title: Drawing Circle with OpenGL
body: <p>I'm trying to draw simple circle with C++/OpenGl</p>
  
  <p>my code is:</p>
  
  <pre><code>#include &lt;GL/glut.h&gt;
  #include &lt;math.h&gt;
  
  void Draw() {
      glClear(GL_COLOR_BUFFER_BIT);
      glColor3f(1.0, 1.0, 1.0);
  
      glBegin(GL_QUADS);
        glColor3f (0.0, 0.0, 0.0);
  
        glVertex3f (0.1, 0.1, 0.0);
        glVertex3f (0.9, 0.1, 0.0);
        glVertex3f (0.9, 0.9, 0.0);
        glVertex3f (0.1, 0.9, 0.0);
  
      glEnd();
  
      glFlush();
  }
  
  void DrawCircle(float cx, float cy, float r, int num_segments)
  {
      glBegin(GL_LINE_LOOP);
      for(int ii = 0; ii &lt; num_segments; ii++)
      {
          float theta = 2.0f * 3.1415926f * float(ii) / float(num_segments);//get the current angle
  
          float x = r * cosf(theta);//calculate the x component
          float y = r * sinf(theta);//calculate the y component
  
          glVertex2f(x + cx, y + cy);//output vertex
  
      }
      glEnd();
  }
  
  
  void Initialize() {
      glClearColor(1.0, 1.0, 1.0, 0.0);
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
      glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
  }
  
  int main(int iArgc, char** cppArgv) {
      glutInit(&amp;iArgc, cppArgv);
      glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
      glutInitWindowSize(950, 500);
      glutInitWindowPosition(200, 200);
      glutCreateWindow("Universum");
      Initialize();
      glutDisplayFunc(Draw);
      glClear(GL_COLOR_BUFFER_BIT);
      glColor3f(1.0, 1.0, 1.0);
      DrawCircle(0.5, 0.5, 0.2, 5);
  
      glutMainLoop();
      return 0;
  
  }
  </code></pre>
  
  <p>I'm beginner with OpenGL and now i'm starting to learn, 
  Can someone please explain me why i don't get the circle (i only see the black box), 
  Thanks!</p>