Операция объединения для графических примитивов

#include <Windows.h>
#include <tchar.h>
 
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);
 
int APIENTRY _tWinMain(HINSTANCE hInstance, HINSTANCE hPrev, LPTSTR cmd, int mode)
{
    const TCHAR szClassMainName[] = _T("Main");
    HWND  hWnd;
    MSG   msg;
    WNDCLASS wc;
 
    wc.style            = CS_VREDRAW | CS_HREDRAW;
    wc.lpfnWndProc      = WndProc;
    wc.cbClsExtra       = 0;
    wc.cbWndExtra       = 0;
    wc.hInstance        = hInstance;
    wc.hIcon            = LoadIcon(NULL, IDI_APPLICATION);
    wc.hCursor          = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground    = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wc.lpszMenuName     = NULL;
    wc.lpszClassName    = szClassMainName;
 
    if (!RegisterClass(&wc))
        return 1;
 
    hWnd = CreateWindow(szClassMainName, _T(""), WS_OVERLAPPEDWINDOW, 0, 0, 500, 500, NULL, NULL, hInstance, NULL);
 
    if (!hWnd)
      return 1;
 
    ShowWindow(hWnd, mode);
    UpdateWindow(hWnd); 
 
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
 
    return 0;
}
 
 
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    HDC  hDC;
    PAINTSTRUCT ps;
 
    static int   clientWidth,  clientHeight;
    static HPEN   pen = NULL;
    static HBRUSH  brush = NULL;
 
    RECT  rectangle = {-50, 50, 50, -50};
    RECT  circle = {-20, 20, 150, -150};
    HRGN  region1, region2;
 
    switch (message)
    {
        case WM_CREATE:
            pen   = CreatePen(PS_INSIDEFRAME, 1, RGB(0,128,0));
            brush = CreateSolidBrush(RGB(0,0,128));
            break;
 
        case WM_SIZE:
            clientWidth  = LOWORD(lParam);
            clientHeight = HIWORD(lParam);
            break;
 
        case WM_PAINT:
            hDC = BeginPaint(hWnd, &ps);
 
            SetMapMode(hDC, MM_ANISOTROPIC);
            SetWindowExtEx(hDC, clientWidth, -clientHeight, NULL);
            SetViewportExtEx(hDC, clientWidth, clientHeight, NULL);
            SetViewportOrgEx(hDC, clientWidth/2, clientHeight/2, NULL);
 
            SelectObject(hDC, pen);
            SelectObject(hDC, brush);
 
            region1 = CreateRectRgnIndirect(&rectangle);
            region2 = CreateEllipticRgnIndirect(&circle);
            CombineRgn(region1, region1, region2, RGN_XOR);
            FillRgn(hDC, region1, brush);
 
            DeleteObject(region1);
            DeleteObject(region2);
            EndPaint(hWnd, &ps);
            break;
 
        case WM_DESTROY:
            DeleteObject(pen);
            DeleteObject(brush);
            PostQuitMessage(0);
            break;
 
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}