public static GraphicRenderingBlock Create(byte[] data, ref int offset)
{
GraphicRenderingBlock grb = new GraphicRenderingBlock();
//
// Image descriptor.
//
Debug.Assert(data[offset] == IMAGE_SEPARATOR);
offset++;
grb.ImageLeftPosition = RawData.FetchLittleUshort(data, ref offset);
grb.ImageTopPosition = RawData.FetchLittleUshort(data, ref offset);
grb.ImageWidth = RawData.FetchLittleUshort(data, ref offset);
grb.ImageHeight = RawData.FetchLittleUshort(data, ref offset);
byte pak = data[offset++];
grb.LocalColorTableFlag = (pak & 0x80) != 0;
grb.InterlaceFlag = (pak & 0x40) != 0;
grb.SortFlag = (pak & 0x20) != 0;
grb.LocalColorTableSize = (byte)(pak & 0x07);
//
// Local color table (optional).
//
if (grb.LocalColorTableFlag)
{
// Size is expressed as a power of 2. TableSize=7 is 256 entries, 3 bytes each.
int tableLen = 1 << (grb.LocalColorTableSize + 1);
grb.LocalColorTable = new byte[tableLen * 3];
for (int i = 0; i < tableLen * 3; i++)
{
grb.LocalColorTable[i] = data[offset++];
}
}
else
{
grb.LocalColorTable = new byte[0];
}
//
// Table based image data.
//
grb.ImageData = data;
grb.ImageStartOffset = offset++;
while (data[offset] != 0)
{
offset += data[offset] + 1;
}
grb.ImageEndOffset = offset++;
return(grb);
}
private bool Unpack(byte[] gifData)
{
//
// Header. Signature ("GIF") + version ("87a" or "89a").
//
if (RawData.CompareArrays(gifData, 0, GIF87A, 0, GIF87A.Length))
{
FileVer = FileVersion.Gif87a;
}
else if (RawData.CompareArrays(gifData, 0, GIF89A, 0, GIF87A.Length))
{
FileVer = FileVersion.Gif89a;
}
else
{
Debug.WriteLine("GIF signature not found");
return(false);
}
//Debug.WriteLine("GIF: found signature " + FileVer);
byte pak;
//
// Logical screen descriptor.
//
int offset = GIF87A.Length;
LogicalScreenWidth = RawData.FetchLittleUshort(gifData, ref offset);
LogicalScreenHeight = RawData.FetchLittleUshort(gifData, ref offset);
pak = gifData[offset++];
GlobalColorTableFlag = (pak & 0x80) != 0;
ColorResolution = (byte)((pak >> 4) & 0x07);
SortFlag = (pak & 0x08) != 0;
GlobalColorTableSize = (byte)(pak & 0x07);
BackgroundColorIndex = gifData[offset++];
PixelAspectRatio = gifData[offset++];
//
// Global color table.
//
if (GlobalColorTableFlag)
{
// Size is expressed as a power of 2. TableSize=7 is 256 entries, 3 bytes each.
int tableLen = 1 << (GlobalColorTableSize + 1);
GlobalColorTable = new byte[tableLen * 3];
for (int i = 0; i < tableLen * 3; i++)
{
GlobalColorTable[i] = gifData[offset++];
}
}
else
{
GlobalColorTable = new byte[0];
}
//
// Various blocks follow. Continue until EOF is reached.
//
GraphicControlExtension lastGce = null;
while (true)
{
if (offset >= gifData.Length)
{
Debug.WriteLine("Error: GIF unpacker ran off end of buffer");
return(false);
}
if (gifData[offset] == GIF_TRAILER)
{
break;
}
else if (gifData[offset] == EXTENSION_INTRODUCER)
{
if (gifData[offset + 1] == GRAPHIC_CONTROL_LABEL)
{
lastGce = GraphicControlExtension.Create(gifData, ref offset);
}
else
{
Debug.WriteLine("Skipping unknown extension 0x" +
gifData[offset + 1].ToString("x2"));
offset += 2;
while (gifData[offset] != 0)
{
offset += gifData[offset] + 1;
}
offset++;
}
}
else if (gifData[offset] == IMAGE_SEPARATOR)
{
GraphicRenderingBlock grb = GraphicRenderingBlock.Create(gifData, ref offset);
if (grb != null)
{
if (lastGce != null)
{
grb.GraphicControlExt = lastGce;
}
ImageBlocks.Add(grb);
}
// this resets after the image
lastGce = null;
}
else
{
Debug.WriteLine("Found unknown block start 0x" +
gifData[offset].ToString("x2"));
return(false);
}
}
return(true);
}
