Draw shape to alpha channel only

Question

I want to draw a shape to the alpha channel only of my image. I can use a colour matrix when drawing an image with Graphics.DrawImage(..), as it has a parameter to pass in the ImageAttributes. Is it possible to do this with the FillEllipse method? Or would I have to draw it to a separate image, then use DrawImage to apply it to my main image?

Cheers, Dan.

Answer 1

You may try something like:

Graphics g;
g.CompositionMode = CompositingMode.SourceOver;

then fill with:

Color c = Color.FromArgb(100, Color.White);

Answer 2

I wrote a blog post on this a couple of years ago:

http://danbystrom.se/2008/08/24/soft-edged-images-in-gdi/

In short: you have to first draw your shape to a separate bitmap and then transfer one of its RGB channels over to the destination's alpha channel.