画一个圆形

#include <glut.h>
#include <math.h>
const int n = 20;
const GLfloat R = 0.5f;
const GLfloat Pi = 3.1415926536f;

void myDisplay(void) {
    glClear(GL_COLOR_BUFFER_BIT);
    glBegin(GL_POLYGON);
    for (int i = 0; i<n; ++i)
        glVertex2f(R*cos(2 * Pi / n*i), R*sin(2 * Pi / n*i));
    glEnd();

    glFlush();
}

int main(int argc, char *argv[]){
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);
    glutInitWindowPosition(10, 10);
    glutInitWindowSize(400, 400);
    glutCreateWindow("the first opengl program");
    glutDisplayFunc(&myDisplay);
    glutMainLoop();
    return 0;
}