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Slime wrote:
| I would guess that what is happening is in test1, the first sample
of a ray
| is well lit and the other 9 are poorly lit or dark (due to self
shadowing).
| While in test2, the first sample is well lit and the other 199 are
poorly
| lit or dark. So the percentage of lit samples in the first is much
greater,
| causing it to be significantly brighter. This is only happening
because the
| media is so dense that it becomes dark at a very shallow level.
|
	You are essentially correct, except that the adaptive sampling
*should* take care of that. However it looks like there are some
speed optimizations in POV that prevent it from working in the case
of a constant density media (see my reply to Jellby).

		Jerome
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*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
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Jellby wrote:
| In my experiments, I did not see any significant difference in the
output by
| fiddling with aa. According to your explanation, "samples 10 aa_level 8
| aa_threshold 0.0001" should give similar results to "samples 160
aa_level 4
| aa_threshold 0.0001", and I don't see that with the above example.
Could
| you give a more clear example?
|
	Like Slime said, you'd need to have aa_threshold set to 0 to get the
same results (if pov accepted). You can however get the same effect
by using a ridiculously small value. Moreover I wrote a little too
fast yesterday: the precise formula is (min_samples-1) * 2^aa_level +
1, assuming of course that min_samples is at least 3.

|
|>Note that this only applies to subsurface scattering simulations: eg
|>in cases when you have a very dense nearly uniform media.
|
|
| Yes, my experiments are confined to uniform media. I wonder if this
adaptive
| sampling and antialiasing is only valid for non-uniform media
(opposite to
| your sentence above), maybe uniform media makes adaptive sampling
| unnecessary...
|
| Or maybe I'm making some mistakes.
|
	It looks like povray does some optimizations in the case of a
constant density media. I did a quick check by adding the following
density to your code:

density {
~  spherical
~  color_map {
~    [ 0.0 rgb 1.0 ]
~    [ 1.0 rgb 1.0 ]
~  }
}

	Since the colormap is rgb 1.0 everywhere, this should give the same
results as the original but it doesn't. The result is pretty much
what I described in my previous post (except that samples 129 takes
much longer to render since there are a lot of pure black samples
taken that get culled in the samples 3 case). You may take a look for
example at those pictures done with aa_threshold 1e-200:

aa_level 8
samples 3
http://jeberger.free.fr/tmp/media-3-8.png
aa_level 2
samples 129
http://jeberger.free.fr/tmp/media-129-2.png

	I'd say that this is a bug.

	Note that you can probably get the correct result with a faster
render time if you disable this optimization by adding a pseudo
density like I just did...

		Jerome

PS: If the pictures aren't there, try again a bit later: as of this
posting, the second one hadn't finished rendering...

- --
******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
******************************
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	If your file doesn't contain a #version directive, or if the version
specified is higher than 3.5, then the default for media are:
method 3
intervals 1
samples 10, 10

	Note that this means that if you only change the method, you'll
still get 1 interval and 10 samples unless you change them too. For
compatibility reason, if your file has a #version lower than 3.5, the
defaults are those written in the doc. I guess that the doc should be
updated...

		Jerome
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******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
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> You are essentially correct, except that the adaptive sampling
> *should* take care of that.

I was under the impression that the adaptive sampling only took into account
the values of the media density, and not the lighting.

 - Slime
 [ http://www.slimeland.com/ ]

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> Which leads me to conclude that the method 3 default intervals is 1 and
> samples 10, if the media samples in the render stats are an appropriate
> indicator.

Ah, thanks.

 - Slime
 [ http://www.slimeland.com/ ]

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> It looks like povray does some optimizations in the case of a
> constant density media. I did a quick check by adding the following
> density to your code:
> 
> density {
> ~  spherical
> ~  color_map {
> ~    [ 0.0 rgb 1.0 ]
> ~    [ 1.0 rgb 1.0 ]
> ~  }
> }
> 
> Since the colormap is rgb 1.0 everywhere, this should give the same
> results as the original but it doesn't. The result is pretty much
> what I described in my previous post (except that samples 129 takes
> much longer to render since there are a lot of pure black samples
> taken that get culled in the samples 3 case). You may take a look for
> example at those pictures done with aa_threshold 1e-200:

Which POV-Ray version and platform are you using? I could not reproduce your
results, I got quite the same effect I was getting with constant density:
overbright media with "aa_level 8 samples 3" even if I introduce a slope in
the color_map (then the media is different, of course, but still not
similar when the samples and aa_level are varied).

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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Slime wrote:
| I was under the impression that the adaptive sampling only took
into account
| the values of the media density, and not the lighting.
|
	Nope, adaptive sampling takes into account the total contribution of
the samples to the resulting color. For emission media, this is only
the density, for absorption it depends on whatever's behind the
media, and for scattering you add the light source(s).

		Jerome
- --
******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
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In article <41db3ff6@news.povray.org>, "Slime" <fak### [at] emailaddress> 
wrote:

> > You are essentially correct, except that the adaptive sampling
> > *should* take care of that.
> 
> I was under the impression that the adaptive sampling only took into account
> the values of the media density, and not the lighting.

If it does so, it should probably be considered a bug...I can't see how 
it would discard any samples that don't actually need to be taken.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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Jellby wrote:
| Which POV-Ray version and platform are you using? I could not
reproduce your
| results, I got quite the same effect I was getting with constant
density:
| overbright media with "aa_level 8 samples 3" even if I introduce a
slope in
| the color_map (then the media is different, of course, but still not
| similar when the samples and aa_level are varied).
|
	I was using MegaPov 0.7 on linux. A quick try with version 3.6.1
shows the same results as you. I think MP0.7 has the correct behavior
here...

		Jerome
- --
******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
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Mike Raiford <mra### [at] hotmailcom> wrote:
> Someone care to explain the following from the manual:
>
> "It's usually best to only use one interval with method 3. Too many
> intervals can lead to artefacts, and POV will create more intervals if
> it needs them."
>
> It seems with one interval, I don't get good results. If I up the
> intervals to 5 or 10, it seems to work better, but is slower in general.
>
> Anyone have any clue about the relationship and why the manual asks to
> use only 1 interval?
>
>
> --
> ~Mike

Btw, I think it would be a great idea to put the valuable info learned here
into the new POV-Ray Wiki at http://www.wikipov.org/

(see this thread:
http://news.povray.org/povray.general/thread/%3C41dd4071%40news.povray.org%3E/?mtop=13
)

George

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