WM_PAINT message and EnumDisplayMonitors

Question

I'm trying to adapt a screensaver written in C++ with WinAPIs to work for multiple monitors. I found this article that suggests to rewrite this basic WM_PAINT handler:

case WM_PAINT:
{
    PAINTSTRUCT ps = {0};
    HDC hdc = BeginPaint(hWnd, &ps );

    DoDrawing(hdc, ps.rcPaint);

    EndPaint(hWnd, &ps);
}
break;

void DoDrawing(HDC hDC, RECT rcDraw)
{
    //Do actual drawing in 'hDC'
}

Into something like this to incorporate drawing for multiple screens:

case WM_PAINT:
{
    PAINTSTRUCT ps = {0};
    HDC hdcE = BeginPaint(hWnd, &ps );

    EnumDisplayMonitors(hdcE,NULL, MyPaintEnumProc, 0);

    EndPaint(hWnd, &ps);
}
break;

BOOL CALLBACK MyPaintEnumProc(
      HMONITOR hMonitor,  // handle to display monitor
      HDC hdc1,     // handle to monitor DC
      LPRECT lprcMonitor, // monitor intersection rectangle
      LPARAM data       // data
      )
{
    RECT rc = *lprcMonitor;
    // you have the rect which has coordinates of the monitor

    DoDrawing(hdc1, rc);

    // Draw here now
    return 1;
}

But the question I have is what about special optimization/clipping that BeginPaint() sets up in the DC after processing WM_PAINT message? With this approach it will be lost. Any idea how to preserve it across the EnumDisplayMonitors() call?

Answer 1

The answer is actually described in the MSDN docs for EnumDisplayMonitors . When you pass an HDC parameter to EnumDisplayMonitors, then the DC it passes to your callback function is a subset of the DC that you originally passed in with these changes:

• The clip has been further reduced to only cover the intersection of the original clip with the monitor's clip rectangle.
• The color format of the DC is for the specific monitor rather than for the "primary" monitor of the window.

Note that in modern Windows (at least since Win8), you will never really see different color formats in practice for Window DCs since GDI always runs with 32-bit color.