Thread: BitBlt turns entire image black?

Ok, I have the following code in my window processes function (where you would expect it in the switch-case block):

Code:

case WM_PAINT:
      {
         PAINTSTRUCT Ps;
         HDC hDC = BeginPaint(hwnd, &Ps);

        HBITMAP graph = CreateCompatibleBitmap(hDC, 400, 300);
         int x, y;
        SelectObject(hDC, graph);
        for( x = 0; x < 400; x++)
        for( y = 0; y < 300; y++)
        SetPixel(hDC, x, y, RGB(255, 0, 0));

		EndPaint(hwnd, &Ps);
         break;
       }

And it does exactly what I expect it to, draw a big red rectangle in the window.

However, if I do the following:

Code:

case WM_PAINT:
      {
         PAINTSTRUCT Ps;
         HDC hDC = BeginPaint(hwnd, &Ps);
         HDC testbit = CreateCompatibleDC(hDC);

        HBITMAP graph = CreateCompatibleBitmap(testbit, 400, 300);
         int x, y;
        SelectObject(testbit, graph);
        for( x = 0; x < 400; x++)
        for( y = 0; y < 300; y++)
        SetPixel(testbit, x, y, RGB(255, 0, 0));

         BitBlt(hDC,0,0,400,300,testbit,0,0,SRCCOPY);
         
      DeleteDC(testbit);
		EndPaint(hwnd, &Ps);
         break;
      }

The rectangle I get is black. Can anybody tell me why this is?

Ok, so I changed a line in the non-functioning section to

Code:

HBITMAP graph = CreateCompatibleBitmap(hDC, 400, 300);

and it seems to work fine. I guess it makes sense that I should be making it compatible with the window's device context itself, but I would think that compatibility would be transitive in that if Bitmap A is transitive with Device Context B, and Device Context B is compatible with Device Context C, A would be just as compatible with C as it is with B. Could anyone explain exactly why it is that way?

My guess is that while Device Context B is compatible, it's still a memory device context, but Device Context A is a screen device context.