Added support for mouse-wheel events on Windows.

To be used in the mouse callback like this:
    if (CV_EVENT_MOUSEWHEEL == CV_GET_MOUSEWHEEL_EVENT(event))
    {
       int delta= CV_GET_WHEEL_DELTA(event);
       // use delta...
    }

modules/highgui/include/opencv2/highgui/highgui_c.h

@@ -170,7 +170,9 @@ enum
     CV_EVENT_MBUTTONUP      =6,
     CV_EVENT_LBUTTONDBLCLK  =7,
     CV_EVENT_RBUTTONDBLCLK  =8,
-    CV_EVENT_MBUTTONDBLCLK  =9
+    CV_EVENT_MBUTTONDBLCLK  =9,
+    CV_EVENT_MOUSEWHEEL     =10,
+    CV_EVENT_MOUSEHWHEEL    =11
 };
 
 enum
@@ -183,6 +185,11 @@ enum
     CV_EVENT_FLAG_ALTKEY    =32
 };
 
+
+#define CV_GET_WHEEL_DELTA(event) ((short)((event >> 16) & 0xffff)) // upper 16 bits
+#define CV_GET_MOUSEWHEEL_EVENT(event) (event & 0xffff) // lower 16 bits
+
+
 typedef void (CV_CDECL *CvMouseCallback )(int event, int x, int y, int flags, void* param);
 
 /* assign callback for mouse events */

modules/highgui/src/window_w32.cpp

@@ -40,6 +40,7 @@
 //M*/
 
 #include "precomp.hpp"
+#include <windowsx.h> // required for GET_X_LPARAM() and GET_Y_LPARAM() marco
 
 #if defined WIN32 || defined _WIN32
 
@@ -1377,6 +1378,43 @@ MainWindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam )
             SetFocus(window->hwnd);
         break;
 
+    case WM_MOUSEWHEEL:
+    case WM_MOUSEHWHEEL:
+       if( window->on_mouse )
+       {
+          int flags = (wParam & MK_LBUTTON      ? CV_EVENT_FLAG_LBUTTON  : 0)|
+                      (wParam & MK_RBUTTON      ? CV_EVENT_FLAG_RBUTTON  : 0)|
+                      (wParam & MK_MBUTTON      ? CV_EVENT_FLAG_MBUTTON  : 0)|
+                      (wParam & MK_CONTROL      ? CV_EVENT_FLAG_CTRLKEY  : 0)|
+                      (wParam & MK_SHIFT        ? CV_EVENT_FLAG_SHIFTKEY : 0)|
+                      (GetKeyState(VK_MENU) < 0 ? CV_EVENT_FLAG_ALTKEY   : 0);
+          int event = (uMsg == WM_MOUSEWHEEL    ? CV_EVENT_MOUSEWHEEL    : CV_EVENT_MOUSEHWHEEL);
+
+          // Set the wheel delta of mouse wheel to be in the upper word of 'event'
+          int delta = GET_WHEEL_DELTA_WPARAM(wParam);
+          event |= (delta << 16);
+
+          POINT pt;
+          {
+             // since The coordinates are relative to screen so get screen size.
+             RECT windowRect;
+             ::GetWindowRect( window->hwnd, &windowRect );
+             pt.x = GET_X_LPARAM( lParam ) - windowRect.left;
+             pt.y = GET_Y_LPARAM( lParam ) - windowRect.top;
+          }
+
+          RECT rect;
+          GetClientRect( window->hwnd, &rect );
+
+          SIZE size = {0,0};
+          icvGetBitmapData( window, &size, 0, 0 );
+
+          window->on_mouse( event, pt.x*size.cx/MAX(rect.right - rect.left,1),
+                                   pt.y*size.cy/MAX(rect.bottom - rect.top,1), flags,
+                                   window->on_mouse_param );
+       }
+       break;
+
     case WM_ERASEBKGND:
         {
             RECT cr, tr, wrc;