hasGlobalColourTable)
{
_colourTable = new GifColourTable();
_colourTable.ReadFromFile(reader, sizeOfGlobalColourTable);
}
else
{
_colourTable = null;
}
}
internal void ReadImageDescriptorFromFile(BinaryReader reader, GifGraphicsControlExtension extensionOrNull)
{
_extensionOrNull = extensionOrNull;
_bounds.X = (int)reader.ReadUInt16();
_bounds.Y = (int)reader.ReadUInt16();
_bounds.Width = (int)reader.ReadUInt16();
_bounds.Height = (int)reader.ReadUInt16();
byte flags = reader.ReadByte();
bool hasLocalColourTable = (flags & 0x80) != 0;
_isInterlaced = (flags & 0x40) != 0;
_isSorted = (flags & 0x20) != 0;
int sizeOfLocalColourTable = 1 << (1 + (flags & 0x07));
if (hasLocalColourTable)
{
_colourTable = new GifColourTable();
_colourTable.ReadFromFile(reader, sizeOfLocalColourTable);
}
else
{
_colourTable = null;
}
ReadImageDataFromFile(reader);
}