LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    HDC hdc;
    PAINTSTRUCT ps;
    RECT rect;
    static int cxClient, cyClient;
    HRGN hRgn[6];
    static HRGN hTotRgn;
    double fAngle, fRadius;
    HCURSOR hCursor;

    switch (message)
    {
    case WM_SIZE:
        cxClient 
 
= LOWORD(lParam);
        cyClient = HIWORD(lParam);

        hCursor = SetCursor(LoadCursor(NULL, IDC_WAIT));
        ShowCursor(TRUE);

        //創建四個區域
        //先左右兩個
        hRgn[0]=CreateEllipticRgn(0, cyClient / 3, cxClient / 2, cyClient * 2 / 3);
        hRgn[1] = CreateEllipticRgn(cxClient, cyClient / 3
 
, cxClient / 2, cyClient * 2 / 3);
        //後上下兩個
        hRgn[2] = CreateEllipticRgn(cxClient / 3, 0, cxClient * 2 / 3, cyClient / 2);
        hRgn[3] = CreateEllipticRgn(cxClient / 3, cyClient/2, cxClient * 2 / 3, cyClient);
        //其余的先設為一個較小的區域（空）
        hRgn[4] = CreateEllipticRgn(0, 0, 1, 1);
        hRgn[
 
5] = CreateEllipticRgn(0, 0, 1, 1);
        hTotRgn = CreateEllipticRgn(0, 0, 1, 1);
        //調用combinRgn後原來這些區域會被銷毀

        //先合並左右
        CombineRgn(hRgn[4], hRgn[0], hRgn[1], RGN_OR);
        CombineRgn(hRgn[5], hRgn[2], hRgn[3], RGN_OR);

        //合並為一個總區域句柄
        CombineRgn(hTotRgn, hRgn[4], hRgn[5], RGN_XOR);

        for (int i = 0; i < 6; i++)
            DeleteObject(hRgn[i]);

        SetCursor(hCursor);
        ShowCursor(TRUE);
        break;
    case WM_PAINT:
        hdc = BeginPaint(hwnd, &ps);
        GetClientRect(hwnd, &rect);

        SetViewportOrgEx(hdc, cxClient / 2, cyClient / 2, NULL);
        //選中剪切區域後，會在這個區域作圖
        SelectClipRgn(hdc, hTotRgn);

        FrameRgn(hdc, hTotRgn, (HBRUSH)GetStockObject(BLACK_BRUSH),1,1);

        //內部畫陰影
        fRadius = _hypot(cxClient / 2, cyClient / 2);//直角三角形斜邊

        for (fAngle = 0.0; fAngle < PI * 2;fAngle += PI*2/360)
        {
            MoveToEx(hdc, 0, 0, NULL);
            LineTo(hdc, (int)(fRadius*cos(fAngle) + 0.5), (int)(-fRadius*sin(fAngle) + 0.5));
        }

        EndPaint(hwnd, &ps);
        break;
    case WM_DESTROY:
        DeleteObject(hTotRgn);
        PostQuitMessage(0);
        return 0;
    }


    return DefWindowProc(hwnd, message, wParam, lParam);
}

學習windows編程 day5 之 區域裁剪