Can't seem to make double-buffered painting with multiple monitors - c++

Question

I'm trying to remake my Windows screensaver written with C++ and WinAPIs to work on multiple monitors. I found this article that gives the basics. But when I implement it in my own code, I get a weird result. Take a look at this code:
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
)
{
MONITORINFO mi = {0};
mi.cbSize = sizeof(mi);
if(GetMonitorInfo(hMonitor, &mi))
{
//Is it a primary monitor?
BOOL bPrimary = mi.dwFlags & MONITORINFOF_PRIMARY;
DoDrawing(bPrimary, hdc1, &mi.rcMonitor);
}
return 1;
}
void DoDrawing(BOOL bPrimaryMonitor, HDC hDC, RECT* pRcMonitor)
{
//#define DIRECT_PAINT //Comment out for double-buffering
int nMonitorW = abs(pRcMonitor->right - pRcMonitor->left);
int nMonitorH = abs(pRcMonitor->bottom - pRcMonitor->top);
HDC hMemDC = ::CreateCompatibleDC(hDC);
if(hMemDC)
{
HBITMAP hMemBmp = ::CreateCompatibleBitmap(hDC, nMonitorW, nMonitorH);
if(hMemBmp)
{
HBITMAP hOldBmp = (HBITMAP)SelectObject(hMemDC, hMemBmp);
COLORREF clr, clrBorder;
if(bPrimaryMonitor)
{
clr = RGB(0, 128, 0); //Green
clrBorder = RGB(255, 0, 0);
}
else
{
clr = RGB(128, 0, 0); //Red
clrBorder = RGB(0, 255, 0);
}
RECT rcRect;
#ifndef DIRECT_PAINT
//With double-buffering
rcRect.left = 0;
rcRect.top = 0;
rcRect.right = nMonitorW;
rcRect.bottom = nMonitorH;
#else
rcRect = *pRcMonitor;
#endif
HBRUSH hBrush = ::CreateSolidBrush(clr);
#ifndef DIRECT_PAINT
//With double-buffering
::FillRect(hMemDC, &rcRect, hBrush);
#else
::FillRect(hDC, &rcRect, hBrush);
#endif
#ifndef DIRECT_PAINT
//With double-buffering
::BitBlt(hDC, pRcMonitor->left, pRcMonitor->top, nMonitorW, nMonitorH, hMemDC, 0, 0, SRCCOPY);
#endif
//Debugging output
CString _s;
_s.Format(_T("%s\n")
_T("%s\n")
_T("hDC=0x%X\n")
_T("hMemDC=0x%X\n")
_T("RcMonitor: L=%d, T=%d, R=%d, B=%d")
,
bPrimaryMonitor ? _T("Primary") : _T("Secondary"),
#ifndef DIRECT_PAINT
_T("Double-buffering"),
#else
_T("Direct paint"),
#endif
hDC,
hMemDC,
pRcMonitor->left,
pRcMonitor->top,
pRcMonitor->right,
pRcMonitor->bottom);
::DrawText(hDC, _s, _s.GetLength(), pRcMonitor, DT_NOCLIP | DT_NOPREFIX);
SelectObject(hMemDC, hOldBmp);
::DeleteObject(hMemBmp);
}
::DeleteDC(hMemDC);
}
}
Painting always works on a primary monitor. But when I paint to the secondary monitor, I can only paint directly to its DC. When I use double-buffering technique (with DIRECT_PAINT pre-processor directive commented out) I only get a black screen on a secondary monitor when it should've been red.
I'm attaching two screenshots here.
First one with direct painting that works:
And then the one with double-buffering that fails:
Any ideas what am I doing wrong here?

Answer 1

Replace the code for WM_PAINT from
case WM_PAINT:
hdc = BeginPaint(hWnd, &ps);
EnumDisplayMonitors(hdc, NULL, MyPaintEnumProc, 0);
EndPaint(hWnd, &ps);
with
case WM_PAINT:
hdc = GetDC(NULL);
EnumDisplayMonitors(hdc, NULL, MyPaintEnumProc, 0);
ReleaseDC(NULL, hdc);
and it will work.
See this http://msdn.microsoft.com/en-us/library/windows/desktop/dd162610(v=vs.85).aspx.