gluLookAt seems to be producting wrong view, OpenGL - c++

Question

I am working on an nbody simulator and I want to display it with OpenGL. I want to always be looking at the centre of mass reference frame. I have the following code. I calculate the COM and I set the center coordinate in the gluLookAt function to be the center of mass. I then subtract the "zoom" from the z coordinate to get the eye position. By logic this should ensure that I am always looking at whatever value the center of mass is. The only issue is that I marked where the center of mass should be on the screen with a red dot and it is moving. Shouldn't it never move if I am always looking at it from the same relative position? Here is my code. Focus on the display function since I assume that is where the error will be. I had similar code working in another project and I can't really find any differences.
#include "Universe.cuh"
#include <iostream>
#include <cstdlib>
#include <ctime>
#include "timer.hpp"
#include <GL/glut.h>
Universe u;
float* vbuf;
double angle = 0.0, zoom = 1000;
void display()
{
glClearColor(0, 0, 0, 0);
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
float3 c = u.getCenterOfMass();
gluLookAt(c.x, c.y, c.z - zoom, c.x, c.y, c.z, 0, 1, 0);
glScalef(0.1, 0.1, 0.1);
glRotated(angle, 1, 0, 0);
glColor4f(1, 1, 1, 0.25);
glBegin(GL_POINTS);
{
glColor3f(1.0, 0.0, 0.0);
glVertex3d(c.x, c.y, c.z);
}
glEnd();
glutSwapBuffers();
}
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60, (double)w / (double)h, 1.0, zoom * 1e9);
glMatrixMode(GL_MODELVIEW);
}
void copy_to_vbuf()
{
for(int i = 0; i < u.size(); i++)
{
vbuf[3 * i + 0] = u.getObjects()[i].p.x;
vbuf[3 * i + 1] = u.getObjects()[i].p.y;
vbuf[3 * i + 2] = u.getObjects()[i].p.z;
}
}
void keyboard(unsigned char c, int x, int y)
{
if(c == 'w')
angle += 1;
else if(c == 's')
angle -= 1;
else if(c == '=')
zoom /= 1.2;
else if(c == '-')
zoom *= 1.2;
glutPostRedisplay();
}
void idle()
{
u.timeStep();
copy_to_vbuf();
glutPostRedisplay();
}
int main(int argc, char** argv)
{
cudaSetDevice(0);
srand(time(0));
u.getConfiguration().max_velocity = 10;
u.getConfiguration().softening_factor = 0.01;
u.getConfiguration().threshold_angle = 35;
u.getConfiguration().time_step = 0.1;
const int N = 5;
vbuf = new float[3 * N];
for(int i = 0; i < N; i++)
{
Object o;
o.m = rand() % 100 + 1;
o.p.x = 500.0 * rand() / RAND_MAX - 250.0;
o.p.y = 500.0 * rand() / RAND_MAX - 250.0;
o.p.z = 500.0 * rand() / RAND_MAX - 250.0;
u.addObject(o);
}
copy_to_vbuf();
glutInit(&argc, argv);
glutInitDisplayMode(GL_DOUBLE);
glutInitWindowSize(1000, 1000);
glutCreateWindow("N-Body");
glutReshapeFunc(reshape);
glutDisplayFunc(display);
glutIdleFunc(idle);
glutKeyboardFunc(keyboard);
glEnable(GL_POINT_SMOOTH);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glPointSize(1.0);
glutMainLoop();
return 0;
}

Answer 1

Two points regarding:
glScalef(0.1, 0.1, 0.1);
glRotated(angle, 1, 0, 0);
Since your axis is not centered on the 'COM', when you apply rotation the COM point will not stay in place and logically would move around the screen.
AFIK the normal order is scale,rotate,translate for transformations. This will apply the rotation and then scale.
EDIT:
To expand on that: Currently you take an arbitrary point rotate it, scale it and then focus on the point where it used to be. If you want to rotate your model (e.g. point marking the 'COM') around itself, it needs to be centered at (0,0,0).