要用常规的方法画出这个图形，就必须根据椭圆的边线公式计算出每条直线的端点。利用复杂的剪裁区域，可以直接画出这些线条，而让Windows确定其端点。CLOVER如程式5-8所示。
程式5-8 CLOVER
CLOVER.C
/*--------------------------------------------------------------------------
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) ;
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)) ;
}
EndPaint (hwnd, &ps) ;
return 0 ;
case WM_DESTROY:
DeleteObject (hRgnClip) ;
PostQuitMessage (0) ;
return 0 ;
}
return DefWindowProc (hwnd, iMsg, wParam, lParam) ;
}
