《windows程序设计》三叶草程序画线部分为何要+0.5？

这就是效果图，我自己实验发现加不加0.5没什么区别，有人说加0.5再强制转换为int是为了四舍五入，我还是不懂这是什么意思

+0.5的部分见代码最后部分

/*--------------------------------------------------
CLOVER.C -- Clover Drawing Program Using Regions
(c) Charles Petzold, 1998
--------------------------------------------------*/

#include <windows.h>
#include <math.h>

#define TWO_PI (2.0 * 3.14159)

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR szCmdLine, int iCmdShow)
{
    static TCHAR szAppName[] = TEXT("Clover");
    HWND         hwnd;
    MSG          msg;
    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("This program requires Windows NT!"),
            szAppName, MB_ICONERROR);
        return 0;
    }

    hwnd = CreateWindow(szAppName, TEXT("Draw a Clover"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hInstance, NULL);

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(&msg, NULL, 0, 0))
    {
        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;

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

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

        for (fAngle = 0.0; fAngle < TWO_PI; fAngle += TWO_PI / 360)
        {
            MoveToEx(hdc, 0, 0, NULL);
            LineTo(hdc, (int)(fRadius * cos(fAngle) +0.5),
                (int)(-fRadius * sin(fAngle) +0.5));
                //就是这两行！！！
        }
        EndPaint(hwnd, &ps);
        return 0;

    case WM_DESTROY:
        DeleteObject(hRgnClip);
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd, iMsg, wParam, lParam);
}

---

c语言中int强制转换，用的是取整，也就是说：

(int)1.4 == 1
(int)1.8 == 1

加了0.5以后则变成了普通的四舍五入的方式来取整：

(int)(1.4 + 0.5) == 1
(int)(1.8 + 0.5) == 2