The reflectance model can be computed in the vertex or the fragment shader.

When it is computed in the vertex shader, one is said to be implementing per-vertex lighting.

When it is computed in the fragment shader, one is said to be implementing per-fragment lighting.

For many scenes, there are fewer vertices than fragments to be processed.  Therefore, per-vertex lighting is typically faster.

However, in per-vertex lighting, the lighting is only evaluated at the vertices and interpolated along the primitive.  In this case, the tessellation of the surface affects the quality of the shading.

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