Drawing a Custom Window Background

You can draw your own window background rather than having the system draw it for you. Most applications specify a brush handle or system color value for the class background brush when registering the window class; the system uses the brush or color to draw the background. If you set the class background brush to NULL, however, the system sends a WM_ERASEBKGND message to your window procedure whenever the window background must be drawn, letting you draw a custom background.

In the following example, the window procedure draws a large checkerboard pattern that fits neatly in the window. The procedure fills the client area with a white brush and then draws thirteen 20-by-20 rectangles using a gray brush. The display device context to use when drawing the background is specified in the wParam parameter for the message.

HBRUSH hbrWhite, hbrGray; 
 
  . 
  . 
  . 
 
case WM_CREATE: 
    hbrWhite = GetStockObject(WHITE_BRUSH); 
    hbrGray  = GetStockObject(GRAY_BRUSH); 
    return 0L; 
 
case WM_ERASEBKGND: 
    hdc = (HDC) wParam; 
    GetClientRect(hwnd, &rc); 
    SetMapMode(hdc, MM_ANISOTROPIC); 
    SetWindowExtEx(hdc, 100, 100, NULL); 
    SetViewportExtEx(hdc, rc.right, rc.bottom, NULL); 
    FillRect(hdc, &rc, hbrWhite); 
 
    for (i = 0; i < 13; i++) 
    { 
        x = (i * 40) % 100; 
        y = ((i * 40) / 100) * 20; 
        SetRect(&rc, x, y, x + 20, y + 20); 
        FillRect(hdc, &rc, hbrGray); 
    } 
  return 1L; 
 

If the application draws its own minimized window, the system also sends the WM_ERASEBKGND message to the window procedure to draw the background for the minimized window. You can use the same technique used by WM_PAINT to determine whether the window is minimized — that is, call the IsIconic function and check for the return value TRUE.