The LookAt target location doesn't matter if it is z = 0 or z = 1000 or -1000?

Question

I am trying LookAt in OpenGL ES,

    gluLookAt(512, 384, 2000,
              512, 384, 0,
              0.0f, 1.0f, 0.0f);

The second row is the Target's position... so I wonder, if I change the z from 0 to 1000 or -1000, shouldn't what is seen different? They turn out to be all the same, why is that?

Answer 1

From the OpenGL spec for gluLookAt, it states the inputs as:

gluLookAt(GLdouble eyeX , GLdouble eyeY , GLdouble eyeZ , 
          GLdouble centerX , GLdouble centerY , GLdouble centerZ , 
          GLdouble upX , GLdouble upY , GLdouble upZ );

Your current values only move the center vector along the z plane so you're actually looking down from your eye vector. Depending on what it is that you're rendering, you might not see any change at all (a cube will look the same from either the top or the bottom).

Try changing your x and y values instead to move the camera to that it is not perpendicular to the vector you're trying to look at.

Answer 2

As the x and y coordinates are the same for the camera and the target, changing the z coordinate doesn't affect the direction that the view is in at all and for the purposes of this it's only the direction that matters.