Warp <war### [at] tagpovrayorg> wrote:
>   Are you sure that radiosity taking into account normal perturbation
> requires additional sampling? Why would it?

Yes, I'm positively sure. I actually did a bit of work on the code, you know :P

The surface normal vector affects radiosity in multiple ways:

(1) It defines the hemisphere from which indirect illumination is sampled at
all.

(2) It defines the direction from which indirect illumination has the strongest
effect. (Indirect illumination is cosine-weighted with respect to the angle of
incidence.)

(3) The deviation in the surface normal vector is one of the key criteria for
the "reusability" of an existing samples for a certain surface point.

(4) Surface normal vectors of other objects may affect the actual sample values.


Specifying "radiosity { normal on }" tells POV-Ray to work with the fake surface
normal for the sake of all (1) through (3).

(3) is the one that causes a higher sample density, because even on a plane the
samples' surface normals now don't always match.


>   (Or do you mean that normal perturbation may create more changes in
> the lighting, in which case the error_bound is triggered more easily,
> resulting in more sampling?)

I guess you're referring to (4) here. Ironically, neither of your assumptions
here is true: (a) fake normals are *always* taken into account in this respect,
regardless of the "normal" setting (maybe there is some potential for speedup
here); and (b) actual changes in the lighting do *not* affect the sample
density; instead, sampling is controlled by an *estimate* of *potential*
lighting changes, derived solely from geometric properties.

Post a reply to this message