winapi：WM_PAINT内具有bRepaint TRUE的GetUpdateRect（）不会清除绘制区域，但是WM_PAINT之外的InvalidateRect（）可以吗？

Question

My custom drawing area's WM_PAINT looks something like this:

// TRUE to clear the background
if (GetUpdateRect(hwnd, &r, TRUE) == 0)
    return; // no update rect; do nothing
dc = BeginPaint(hwnd, &ps);
// check return
// paint some RGBA image data with GDI+
EndPaint(hwnd, &ps);

(Disregard the sending a HDC to GDI+ for now.)

I use GetUpdateRect() here instead of getting the update rect from BeginPaint() because I always want to draw over a blank canvas, especially since I'm doing alpha-mixed drawing here. Furthermore, the docs for WM_PAINT say not to call BeginPaint() if GetUpdateRect() fails.

However, when my custom drawing area actually does this WM_PAINT, it isn't clearing the drawing area. I can tell because the alpha-enabled image is drawn on top of itself, resulting in darker colors.

In another part of my code, I respond to a keyboard event by calling InvalidateRect() to force an update of the whole area. If I change the equivalent of

InvalidateRect(hwnd, NULL, FALSE);

to

InvalidateRect(hwnd, NULL, TRUE);

the area does get cleared properly.

So what exactly AM I doing wrong here? Thanks.

(The actual code is this; it's written in Go. This is also why I am working with GDI+ directly. (I have yet to figure out how to do alpha-mixed stuff with regular GDI.))

Answer 1

Thanks to both of you. Stack Overflow will only let me award one check at a time though... And yep, after doing the clearing properly I got the flicker mentioned in the checked answer.

Ultimately I decided to throw out what I have so far, especially at the advice of Treeki/Ninjifox, who suggested I use the AlphaBlend() function instead. Now, though, I have more problems... but that'll be for another question, with a standalone example program.