c# Bitmap.Save transparancy doesn't save in png

Question

I'm trying to save a Bitmap class that has transparancy as a png file with transparancy. I'm having no luck.

The bitmap has transparancy, it just doesn't save with transparancy.

this is what I'm doing

bitmap setup

Bitmap ret = new Bitmap(bWidth, bHeight, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

saveing

ret.Save(filename, ImageFormat.Png);

I also tried saving the file with a filestream, and that made no difference.

When the image is in a Picture box the transparancy exists, but when I save i I just get a black background.

I really don't want to use any 3rd party code, they found a way to do it, I'd like to also.

thanks.

Answer 1

Been a while since I've done image editing/saving but if I remember right PNGs are different than most. I think you have to use an actual FileStream.

EDIT: Ah, found an example here

FileStream imageStream= new FileStream( filename, FileMode.Create );
myBitmap.Save( imageStream, ImageFormat.Png );
imageStream.Close();

EDIT2: After more research on this I think the intermediary step is only required under certain circumstances.

It's also possible that because you're using "MakeTransparent" it's catching an indexed alpha, but trying to save based on the actual alpha value of each pixel. You might try actually setting the alpha values of the image.

Answer 2

ret.MakeTransparent(...);

Answer 3

Have you tried using Bitmap.MakeTransparent() method?

Answer 4

basically I assumed that the FilterIndex of a dialog box started at 0...but for some dumb reason it starts at 1, so my images were being saved as Gifs using alpha transparancy, and gif doesn't support alpha transparancy. My bad.

Answer 5

Saving as PNG REQUIRES seekable stream like FileStream or MemoryStream. If you save into one of there and get from there there will be noe GDI+ exception or similar. Hope this helps.