On 2/8/21 2:38 PM, Bald Eagle wrote:
> While working out some media stuff for Mike, I ran a a quick media test to just
> make sure I had the standard part working, and I got some odd results.
> Perhaps someone could render this and tell me why the center gives me a nice x
> gradient, but off to the sides I get those weird fans / lines.
Been now some years since I was digging around in that code, but if I'm
remembering my "things-look-fishy-there" list correctly, the short
answer is you need to increase the sampling. Try:
or perhaps more (with stronger AA tend to need less). The second value
is not used with method 3. The aa_level (recursion limit) and
aa_threshold stop method 3.
---- Details... Suspicions... on my look at it more later list.
- Why did I jump to the odd number of samples? With method three you get
+2 samples per sampling segment for odd sample specification and +1 for
even. Where the decent and the adaptive mode is working, I think the
code is slightly more symmetric with odd values - and 'probably' creates
- I think the adptive sampling mechanism of method 3 is not working for
absorbtion/emissive only media!! At least not working in the same way as
scattering. Blah, blah,.... lots more details. Comes to needing to force
a higher level of sampling - which, you're right, won't always be adaptive.
Aside: You can sort of see this by looking at the number in parens after
the number of media samples. You can twiddle all you want with the
samples values and that number will always be +1 or +2 of the initial
samples setting because it is the number or samples per segment. If you
have scattering media it comes to be +1 or +2 the initial samples, plus
some 0-1 float adder. Further, the aa_level and aa_threshold values do
something if you have scattering media.
- The aa_threshold value I suspect, should have been adjusted on the
changes to assumed_gamma 1.0 as was done eventually in v3.8 for the
ambient default (there zero makes more sense, but to match ambient of
earlier scenes it should not be the previous 0.1, but something much
smaller like 0.004).
- Though absorption works alone I wonder if it wasn't originally
supposed to always be paired with scattering.
- When emission was added, a second absorption value spec should have
been added specifically for it's contribution. Having one value for both
scattering and emission sometimes just won't work out - and it's harder
to use even when you can make it work.
Aside: You can get interesting patterns with densities and medias like
yours by setting the samples low. As low as 2 even.
- Even where the adaptive is working you sometimes need to increase the
initial samples count quite a bit. As with other gradient sensitive
adaptive algorithms (isosurfaces, AA method 3(a)) you sometimes have to
set the initial sample higher to pick up enough change to 'trip' the
adaptive algorithm. You have to sample rapidly enough to see
(a) - AA method 3, is currently missing this capability except by
increasing the size of your initial render - which is sort of a back
door method of increasing the initial number of samples. This something
Christoph recognized. There are comments in the code.
- Something I do not think made clear in the documentation is that when
the density is constant method 3 is not used but rather a part of method
1 or 2 (I don't recall) which is very fast. What this means in practice:
For constant medias you can get away with settings that are quite
aggressive. The instant that media is not constant things will either
not render correctly with the aggressive (cheap) settings or suddenly
slow significantly if the settings are correct for non-constant media.
The big slow down happens where the adaptive algorithm gets tripped. In
other words, the fast path is no longer used - and the method 3 settings
- It's true with method 3, intervals should be kept at 1. :-) Here it
happens you can actually set it to 3, 4 or more maybe and get much
better results. The issue with it, that I saw, is when intervals are >1
the math gluing together all the intervals is buggy. You get media
results which are less accurate - sometimes to the point of obvious
artifacts. Aside. That old Persistence, submarine scene actually used
the artifacts for effect - but if you look carefully, you can see them.
...There is more, and somewhere I have notes. But! I'm going to worry
about it all some other day and go grab more coffee. :-)
Disclaimer. I've listed some of my initial media 'suspects'. If I get
back to working on the media code, I'm sure I'll find myself to be
right, wrong and partly right/wrong about the different details.
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