Redirecting Window Output

[iPick12]

Hi, I'm trying to render the contents of a window, with desktop composition disabled, on an off-screen buffer. I have no control over the window and the only thing I have is the window handle and the DC. I thought about allocating an off-screen DC and doing something like:

/* The window handle is stored in the
hWnd variable and the offscreen DC,
is in hDC */
PostMessage(hWnd, WM_ERASEBKGND,
(WPARAM)hMemDC, 0);
PostMessage(hWnd, WM_PAINT, 0, 0);

But it doesn't seem to work. Please note that I don't want the application to render on the primary display DC. In other words I want to redirect it's graphical output, and blit the result in my window.