Bitmap.Make​Transparent

System.Drawing.Bitmap outBmp;
string FilePath = MapPath(Request.QueryString["File"]);
sigImg = System.Drawing.Image.FromFile(FilePath);
outBmp = new System.Drawing.Bitmap(sigImg);

// Sending Response GIF type to the browser.
Response.ContentType = "image/gif";
outBmp.MakeTransparent(System.Drawing.Color.White);//(outBmp.GetPixel(1,1));

outBmp.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif);

// Disposing the objects.
outBmp.Dispose();

Why would the resulting image have black wherever white appears in the image?

qwert231 wrote:



System.Drawing.Bitmap outBmp;
string FilePath = MapPath(Request.QueryString["File"]);
sigImg = System.Drawing.Image.FromFile(FilePath);
outBmp = new System.Drawing.Bitmap(sigImg);

// Sending Response GIF type to the browser.
Response.ContentType = "image/gif";
outBmp.MakeTransparent(System.Drawing.Color.White);//(outBmp.GetPixel(1,1));

outBmp.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif);

// Disposing the objects.
outBmp.Dispose();

Why would the resulting image have black wherever white appears in the image?

Just a guess (seriously, a guess), but does


MakeTransparent(System.Drawing.Color.White);
 
make everything that matches System.Drawing.Color.White in the image transparent?

And if so, is your background (behind the image) black

The background is gray. The image is Red and White. All the white in the image turns to black.

qwert231 wrote:

The background is gray. The image is Red and White. All the white in the image turns to black.

Is the Transparent key color part of the GIF's color pallette? If not, it isn't going to work and GDI+ will just use the closest color. Are you using any form of quantization on the image either?

Save the image to disk, then open it in Photoshop (in Indexed Color mode) and take a look at the color table. If the Transparency Key color doesn't turn up, you're not quantizing the image properly.

This is a limitation of GDI+ library. You have to manually change a particular color in the palette to make it transparent.

See this entry in the GDIPlus FAQ.

See
http://www.nathansokalski.com/code/TransparencyClass.aspx
It has a class used to add transparency to a System.Drawing.Bitmap object