"Kenneth" <kdw### [at] gmailcom> wrote:
(BEWARE:
> This scene renders *REALLY* slowly at the threshold-- 42 minutes, vs. 5 seconds
> at a more normal rad brightness(!) Try reducing the number of boxes in the #for
> loop; that *should* help somewhat.)
>

Well, I just tried that: Even with only FIVE boxes in the scene, the threshold
render takes 44 minutes-- only 1 minute less than using 900 boxes.

I then decided to substitute a MUCH-smaller 'ground box' for my plane{y,0...}...
         box{<-1,-.1,-1>,<200,0,200> no_shadow
               pigment{rgb .4}
            }

.... my quasi-theory being that it would present FAR less of a surface area for
radiosity to check for samples vis-a-vis the five boxes, rather than the
infinite expanse of a plane. That helped *somewhat*-- a 33-minute render vs. 44
minutes. But not what I was expecting.

HOWEVER... using NEITHER the ground box nor the plane-- and just letting the
sky_sphere contribute its radiosity to the five box objects-- the render time
dropped to an astonishing 1.5 seconds! In other words, the 'threshold'
completely disappeared (even when using rad 'brightness' .001)

*That* makes me think that there may be some hidden 'radiosity shadow'
calculations going on behind-the-scenes, when there shouldn't be-- the ground
box or plane somehow casting shadows that radiosity is 'seeing.' (Yet my scene
has NO shadows, AFAIK.) Or if not that, then something that's having a similar
effect on the radiosity algorithm.

I don't know if these insights are *useful*-- but again, the results are quite
interesting.

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