Op 12/08/2020 om 16:41 schreef William F Pokorny:
> Interesting. Not sure.

I am even less sure now to tell the truth. Rendering again a vertical 
slice covering the upper right area where the effect was most visible, I 
could not recreate the effect! I am continuing to do a couple of tests 
today, but I wonder now if this is not a clear sign of the Jekyll/Hyde 
Syndrome ;-) I /may/ have made a slight change, in particular to the 
haze media code I use, between render sessions... Only I do not remember 
having done that at all. You could even hold a gun to my head and I 
would say the same ;-)

> 
> While you were experimenting with continuations, did the discontinuity 
> at the continuation move around substantially enough to be sure it's not 
> related to camera rays and slope in some fashion? Guessing you are using 
> a slope based pattern?

In a slightly different setting (of which I shall talk later) the 
discontinuity moved with the location where I interrupted/restarted the 
render. That seemed clearly to be related to the isosurface somehow.

> 
> There are what I think of as 2.5 bugs in the shadow cache mechanism of 
> v37 and v38 which are fixed in povr. And with povr, users can set the 
> shadow tolerance which is in play with the bugs too. These bugs / 
> limitations have the potential to behave differently depending upon the 
> objects in play and so too isosurfaces over other objects. The shadow 
> cache state wouldn't be carried through on a continuation so 'suppose' 
> those 2.5 shadow cache bugs could be the cause.
> 
> There is too an isosurface/thread based cache mechanism specific to the 
> isosurface, but looking at the code doubt it could cause the discontinuity.
> 
> I don't really have the machine for this scene..., but is the scene 
> small enough you could package it up  - without the cloud media if 
> possible? Maybe, if I render vertical slices or something I could 
> reproduce it in v38, then try a similar continuation with povr without 
> tying my machine up too long.
> 

Thanks, I shall do that if my own suspicions are cleared away. The scene 
is not large at all. I shall report back.

-- 
Thomas

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