Обработкой регионов

#include "windows.h"
#include <stdlib.h>
#include <math.h>
#include <stdio.h>
 
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
 
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   LPSTR cmdString, int cmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
 
    WNDCLASSEX wc;
    HWND hWnd;
    MSG msg;
    CHAR CLASS_NAME[] = "CLASS_NAME";
 
    wc.cbClsExtra = 0;
    wc.cbSize = sizeof(wc);
    wc.cbWndExtra = 0;
    wc.hbrBackground = (HBRUSH) GetStockObject(WHITE_BRUSH);
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
    wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
    wc.hInstance = hInstance;
    wc.lpfnWndProc = WndProc;
    wc.lpszClassName = CLASS_NAME;
    wc.lpszMenuName = NULL;
    wc.style = CS_HREDRAW | CS_VREDRAW;
 
    RegisterClassEx(&wc);
 
    hWnd = CreateWindow(CLASS_NAME, "Let's try regions!",
                        WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        NULL, NULL, hInstance, NULL);
 
   ShowWindow(hWnd, cmdShow);
   UpdateWindow(hWnd);
 
    while (GetMessage(&msg, NULL, 0, 0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
 
    return (int) msg.wParam;
}
 
HRGN createCircleRegion(int x0, int y0, int r)
{
    int dr = (int) (r / 1.41);
 
    return CreateEllipticRgn(x0 - dr, y0 - dr, x0 + dr, y0 + dr);
}
 
void setCoordinateSystem(HDC hDC,
                         int xOrg, int yOrg,
                         int viewportExtX, int viewportExtY,
                         int windowExtX, int windowExtY)
{
    SetMapMode(hDC, MM_ANISOTROPIC);
    SetViewportExtEx(hDC, viewportExtX, viewportExtY, NULL);
    SetWindowExtEx(hDC, windowExtX, windowExtY, NULL);
    SetViewportOrgEx(hDC, xOrg, yOrg, NULL);
}
 
LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    HRGN rgns[5];
    static HRGN clipRgn;
    int i, x, y;
    float deg;
    HDC hDC;
    PAINTSTRUCT ps;
    static int cxClient, cyClient, r;
 
    switch (msg)
    {
    case WM_SIZE :
        cxClient = LOWORD(lParam);
        cyClient = HIWORD(lParam);
        r = min(cxClient / 4, cyClient / 4);
 
        if (clipRgn)
            DeleteObject(clipRgn);
 
        rgns[0] = createCircleRegion(0, 0, r);
        rgns[1] = createCircleRegion(-r  * 3 / 4, 0, r / 2);
        rgns[2] = createCircleRegion(0, r * 3 / 4, r / 2);
        rgns[3] = createCircleRegion(r * 3 / 4, 0, r / 2);
        rgns[4] = createCircleRegion(0, -r * 3 / 4, r / 2);
        clipRgn = CreateRectRgn(0, 0, 1, 1);
 
        for (i = 0; i < 5; i++)
        {
            CombineRgn(clipRgn, clipRgn, rgns[i], RGN_OR);
            DeleteObject(rgns[i]);
        }
 
        return 0;
 
    case WM_PAINT :
        hDC = BeginPaint(hWnd, &ps);
 
        setCoordinateSystem(hDC, cxClient / 2, cyClient / 2, 1, 1, 1, -1);
        SelectClipRgn(hDC, clipRgn);
 
    
        for (deg = 0; deg < 360; deg+=5)
        {
            MoveToEx(hDC, 0, 0, NULL);
            x = (int) (r * cos(deg));
            y = (int) (r * sin(deg));
            LineTo(hDC, x, y);
        }
        MoveToEx(hDC, 0, 0 ,NULL);
        LineTo(hDC, 1000, 1000);
 
        EndPaint(hWnd, &ps);
         
        return 0;
 
    case WM_DESTROY :
        DeleteObject(clipRgn);
        PostQuitMessage(0);
        return 0;
    }
 
    return DefWindowProc(hWnd, msg, wParam, lParam);
}