Using AlphaBlend() and FillRect()

Question

So, I'm using AlphaBlend() to copy a rectangle from one HBITMAP to another.

It works, but there is a problem. Whenever I use the FillRect() function, the alpha values in the HBITMAP get slammed out to 0. Everytime.

So I have to GetDIBits(), reset the alpha back to 255, and then SetDIBits(), after every call to the Win32 api functions like FillRect().

So, is there a way to create an HBRUSH or otherwise tell FillRect() not to touch the alpha channel values in the HBITMAP it is going to be drawing to?

Answer 1

With the exception of AlphaBlend... BitBlt is the only other GDI function that will preserve the alpha channel in any way.

Your options basically therefore are:

1. Switch to using DIBSections. This will not solve the basic problem of GDI apis overwiting the alpha channel, but as a DIBSection you can avoid the costly DDB -> DIB -> DDB transformation needed to patch up the alpha channel. DIBSections give you access to both an HBITMAP and a memory section where the bitmaps bits are stored.

2. Use an alpha aware painting API like GdiPlus instead of GDI.