[mattpharr]

> I would expect a shader could do a credible job of emulating all of these.

The key is that it is necessary to know the coherence of the light arriving at the surface in order for a shader to accurately model reflection. (See Figure 13 in the paper for an example that shows how this matters.) Otherwise a shader would have to make up a guess about the light's coherence.

The main contribution of the paper is a much more efficient approach than was known before for finding light's coherence, even in the presence of complex light transport (multiple reflections, etc), and one that further allows the application of traditional ray-optics computer graphics techniques for sampling light paths through the scene. (For example, previously if one wanted to track coherence, it was necessary to sample light paths starting from the light sources rather than from the camera, as is done in path tracing.)