OpenGL rotating a camera around a point

Question

In OpenGL I'm trying to rotate a camera around a point, with camera being distance r from the point and facing the point when it rotates. In other words, I want the camera to move along the circumference of a circle at a radius r from the center, with the camera facing the center at any point along the circumference.

Lets say that in 3d space the center of the circle is (3, 0, 3);

I've tried:

// move to center of circle    
glTranslatef(-3, 0, -3)
// move a distance away from the circle
glTranslatef(0, 0, r);
// rotate along the y "up" axis
glRotatef(CameraAngle, 0, 1, 0);

where CameraAngle is the degrees being moved around the circle.

My end result is the camera is still rotating along the origin, not the center of the circle. Can anyone help me fix this problem? Thanks!

Answer 1

You need to either:

• rotate the camera around the origin and then translate it (*)

or:

• use gluLookAt to keep the camera pointing at the center of the circle

(*) rotation functions normally rotate about the origin. To rotate around another point P you have to:

• translate(-P)
• rotate
• translate(P)

Answer 2

it is a little confusing, but i think you should:

// move camera a distance r away from the center
glTranslatef(0, 0, -r);

// rotate 
glRotatef(angley, 0, 1, 0);
glRotatef(anglex, 1, 0, 0);

// move to center of circle    
glTranslatef(-cx, -cy, -cz)

note the order must NOT be changed.

Answer 3

Why bother with all the trouble of rotating the camera and not rotate the scene itself?
It's much more straight forward. just rotate the modelview matrix around the origin. You'll get the exact same result.

Answer 4

I find problems like this much easier to solve with gluLookAt(). You define a path for the camera (a circle is easy!) and keep the "center" point fixed (i.e. the thing you're looking at).

The only possible trick is defining a good up vector--but not usually too much work. If the path and the target point are in the same plane, you can use the same up vector each time!