Ball Collisions with OpenGL. 1 -> 2 -> 1

balls_gl.cpp

#define GLUT_DISABLE_ATEXIT_HACK
#include <windows.h>
#include <gl\glut.h>
#include <iostream>

float x1=-2.0, x2=2.0;

static int flag=1;
static int i=0;

void reshape(int w,int h)
{
    glViewport(0,0,w,h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45, w/h, 1, 200);
}

void drawBall(float r, float g, float b, float x, float radius)
{
    glColor3f(r, g, b);
    glPushMatrix();
    glTranslatef(x, 0.0, -5.0);
    glutSolidSphere(radius, 20, 20);
    glPopMatrix();    
}

void update()
{
    if(flag)
    {
        x1 += 0.0007;
        x2 -= 0.0007;

        if(x2-x1 < 0.8) 
        { 
            flag=0;
            i=(i+1)%2;
        }        
    }
    else
    {
        x1-=0.0007;
        x2+=0.0007;

        if(x1 < -2.0)
            flag=1;
    }
    
    
}
void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    if (i)
    { 
        drawBall(1.0,1.0,0.0, x1, 0.2); 
        drawBall(1.0,1.0,0.0, x1, 0.2);
        drawBall(1.0,0.0,1.0, x2, 0.2);
        drawBall(1.0,0.0,1.0, x2, 0.2);
    }
    else 
    {
        drawBall(1.0,1.0,0.0, x1, 0.4);
        drawBall(1.0,0.0,1.0, x2, 0.4);
    }
    
    update();
    glutSwapBuffers();
}

int main(int argc,char **argv)
{
    glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);
    glutInitWindowSize(400,400);
    glutCreateWindow("Collision Window");
    glEnable(GL_DEPTH_TEST);
    glutDisplayFunc(display);
    glutIdleFunc(display);
    glutReshapeFunc(reshape);
    glutMainLoop();
    return(0);
}