This is something that's really cooking my noodle. When I'm computing the light response for a material in my fragment shader, I'm using the standard 3 normalized vectors, in eye space: fragment normal, light direction, eye direction.
However, because the 3 vectors are in eye space, I've seen the eye direction represented as the following, because the camera is looking down -z:
vec3(0, 0, 1)
While the light direction is:
normalized(light_eyespace_position - fragment_eyespace_position)
This makes sense at first glance, but the more I think about it, I'm trying to figure out why the eye direction isn't computed similarly to the lighting direction:
In other words, why does the vector from the light to the surface dependent on fragment position, but the vector from the camera to the surface is independent on the fragment?
The only thing I can work out is that using vec3(0, 0, 1) is correct only for ortho cameras, while normalized(-fragment_eyespace_position) is correct for perspective cameras. Is this correct? Can anyone shed some light on this?