Plasfunc surface normal

I have a question regarding plasfunc. In the manual I see the following sentence:

“As in most material types, the surface normal is always altered to face the incoming ray”.

I’m a bit confused by that sentence. Why is the normal always directed towards the incoming ray? Doesn’t that mean we discard important information about the object we are rendering?

I think I might be reading this all wrong.

Thanks in advance!

Most materials in Radiance are “one-sided” in the sense that they appear the same on the back-side of the surface, unless backface visibility is turned off (rendering option -bv0) in which case they disappear entirely. To do this, the surface normal is treated as if you hit the front side, which affects things like calculations involving the surface normal (rendering variables Nx, Ny, Nz).

There are some two-sided materials, like BRTDfunc and BSDF that do not reorient the surface normal, and for these types, you can have different properties on each side if you need it. You can also use the “mixfunc” type to select different materials for the front and back sides.

That helped a lot, thanks!

I took this phrase literally as if the normal is oriented to be equal to the ray.

Again, thanks!

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