#include <GL/glut.h>
#include <math.h>

#define PI 3.1415

void drawCircle(GLdouble x, GLdouble y, GLdouble r, GLint circle_points)
{
    double red = 1.0;
    double green = 0.0;
    double blue = 0.0;
    double alpha = 2 * PI / circle_points;
    glBegin(GL_POLYGON);
    //  glBegin(GL_LINE_LOOP);
    for (int i= 0; i< circle_points; i++)
    {
        glColor3f(red, green, blue);
        red -= 1.0 / circle_points;
        green += 1.0 / circle_points;
        double angle = i * alpha;
        glVertex2f(x + r * cos(angle), y + r * sin(angle));
    }
    glEnd();
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);

    drawCircle(0.5, 0.5, 0.2, 36);
    glFlush();
}

void init(void)
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(650, 650);
    glutInitWindowPosition(670, 330);
    glutCreateWindow("hello");
    init();
    glutDisplayFunc(display);
    glutMainLoop();
	
    return 0;
}