Circles in the GL/OpenGL

Source code:

#include<GL/freeglut.h>

#include <math.h>

void drawCircle(float cx, float cy, float r, int num_segments) {

      

       const float PI = 3.14159;

       glBegin(GL_TRIANGLE_FAN);

       for (int i = 0; i < num_segments; i++)  {

             

              float theta = i * (2.0f * PI / num_segments);

              float x = r * cos(theta);

              float y = r * sin(theta);

              glVertex2f(x + cx, y + cy);

       }

       glEnd();

}

void display(void) {

      

       glClear(GL_COLOR_BUFFER_BIT);

       drawCircle(0.0, 0.0, 0.5, 36);

      

       glColor3b(2.0, 2.0, 2.0);

       drawCircle(0.0, 0.0, 0.4, 36);

       glColor3b(100.0, 100.0, 100.0);

       drawCircle(0.0, 0.0, 0.4, 36);

      

       glColor3b(100.0, 200.0 , 100.0);

       drawCircle(0.0, 0.0, 0.3, 36);

       glColor3b(200.0, 100.0, 100.0);

       drawCircle(0.0, 0.0, 0.2, 36);

       glColor3b(200.0, 100.0, 200.0);

       drawCircle(0.0, 0.0, 0.2, 36);

      

       glEnd();

       glFlush();

}

//

void init(void) {

       // select background color

       glClearColor(0.0, 0.0, 0.0, 0.0); // (red, green, blue, alpha), used by glClear

       // Define world coordinate frame

       glMatrixMode(GL_PROJECTION);

       glLoadIdentity();

       glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); // model in real word units

       // (left, right, bottom, top, near, far)

}

int main(int argc, char** argv) {

       //Step#1 Create and initialize glut windowing system

       glutInit(&argc, argv);

       glutInitWindowSize(600, 400);

       glutInitWindowPosition(100, 100);

       glutCreateWindow("Let's Programming");

       //Step#2 Do Open GL related initializations

       init();

       //Step#3 Register Call back methods or Event Handler methods

      

       glutDisplayFunc(display);

       //Step#4 Enter in  main loop (an infinite loop listening to registered events and then call their registered methods if any)

       glutMainLoop();

       return 0;

}