>>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.

Cool. :-)

So... the pretrace step... Is the video from that used for anything? Or 
is the only purpose of this step to fill up the cache?

>>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.
> 
> 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.
>
> 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.

...does all of this mean that the diffuse highlight options don't affect 
radiosity?

Also, while we're on the subject, does radiosity take (specular) 
reflection into account as well? (Never tried it, but I assume it does...)

>>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.

So there's a separate cache for each depth?

>>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.

Yeah, I might have a go at it too at some point...

Andrew.

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