On 04/28/2016 07:04 AM, Bill Pragnell wrote:
> Jaime Vives Piqueres <jai### [at] ignoranciaorg> wrote:

>>> Awesome. I look forward to this time next year when the full size
>>> version finishes rendering!
>>
>>     Thanks, but it's not that slow... that test took just 29 min, and the
>> one attached here, using +a, took 1h45m using only 4 out of 6 cores. I
>> guess the full render at 1600x900 will take about 4 hours with 6 cores.
>> The idea of using a blob as container, instead a box or sphere, works
>> pretty well... it's just a shame it generates some artifacts. :(
>
> I guess the blob containers makes for fewer media calculations - that's a decent
> time for such a render.
>
> What causes the artifacts, is it something about the way blobs are implemented?
> How about using a merge of the blob component spheres instead?
>

Answering without going back to refresh my old brain cells...

When Gail Shaw hit this issue back in 2005. At that time Slime wrote:

-------
This is probably just a problem with the accuracy of the blob solver; I 
think it's not noticing the second (backside) surface near the edges, so 
it thinks there's a long distance of media and makes it very bright as a 
result. (In 3.5 this may have been diminished by anti-aliasing.)

A suggestion: Use a small box as your media container instead of a blob, 
and then use a density{} in the media with the object pattern with the 
blob, so that all points outside of the blob have no media.

- Slime
-------

In my digging I've found blob surfaces are actually a bit noisy 
especially as the rays hit the surface in a more tangential way - so I 
think Slime's thinking as to the cause is probably right.

Things that tend to help:

1) Try running with sturm off.

2) Lower the blob threshold - start extreme with 0.0001 say - though you 
may of course lose the blob effect you want, but perhaps in the case of 
these clouds this would be OK?

3) The code change I have been working to verify against blob problem 
cases from years past tends to help a little with bright artifacts - 
more with dark artifacts. Specifically the change in blob.cpp is changing:

const DBL DEPTH_TOLERANCE = 1.0e-2;

to:

const DBL DEPTH_TOLERANCE = 1.0e-4;

4) AA sometimes helps - and sometimes not - depending on how tangential 
the super samples themselves are I guess.

5) What the other Bill P suggested should help too - if the merge 
eliminates internal blob surfaces causing the artifacts.

Hope one or more things here enough to get this cloud method working 
generally.

Bill P.

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