1         public static unsafe void ConvertTransparancyGif(int colorIndex, string baseFile, string outputFile)
 2         {
 3             using (FileStream fs = new FileStream(baseFile, FileMode.Open, FileAccess.Read))
 4             {
 5                 Bitmap img = (Bitmap)Image.FromStream(fs, false, false);
 6                 int width = img.Width;
 7                 int height = img.Height;
 8 
 9                 Bitmap resultbmp = new Bitmap(width, height, PixelFormat.Format8bppIndexed);
10                 ColorPalette palette = resultbmp.Palette;
11                 int n = 0;
12                 foreach (Color tc in img.Palette.Entries)
13                 {
14                     palette.Entries[n] = Color.FromArgb(255, tc);
15                     n++;
16                 }
17 
18                 palette.Entries[colorIndex] = Color.FromArgb(0, palette.Entries[colorIndex]);
19                 resultbmp.Palette = palette;
20 
21                 //now to copy the actual bitmap data 
22                 BitmapData src = img.LockBits(
23                     new Rectangle(0, 0, width, height),
24                     ImageLockMode.ReadOnly,
25                     img.PixelFormat);
26 
27                 BitmapData dst = resultbmp.LockBits(
28                     new Rectangle(0, 0, width, height),
29                     ImageLockMode.WriteOnly,
30                     resultbmp.PixelFormat);
31 
32                 byte* pSrc = (byte*)src.Scan0.ToPointer();
33                 byte* pDst = (byte*)dst.Scan0.ToPointer();
34                 int offset = src.Stride - width;
35 
36                 //steps through each pixel 
37                 for (int y = 0; y < height; y++)
38                 {
39                     for (int x = 0; x < width; x++)
40                     {
41                         pDst[0] = pSrc[0];
42                         pDst++;
43                         pSrc++;
44                     }
45                     pDst += offset;
46                     pSrc += offset;
47                 }
48 
49                 //unlock the bitmaps 
50                 img.UnlockBits(src);
51                 resultbmp.UnlockBits(dst);
52 
53                 resultbmp.Save(outputFile, ImageFormat.Gif);
54 
55                 img.Dispose();
56                 resultbmp.Dispose();
57             } 
59         }