How do I create a bitmap with an alpha channel on the fly using GDI?

Question

I am using layered windows and drawing a rounded rectangle on the screen. However, I'd like to smooth out the jagged edges. I think that I'll need alpha blending for this. Is there a way I can do this with GDI?

Answer 1

CreateDIBSection. Fill in the BITMAPINFOHEADER with 32bpp. Fill in the alpha channel with pre-multiplied alpha and youre good to go.

AlphaBlend is the API to actually blit 32 bpp bitmaps with an aplha channel.