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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:

normalized(-fragment_eyespace_position)

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?

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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 point is, because of the lighting equation. What you need is the vector from the light to the vertex, in other words the light direction relative to the vertex position. This is needed in order to take the dot product between the normal vector and the L vector. This is called the diffuse term and is calculated

float diffuseTerm = (N.L); Where N is the normal vector. L is the light direction. In case of directional light (infinitly far) L will be constant for all the vertices. If the light is not L will be calculated relative to each vertex.

Regarding the camera vector you are in the eye space, which means the camera position is (0,0,0) you can actually think of this

normalized(-fragment_eyespace_position)

As this

normalized( (0,0,0)-fragment_eyespace_position)

Because in eye space the camera is actually (0,0,0)

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  • \$\begingroup\$ Sorry, this doesn't answer the question for me. I understand how lambertian diffuse is calculated, but nothing is clear on how the camera direction is computed PER FRAGMENT. To me, a perspective camera is very similar to a point light source: rays of light converge at a point. It seems to make sense then that a direction vector is from that point to the fragment being considered. I'm looking for reasons why or why not. \$\endgroup\$
    – user48206
    Jun 20, 2014 at 15:07
  • \$\begingroup\$ @user48206 you don't need to think about per fragment or per vertex the calculation are the same, what you care about is the space that you are calculating the direction in. In your case you are calculating the light contribution in Eye Space where the eye position is (0,0,0). If you were calculating the light contribution in world space then you need to use EyePosition- fragmentWorldPosition. \$\endgroup\$
    – concept3d
    Jun 20, 2014 at 18:05

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