Am 08.08.2017 um 23:13 schrieb Kenneth:

> I've been reading the technical article "A Practical Model For Subsurface Light
> Transport," recommended in the docs. Unless I missed something there, it doesn't
> mention anything specific about the use of 'normals' on the mesh triangles.
> FlyerX's 'coarser' mesh-derived example shows odd brightness concentrations
> around what look like the edges of the triangles (the eyes, specifically.) So
> I'm wondering if POV-Ray's implementation of SSLT --or *any* implementation--
> takes 'smooth triangles' into consideration, and shoots its rays along those
> interpolated normals? Just from his example, it seems not. (I apologize for my
> lack of technical understanding of exactly how all of this works.)

There's a twofold problem here:

- Dealing with the edges of supposedly smooth meshes in SSLT is tricky
enough in and of itself: The algorithm needs to look at nearby geometry,
not just a single ray-surface intersection and its surface normal.
Without any additional countermeasures, edges in supposedly smooth
surfaces /will/ cause artifacts. There is no paper on such
countermeasures that I'd be aware of, so careful thought needs to be put
into this.

- Whatever the details of such countermeasure would be, it would
inevitably involve looking at the true geometric surface normal.
Unfortunately, POV-Ray currently only passes around the fake-smoothed
surface normal and the perturbed surface normal.

So in order to prevent the artifacts in question, first POV-Ray's
internal architecture needs to be modified to also pass around the true
geometric surface normal (which is on the agenda anyway to eliminate
certain other artifacts). Once that is done, brainpower needs to be
invested into finding a suitable algorithm to account for smoothed geometry.


BTW, there's also a related problem with photons collected on smooth meshes.

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