syntax membuat lingkaran di openGL; basic4GL

// test2.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <GL/glut.h>
#include "math.h"



void display()
{
 glClear(GL_COLOR_BUFFER_BIT);
 
 float alpha, radius, cx, cy;
 cx=25;
 cy=25;
 alpha = 0;
 radius = 20;

 glBegin(GL_LINE_STRIP);
 {
  for(float i=0; i<=360; i++)
  {
   alpha+=1;
   glVertex2f(radius * cos (alpha/180 * 3.1415) + cx, radius * sin(alpha/180 * 3.1415) + cy);
  }
 }
 glEnd();
 glFlush();
}

void myinit()
{
 glMatrixMode(GL_PROJECTION);
 glLoadIdentity();
 gluOrtho2D(0.0, 50.0, 0.0, 50.0);
 glMatrixMode(GL_MODELVIEW);
 glClearColor(1.0,1.0,1.0,1.0);
 glColor3f(0.0,0.0,0.0);
}

int main(int argc, char* argv[])
{
 glutInit(&argc,argv);
 glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
 glutInitWindowSize(500,500);
 glutInitWindowPosition(100,100);
 glutCreateWindow("testing1");
 glutDisplayFunc(display);
 myinit();
 glutMainLoop();
 
 return 0;
}