C#: Draw one Bitmap onto Another, with Transparency

Question

I have two Bitmaps, named largeBmp and smallBmp. I want to draw smallBmp onto largeBmp, then draw the result onto the screen. SmallBmp's white pixels should be transparent. Here is the code I'm using:

public Bitmap Superimpose(Bitmap largeBmp, Bitmap smallBmp) {     Graphics g = Graphics.FromImage(largeBmp);     g.CompositingMode = CompositingMode.SourceCopy;     smallBmp.MakeTransparent();     int margin = 5;     int x = largeBmp.Width - smallBmp.Width - margin;     int y = largeBmp.Height - smallBmp.Height - margin;     g.DrawImage(smallBmp, new Point(x, y));     return largeBmp; } 

The problem is that the result winds up transparent wherever smallBmp was transparent! I just want to see through to largeBmp, not to what's behind it.

Answer 1

CompositingMode.SourceCopy is the problem here. You want CompositingMode.SourceOver to get alpha blending.