Comments on: Generating a transparent GIF image using C#

By: victor

victor — Thu, 03 Jul 2008 17:28:53 +0000

Here is the code w/o syntax errors.
//
///
/// Returns a transparent background GIF image from the specified Bitmap.
///
/// The Bitmap to make transparent.
/// The Color to make transparent.
/// New Bitmap containing a transparent background gif.
public Bitmap MakeTransparentGif(Bitmap bitmap, Color color)
{
byte R = color.R;
byte G = color.G;
byte B = color.B;
MemoryStream fin = new MemoryStream();
bitmap.Save(fin, System.Drawing.Imaging.ImageFormat.Gif);
MemoryStream fout = new MemoryStream((int)fin.Length);
int count = 0;
byte[] buf = new byte[256];
byte transparentIdx = 0;
fin.Seek(0, SeekOrigin.Begin);
//header
count = fin.Read(buf, 0, 13);
if ((buf[0] != 71) || (buf[1] != 73) || (buf[2] != 70)) return null; //GIF
fout.Write(buf, 0, 13);
int i = 0;
if ((buf[10] & 0×80) > 0)
{
i = 1 < ((buf[10] & 7) + 1) == 256 ? 256 : 0;
}
for (; i != 0; i—)
{
fin.Read(buf, 0, 3);
if ((buf[0] == R) && (buf[1] == G) && (buf[2] == B))
{
transparentIdx = (byte)(256 – i);
}
fout.Write(buf, 0, 3);
}
bool gcePresent = false;
while (true)
{
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
if (buf[0] != 0×21) break;
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
gcePresent = (buf[0] == 0xf9);
while (true)
{
fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
if (buf[0] == 0) break;
count = buf[0];
if (fin.Read(buf, 0, count) != count) return null;
if (gcePresent)
{
if (count == 4)
{
buf[0] |= 0×01;
buf[3] = transparentIdx;
}
}
fout.Write(buf, 0, count);
}
}
while (count > 0)
{
count = fin.Read(buf, 0, 1);
fout.Write(buf, 0, 1);
}
fin.Close();
fout.Flush();
return new Bitmap(fout);
}
//

By: janeks

janeks — Thu, 12 Jun 2008 21:38:40 +0000

I’m wondering wouldn’t be easer to convert GIF to PNG, then use .NET function foradding transparency, then convert PNG back to the GIF?

Or it’s not working?

By: Joe

Joe — Sun, 25 May 2008 20:54:20 +0000

Could someone please post the code without any syntax errors.
Thanks

By: Kim

Kim — Fri, 07 Mar 2008 10:56:17 +0000

Dear Andrei,

Thank you very much! Your article give me a very usedly help.
In order to resovle this issue,I had searched all forums,BBS,Tech. Blog in China almost,but can’t get a best solution yet,so much as I had make a decision to give up it.

Thanks!

Best regards,
Kim

By: Csiga

Csiga — Wed, 06 Feb 2008 16:16:30 +0000

Dear Andrei,

This is excellent! You save me a lot of time, not to mention those hours what I spent searching for such a "converter". I’m developing a web application, where I use a lot of transparent images, but I have to use the png format (because of the Graphics.FromImage function – isn’t able to populate Graphics object from image with indexed format, like gif). Of course, our wonderful browser (let’s call it "ie6.0"), doesn’t support transparent png, only with alphaimageloader, but it isn’t so fast. With this function, I can create gif-s for mozilla and for explorer too from the png files I create. This is the solution for me! So, you rock!

Thanks a lot!

Best regards,
Csiga

By: Vladimir

Vladimir — Thu, 18 Oct 2007 09:56:20 +0000

Hi Andrei,

Great Posting thank you very much for sharing this info.
Although the code published has some syntaxes problems and a extra almost at the beginning.
Referring to this topic, I think you are generating GIFs because you want to display transparent images on the a browser and IE6 is not a good player with PNG.
An alternative would be to let .NET generate a PNG8 that’s well accepted by IE4 onwards!
But I couldn’t find a why to make the .NET framework do it, do you know if that’s possible?

Best regards,

Vladimir