Moving a Sphere Around a Ellipse

Question

I am trying to write a simple program that moves a Sphere around a Ellipse in OpenGL. I thought that if I set translate coordinates to the same as the Ellipse coordinates it would simulate motion.

This is the code I already have:

#include "stdafx.h"
#include <windows.h>
#include <GL/Gl.h>
#include <GL/GLU.h>
#include <GL/glut.h>
#include <math.h>

const float eRad = 6.5;

void drawSphere(void)
{
  float x = 0.5;
  float y = 0.4;
  float z = 0.0;
  glMatrixMode(GL_PROJECTION | GL_MODELVIEW);
  glLoadIdentity();
  glClear(GL_COLOR_BUFFER_BIT);                 


// Draw the Ellipse
glColor3d(1.0, 0.0, 1.0);
glBegin(GL_LINE_STRIP);
for (float i = 0; i <= eRad; i += 0.1)
{
    glVertex2f(x*cos(i), y*sin(i));
}
glEnd();

//Draw the Sphere
glColor3d(1.0, 1.0, 0.0);
glPushMatrix();
glTranslatef(0.5, 0, 0);
glutWireSphere(0.15, 30, 30);
glPopMatrix();
glFlush();
}

 void animation(void)
 {
   float x = 0.2;
   float y = 0.1;
   float z = 0.0;

   for (float i = 0; i <= eRad; i += 0.1)
   {
      glTranslated(x*cos(i), y*sin(i), z);
    }
   drawSphere();
 }

 int main(int argc, char** argv)
 {
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Moving Sphere Test");
   glutDisplayFunc(drawSphere);
   glutIdleFunc(animation);
   glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
   glViewport(0, 0, 600, 600);
   glutMainLoop();
   }

The issue I am having is that the Ellipse is drawn in and so is the sphere, but its just staying at one point on the ellipse. So what am I doing wrong?

Answer 1

Yes, you need to use the functions correctly. The animation happens in glutDisplayFunc, the state setting can happen in glutIdleFunc(It does not have to). The gultTimerFunc() callback is what you want to be called every frame. I made a few modifications to your code. But you might want to use the GLUT correctly.

const float eRad = 6.5;
static float iter = 0.0;

void drawSphere(void)
{
  float x = 0.5;
  float y = 0.4;
  float z = 0.0;
  glMatrixMode(GL_PROJECTION | GL_MODELVIEW);
  glLoadIdentity();
  glClear(GL_COLOR_BUFFER_BIT);

// Draw the Ellipse
glColor3d(1.0, 0.0, 1.0);
glBegin(GL_LINE_STRIP);
for (float i = 0; i <= eRad; i += 0.1)
{
    glVertex2f(x*cos(i), y*sin(i));
}
glEnd();

//Draw the Sphere
glColor3d(1.0, 1.0, 0.0);
glPushMatrix();
glTranslatef(x*cos(iter), y*sin(iter), 0);
glutWireSphere(0.15, 30, 30);
glPopMatrix();
glFlush();
}

 void animation(void)
{
        iter = (iter < 6.5) ? iter+0.0001 : 0.0;
}
void Timer(int value) {
        glutTimerFunc(33, Timer, 0);
        glutPostRedisplay();
}
 int main(int argc, char** argv)
 {
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize(600, 600);
   glutInitWindowPosition(0, 0);
   glutCreateWindow("Moving Sphere Test");
   glutDisplayFunc(drawSphere);
   glutIdleFunc(animation);
   glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
   glViewport(0, 0, 600, 600);
   Timer(0);

   glutMainLoop();
}