Comments on: Generating a transparent GIF image using C#

By: coder

coder — Fri, 13 Aug 2010 23:12:42 +0000

Thanks again, the source code you posted is very helpful!

By: O_O

O_O — Tue, 01 Jun 2010 22:18:39 +0000

to add:
this algorithm recognizes only colors with the same rgb.

By: O_O

O_O — Tue, 01 Jun 2010 21:21:40 +0000

this code doesn’t work. it doesn’t pick a transparent color in any way that uses at least a bit inteligence, so it’s quite useless… .

By: Reader

Reader — Tue, 11 Aug 2009 16:47:55 +0000

Complete code is useless, and only usefull for those whom have a fix requirements.

By: Salman

Salman — Fri, 13 Feb 2009 05:14:05 +0000

I have had success in creating transparent GIFs in ASP.NET. In my code sample I added a hack for forcing GIF transparency and work around the .NET limitation of a pre-defined non-transparent palette. Have a look at this article:
http://911-need-code-help.blogspot.com/2009/02/create-text-images-on-fly-with-aspnet.html

By: Dave

Dave — Fri, 24 Oct 2008 16:05:43 +0000

Thanks for this! Just what I was looking for. Nice job to whomever created it.
A slight change is required if a color exists twice within a gif color map. I changed the declaration of the transparent index to :
byte? transparentIdx = null;
Then altered the for loop that searched for the index:
for (; i != 0; i—)
{
fin.Read(buf, 0, 3);
if ((buf[0] == R) && (buf[1] == G) && (buf[2] == B))
{
if (transparentIdx == null)
transparentIdx = (byte)(256 – i);
}
fout.Write(buf, 0, 3);
}
if (transparentIdx == null)
transparentIdx = 0;
Finally, I changed the assignment at the bottom to convert the nullable index to a standard byte.
if (count == 4)
{
buf[0] |= 0×01;
buf[3] = (byte)transparentIdx;
}
Again, thanks!

By: janos

janos — Thu, 23 Oct 2008 16:14:32 +0000

i think the orginial author of the code can be found here:
http://www.ben-rush.net/blog/PermaLink.aspx?guid=103ed74d-c808-47ba-b82d-6e9367714b3e&dotnet=consultant

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