Clarify handling of rays incident to the surface from the interior about openpbr HOT 3 CLOSED

A related issue is how we deal with rays which are exiting from the surface when the surface is locally (e.g.) a metal.

Arguably one can make sense of this by assuming the metal must be a thin sheet of foil covering the interior dielectric, so we would reflect from this foil back into the interior. This would allow for rendering of e.g. bottles with metallic labels, with the label visible through the glass (without the need for modelling a separate label).

This still seems like the best way to reason about the meaning of a mixture of metal and transmission to me. If the surface is being hit from the inside, then the bulk must have been transparent, so the metal must be a foil on top of the surface.

from openpbr.

A related issue is how we deal with rays which are exiting from the surface when the surface is locally (e.g.) a metal.

Arguably one can make sense of this by assuming the metal must be a thin sheet of foil covering the interior dielectric, so we would reflect from this foil back into the interior. This would allow for rendering of e.g. bottles with metallic labels, with the label visible through the glass (without the need for modelling a separate label).

In general though the interior properties are (currently) ill-defined if different parts of the surface specify different base interiors (i.e. metal, diffuse, subsurface, volume, or a mixture thereof). For the moment it probably has to remain implementation-dependent how this is handled. In the spec we just say something vague about the implementation "doing the best it can to make sense of it":

This physical picture of the mix operation becomes somewhat unrealistic in some cases where the bottom-most bulk materials being blended are not obviously consistent (e.g. blending dielectric and metallic bulks), but in such cases it is understood that the implementation should do the best it can to make sense of the physics (e.g. the metal bulk could be considered to actually be surface metallic flakes on top of a single consistent dielectric).

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I think the effects I described are implicit in the physical description, so we don't need to elaborate on them. Implementations typically won't bother modeling these, but in future renderers which e.g. do full MC light transport in the layer structure, they might attempt it.

I could perhaps add a couple sentences to the spec noting that the entering/exiting rays encounter the layers in different order, which in principle should be accounted for in the layering math, but it isn't strictly necessary.

from openpbr.