一个LineTo函数画出两条线段

Question

经典的《windows程序设计》第五版第五章“Rectangles， Regions， Clipping”， 有一个演示Clipping的例子程序 clover.c。这个程序运行出现奇怪问题。 下面是我对该例子稍加改动后的程序。很明显， 一条LineTo函数调用出现了两条线段(窗口最大化后最明显）。 关闭win7 “桌面组合”后, 这个问题消失， 打开“桌面组合” 问题出现。 这是不是一个bug? 

#include<windows.h>

#include<math.h>

#define TWO_PI (2.0 * 3.14159)

LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)

{

    static TCHAR szAppName[] = TEXT("Clover");

    static TCHAR szWndName[] = TEXT("Draw a clover");

    BOOL bRet;

    MSG msg;

    HWND hwnd;

    WNDCLASS wndclass;

    wndclass.style = CS_HREDRAW | CS_VREDRAW;

    wndclass.lpfnWndProc = WndProc;

    wndclass.cbClsExtra = 0;

    wndclass.cbWndExtra = 0;

    wndclass.hInstance = hInstance;

    wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);

    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);

    wndclass.hbrBackground = (HBRUSH) GetStockObject(WHITE_BRUSH);

    wndclass.lpszMenuName = NULL;

    wndclass.lpszClassName = szAppName;

    if (!RegisterClass(&wndclass)) {

        MessageBox(NULL, TEXT("Program requires Windows NT!"), szAppName, MB_ICONERROR);

        return FALSE;

    }

    hwnd = CreateWindow(szAppName, szWndName, WS_OVERLAPPEDWINDOW,

        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, hInstance, NULL);

    if (!hwnd) {

        MessageBox(NULL, TEXT("Can not create window!"), szAppName, MB_ICONERROR);

        return FALSE;

    }

    ShowWindow(hwnd, nCmdShow);

    UpdateWindow(hwnd);

    while (bRet = GetMessage(&msg, NULL, 0, 0)) {

        if (bRet == -1 ) {

            MessageBox(NULL, TEXT("Can not get message!"), szAppName, MB_ICONERROR);

            return FALSE;

        }               

        TranslateMessage(&msg);

        DispatchMessage(&msg);

    }

    return msg.wParam;

}

LRESULT CALLBACK WndProc(HWND hwnd, UINT iMsg, WPARAM wParam, LPARAM lParam)

{

    static HRGN hRgnClip;

    static int cxClient, cyClient;

    double fAngle, fRadius;

    HCURSOR hCursor;

    HDC hdc;

    HRGN hRgnTemp[6];

    int i;

    PAINTSTRUCT ps;

    HBRUSH hbr;

    static TCHAR szBuffer [50];

    switch (iMsg) {

    case WM_SIZE:

        cxClient = LOWORD(lParam);

        cyClient = HIWORD(lParam);

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

        ShowCursor(TRUE);

        if (hRgnClip)

            DeleteObject(hRgnClip);

        

        hRgnTemp[0] = CreateEllipticRgn(0, cyClient / 3, cxClient / 2, 2 * cyClient / 3);

        hRgnTemp[1] = CreateEllipticRgn(cxClient / 2, cyClient / 3, cxClient, 2 * cyClient / 3);

        hRgnTemp[2] = CreateEllipticRgn(cxClient / 3, 0, 2 * cxClient / 3, cyClient / 2);

        hRgnTemp[3] = CreateEllipticRgn(cxClient / 3, cyClient / 2, 2 * cxClient / 3, cyClient);

        hRgnTemp[4] = CreateRectRgn(0, 0, 1, 1);

        hRgnTemp[5] = CreateRectRgn(0, 0, 1, 1);

        hRgnClip = CreateRectRgn(0, 0, 1, 1);

        CombineRgn(hRgnTemp[4], hRgnTemp[0], hRgnTemp[1], RGN_OR);

        CombineRgn(hRgnTemp[5], hRgnTemp[2], hRgnTemp[3], RGN_OR);

        CombineRgn(hRgnClip, hRgnTemp[4], hRgnTemp[5], RGN_XOR);

        for (i = 0; i < 6; i++)

            DeleteObject(hRgnTemp[i]);

        SetCursor(hCursor);

        ShowCursor(FALSE);

        return 0;

    case WM_PAINT:

        hdc = BeginPaint(hwnd, &ps);

        hbr = GetStockObject(GRAY_BRUSH);

        FillRgn(hdc, hRgnClip, hbr);

        DeleteObject(hbr);

        SetViewportOrgEx(hdc, cxClient / 2, cyClient / 2, NULL);

        SelectClipRgn(hdc, hRgnClip);

        fRadius = hypot(cxClient / 2.0, cyClient / 2.0);

        fAngle = 3 * TWO_PI / 16;

        //for (fAngle = 0.0; fAngle < TWO_PI; fAngle += TWO_PI / 360) {

            MoveToEx(hdc, 0, 0, NULL);

//          LineTo(hdc,(int) (fRadius * cos(fAngle) + 0.5) / 3, (int) (-fRadius * sin(fAngle) + 0.5) / 3);

            LineTo(hdc, (int) (fRadius * cos(fAngle) + 0.5), (int) (-fRadius * sin(fAngle) + 0.5));

//          LineTo(hdc, 0, 0);

            //TextOut(hdc, 200, 0, szBuffer, wsprintf(szBuffer, TEXT("x = %d, y = %d"),

            //  (int) (fRadius * cos(fAngle) + 0.5), (int) (-fRadius * sin(fAngle) + 0.5) ));

        //  }

        EndPaint(hwnd, &ps);

        return 0;

    case WM_DESTROY:

        DeleteObject(hRgnClip);

        PostQuitMessage(0);

        return 0;

    }

    return DefWindowProc(hwnd, iMsg, wParam, lParam);

}