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.

>>HBITMAP graph = CreateCompatibleBitmap(testbit, 400, 300);

When created a memory DC contains a 1x1 monochrome bitmap regardless of the colour depth of the DC it was created compatible with/from.

Therefore the line above (in your code) creates a monochrome bitmap compatible with the default bitmap already in the testbit DC.



Look up GDI leaks as you have a few bad ones. You must put the default GDI objects back before the DC will delete. Bitmaps may not delete while selected into a DC. A .NET version IDE will fix most of these leaks.

Try moving another window over your app. It will soon fail to redraw. Can watch the GDI object count in the Task Manager (need to add GDI to the colums displayed).



PS
A quicker/better way would be to create the big red rect DC on a WM_CREATE msg, use it in the WM_PAINT msg and clean it up in WM_CLOSE. Make it a static, if local to the callback or global to the window.