Dual / Multiple Monitor Support in Win32

Introduction

When I was writing the GXTS screensaver for http://www.goheer.com, one of the problems I faced was support for multiple monitors. My goal was to display the same image on all monitors connected to the system regardless of their number.

There are many articles available online relating to multiple monitors, all of which provided no help to me. Somewhere someone is using ::GetSystemMetrics, while another is registering self made classes just to make the multiple monitor thing work. That lead me to go and search for an easy solution as I was not using MFC or any other fancy library. In the end, I was very surprised to find an easy solution in MSDN library. Let's take a look at the solution.

Background

Basic knowledge of Win32 API.

Using the code

Normally, when you are handling the WM_PAINT event, you do this... catch the WM_PAINT and draw whatever you have to in that block.

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

    // Draw your stuff here


    EndPaint(hWnd, &ps);
}
break;

OK, now if we have 2+ monitors and you want to call the paint procedure for each one of it, just use the EnumDisplayMonitors function and supply a callback function where you will be doing painting for each monitor. In this case, I am supplying the callback function MyPaintEnumProc.

case WM_PAINT:
{
    PAINTSTRUCT ps = {0};
    HDC hdcE = BeginPaint(hWnd, &ps );
    // call the function for enumerating all the monitors

    EnumDisplayMonitors(hdcE,NULL, MyPaintEnumProc, 0);

    EndPaint(hWnd, &ps);
}
break;

Now, let's examine the callback function MyPaintEnumProc. By supplying a HDC as first param and NULL as lprect in second argument, the system will call our callback function for each monitor. Every time hMonitor will be the handle to the monitor for which that function is called, hdcl will be the HDC to that monitor, and lprcMonitor will be monitor intersection rectangle. Lastly, in our case, data will be null because we supplied 0 as last argument to EnumDisplayMonitors.

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


    // Draw here now

    return 1;
}

Remember to return true in MyPaintEnumProc to enumerate through all monitors. If you returned false, the enumeration will stop at that point.

Points of Interest

If you think the screenshot of the screensaver "GXTS" is cool then try it and let me know about your comments. It's available free on http://www.goheer.com.

History

First release of this code.