Andrew the Orchid <voi### [at] devnull> wrote:
> OK, so... it works by tracing backwards like normal, but with extra rays
> being spawned for diffuse illumination, approximately? (Just like
> specular reflection and refraction require additional rays.)
>

Yes.

> Well, that answers the main question.
>
> Just to clarify... Are rays fired evenly in all directions? Completely
> at random? Are they set up to "avoid" the object under consideration?
> Are their directions in any way related to the direction of the ray that
> hit this surface in the first place? Does the total computed intensity
> take into account the different angles the light is illuminating the
> surface from?
>

In the ideal case, you would want to use completely random rays, but POV-Ray
does not do this. Instead it uses a pre-computed table of ray directions
whose density falls off as the angle from the normal increases. Were it not
for this density fall-off, they would have roughly uniform coverage, but
with some quasi-randomness in their actual placement. (Jim McElhiney has
posted in more detail how he chose the sample set).

This density falloff places more weight on light that is coming to the point
head on, rather than at an angle, in accordance with Lambert's cosine law.
It saves computing time by tracing fewer rays in angles that contribute
less to the illumination. However, this approach has undesirable side
effects with regards to the irradiance caching process.

No, the directions of the gather rays are the same regardless of angle of
the ray that hit the surface in the first place. Also, before a gather ray
is shot, there is a check to make sure that it does not go behind the
surface in question.

> You say the process traces backwards according to the max trace limit.
> (In fact, I think there might even be a parameter to change it for the
> radiosity system so it's different to the main trace.) Does POV-Ray only
> save the final intensities, or does it cache all the intermediate values
> too?
>

No, I said it uses the radiosity recursion_limit, not the max_trace limit.
POV-Ray caches the intermediate values, too. However, it will only reuse
samples from the irradiance cache when the bounce depth of the stored
sample matches the bounce depth of the sample point in question.

> Andrew.
>
> PS. Some graphical diagram in the help file would be really useful here.
> I /think/ I understand the overall principle now, but for a long time I
> was very confused by this feature.

I agree on that! I'm not on the POV-Ray team, but I'd like to make one when
I get a chance.


George
http://www.gammaburst.net

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