Drawing Circle with OpenGL

Question

I'm trying to draw simple circle with C++/OpenGl

my code is:

#include <GL/glut.h> #include <math.h>  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 < 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(&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;  } 

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!

Answer 1

It looks like immediately after you draw the circle, you go into the main glut loop, where you've set the Draw() function to draw every time through the loop. So it's probably drawing the circle, then erasing it immediately and drawing the square. You should probably either make DrawCircle() your glutDisplayFunc(), or call DrawCircle() from Draw().