如何对现有颜色进行位图透明？

Question

I'm working with Win32. I found here that if you use:

BitBlt(hdc, 0, 0, 32, 32, hdcMem, 0, 0, SRCAND); // hdc has mask in it
BitBlt(hdc, 0, 0, 32, 32, hdcMem, 0, 0, SRCPAINT);`// hdc has regular image in it

you can create a bitmap transparency. My problem is that this relies on the fact that the hdc is already empty on the target location. Mine isn't, so the code erases (makes black) part of the existing image. How do I copy a transparent bitmap to an area that already has stuff drawn on it?

I tried sending both the mask and the image to 1000, 1000 - where there isn't anything drawn in the hdc, and then using SRCCOPY in BitBlt() to copy the result from hdc to hdc. But either nothing got drawn out at 1000, 1000 or it didn't copy back correctly.

Answer 1

You can use TransparentBlt instead.

BOOL TransparentBlt(
  _In_  HDC hdcDest,
  _In_  int xoriginDest,
  _In_  int yoriginDest,
  _In_  int wDest,
  _In_  int hDest,
  _In_  HDC hdcSrc,
  _In_  int xoriginSrc,
  _In_  int yoriginSrc,
  _In_  int wSrc,
  _In_  int hSrc,
  _In_  UINT crTransparent
);

If background color of your source bitmap is black( #000000 ), just set crTransparent as RGB(0,0,0) and it will work.