void start() {
        std::chrono::time_point<std::chrono::system_clock> startTime=std::chrono::system_clock::now();

        //Event thread (main thread)
        bool running = true;
        while (running) {
            sf::Event event;
            while (window.pollEvent(event)) {
                if (event.type == sf::Event::KeyPressed) {
                    if (sf::Keyboard::isKeyPressed(sf::Keyboard::Escape)) {
                        window.close();
                        running=false;
                    }
                }
                else if (event.type == sf::Event::Closed) {
                    window.close();
                    running = false;
                }
            }
            
            std::chrono::duration<double> elapsed_seconds = std::chrono::system_clock::now() - startTime;
            double time=elapsed_seconds.count();
            
            glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
            glLoadIdentity();
        
            gluLookAt(0.0, 30.0, -65.0, //Camera position in World Space
                      0.0, 5.0, 0.0,    //Camera looks towards this position
                      0.0, 1.0, 0.0);   //Up
        
            glPushMatrix();
            glRotatef(-time*50.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            unanimatedAstroBoy.draw();
            glPopMatrix();
            
            glPushMatrix();
            glRotatef(-time*50.0+90.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoy.drawFrame(time);
            glPopMatrix();
            
            glPushMatrix();
            glRotatef(-time*50.0+180.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoyMovingGlasses.drawFrame(time);
            glPopMatrix();
            
            glRotatef(-time*50.0+270.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoyHeadBanging.drawFrame(time);
            
            //Swap buffer (show result)
            window.display();
        }
    }
    void draw() {
        //Activate the window's context
        window.setActive();

        //Various settings
        glClearColor(0.5, 0.5, 0.5, 0.0f);
        glShadeModel(GL_SMOOTH);
        glEnable(GL_DEPTH_TEST);
        glDepthMask(GL_TRUE);
        glClearDepth(1.0f);
        glDepthFunc(GL_LEQUAL);
        glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
        glEnable(GL_TEXTURE_2D);
        
        //Lighting
        GLfloat light_color[] = {0.9, 0.9, 0.9, 1.f};
        glMaterialfv(GL_FRONT, GL_DIFFUSE, light_color);
        glEnable(GL_LIGHTING);
        glEnable(GL_LIGHT0);
        
        //Setup projection matrix
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        GLfloat ratio = float(window.getSize().x) / window.getSize().y;
        glFrustum(-ratio, ratio, -1.0, 1.0, 1.0, 500.0);
        
        //Store start time
        std::chrono::time_point<std::chrono::system_clock> startTime=std::chrono::system_clock::now();
        
        //The rendering loop
        glMatrixMode(GL_MODELVIEW);
        while (window.isOpen()) {
            std::chrono::duration<double> elapsed_seconds = std::chrono::system_clock::now() - startTime;
            double time=elapsed_seconds.count();
            
            glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
            glLoadIdentity();

            glTranslatef(0.0f, -20.0f, -45.0f);

            glPushMatrix();
            glRotatef(-time*50.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            unanimatedAstroBoy.draw();
            glPopMatrix();
            
            glPushMatrix();
            glRotatef(-time*50.0+90.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoy.drawFrame(time);
            glPopMatrix();
            
            glPushMatrix();
            glRotatef(-time*50.0+180.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoyMovingGlasses.drawFrame(time);
            glPopMatrix();
            
            glRotatef(-time*50.0+270.0, 0.0, 1.0, 0.0);
            glTranslatef(20.0, 0.0, 0.0);            
            astroBoyHeadBanging.drawFrame(time);
            
            //Swap buffer (show result)
            window.display();
        }
    }