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Jellby wrote:
| What I don't quite understand is why the number of samples has so much
| effect over the overall brightness of the media (I'm thinking of dense
| scattering media to simulate SSS). Sure, the sample number should
affect
| the quality and presence of possible artifacts, but why is "samples
15" so
| much brighter than "samples 75" (method 3, intervals 1)? Is this
maybe due
| to some bad coding on my part?
|
	That's because the darkening you see is an artefact :) It's due to
the fact that when you don't take many samples, a lot of them
proportionnaly get taken deep inside the media where it's dark.
Increasing the number of samples ensures that more and more get taken
in the thin layer near the surface where the SSS effect occurs which
leads POV to notice that the result should be bright. BTW, you
shouldn't increase the number of samples too much. Instead you should
increase the aa_level and possibly decrease the aa_threshold. This
will ensure that the additional samples get taken in places where
they will have a useful impact on the result.

		Jerome

- --
******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
******************************
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> That's because the darkening you see is an artefact :) It's due to
> the fact that when you don't take many samples, a lot of them
> proportionnaly get taken deep inside the media where it's dark.
> Increasing the number of samples ensures that more and more get taken
> in the thin layer near the surface where the SSS effect occurs which
> leads POV to notice that the result should be bright.

According to this, more samples -> brighter media. But I see the opposite,
as if the influence of the samples was "averaged" (since most samples are
deep inside, the effect of the surface ones is weaker). I would have
thought that the sampling algorithm (and colour calculation) would be
smarter than that, but maybe that's not so easy...

> BTW, you 
> shouldn't increase the number of samples too much. Instead you should
> increase the aa_level and possibly decrease the aa_threshold. This
> will ensure that the additional samples get taken in places where
> they will have a useful impact on the result.

I couldn't get the same effect... Even if the "darkening" due to many
samples (or intervals) is an artifact, I didn't find a reliable way to get
what I needed other than increasing the number of samples.

-- 
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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Jellby wrote:
| According to this, more samples -> brighter media.
	That is correct. If you push this to the extreme, you'll get this
image (10 samples):
http://jeberger.free.fr/mpov/sphere_bl.png

	Increasing the number of samples (to 200) gives this one:
http://jeberger.free.fr/mpov/sphere_no.png

| But I see the opposite,
| as if the influence of the samples was "averaged" (since most
samples are
| deep inside, the effect of the surface ones is weaker). I would have
| thought that the sampling algorithm (and colour calculation) would be
| smarter than that, but maybe that's not so easy...
|
	Do you have a sample code that produces this effect?

|
|>BTW, you
|>shouldn't increase the number of samples too much. Instead you should
|>increase the aa_level and possibly decrease the aa_threshold. This
|>will ensure that the additional samples get taken in places where
|>they will have a useful impact on the result.
|
| I couldn't get the same effect... Even if the "darkening" due to many
| samples (or intervals) is an artifact, I didn't find a reliable way
to get
| what I needed other than increasing the number of samples.
|
	It takes some trial and error, but it should be possible to get the
same effect with a much faster render. Of course, it all depends on
whether you can afford to waste more time fiddling with the
parameters or simply letting the computer render in the background
while you're doing something else ;) To make accurate comparisons,
you also need to make sure that the maximum number of samples is the
same in both cases (I'm not talking of the second parameter of the
"samples" keyword here, but of the number of potential samples which
is something in the order of min_samples * 2^aa_level). Then if you
set aa_threshold to 0 the results should be the same (and take
*forever* to render) so all you need to do is find a value that's low
enough to get the effect you want while not being exactly 0.

	Note that this only applies to subsurface scattering simulations: eg
in cases when you have a very dense nearly uniform media. If you have
a density with areas of very dense media and other areas that are
empty, you will need the increased number of samples

		Jerome
- --
******************************
*      Jerome M. Berger      *
* mailto:jbe### [at] ifrancecom *
*  http://jeberger.free.fr/  *
******************************
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> | But I see the opposite,
> | as if the influence of the samples was "averaged" (since most
> samples are
> | deep inside, the effect of the surface ones is weaker). I would have
> | thought that the sampling algorithm (and colour calculation) would be
> | smarter than that, but maybe that's not so easy...
> |
> Do you have a sample code that produces this effect?

Yep, try this (povray 3.6.1, custom compile):

---------------------------------------
camera { location <0, 0, -10> look_at 0 }

light_source { <-20, 20, -20>, rgb 1 }

sphere {
  0, 3
  hollow
  pigment { color transmit 1 }
  interior {
    media {
      scattering { 5, rgb 3 }
      method 3
      intervals 1
      samples 10 // try also with 200
    }
  }
}
---------------------------------------

I've attached two images: "test1" has samples 10, "test2" has samples 200.

> It takes some trial and error, but it should be possible to get the
> same effect with a much faster render. Of course, it all depends on
> whether you can afford to waste more time fiddling with the
> parameters or simply letting the computer render in the background
> while you're doing something else ;) To make accurate comparisons, 
> you also need to make sure that the maximum number of samples is the
> same in both cases (I'm not talking of the second parameter of the
> "samples" keyword here, but of the number of potential samples which
> is something in the order of min_samples * 2^aa_level). Then if you
> set aa_threshold to 0 the results should be the same (and take
> *forever* to render) so all you need to do is find a value that's low
> enough to get the effect you want while not being exactly 0.

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?

> 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.

-- 
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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Attachments:
Download 'test1.jpg' (3 KB) Download 'test2.jpg' (3 KB)

Preview of image 'test1.jpg'

Preview of image 'test2.jpg'

> Interesting that the defaults for media would be set up as such:


You should see the thread "default media setting inconsistancies (?)" on
12/27/2004 (about 15 threads down). Assuming I gave correct information in
that thread (and I may be wrong; no one's confirmed), it should clarify
things for you.

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

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> I've attached two images: "test1" has samples 10, "test2" has samples 200.

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.

I would assume that the greater number of samples is producing the more
accurate result.

One thing to try: make two nested spheres, the inner one about 4/5 the
diameter of the outer one. Give them the same media, but give the outer one
much more samples. Then the edge of the media (in the outer sphere) will be
well sampled - as it needs to be to avoid this problem, but the inside of
the media (the inner sphere) will be poorly sampled, which is ok since it's
all dark in there anyway. If this works, it may be the fastest way to get
the effect with accuracy. (Be sure, of course, to avoid having the sphere's
volume's overlap (use a difference on the outer sphere to do this), and to
avoid coincident surfaces.)

> 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?

When the media has a constant density, the first case will take 10 samples
and the latter case will take 160 samples. Only if the aa_threshold is set
to 0 (or if the media has a very noisy density) will the two samplings be
equivalent.

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

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Slime wrote:
> You should see the thread "default media setting inconsistancies (?)" on
> 12/27/2004 (about 15 threads down). Assuming I gave correct information in
> that thread (and I may be wrong; no one's confirmed), it should clarify
> things for you.

Its still clear as mud. Whats needed is for someone with a really good 
understanding of how media works in POV-Ray to chime in and let us all 
know whats what.

-- 
~Mike

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Among other things, Slime saw fit to write:

>> I've attached two images: "test1" has samples 10, "test2" has samples
>> 200.

(Sorry for attaching images to this non-binary group. I thought this was
p.b.i. and it was too late when I realized it wasn't)

> 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.
>
> I would assume that the greater number of samples is producing the more
> accurate result.

That was my impression and that could be the reason. There's little
difference between "samples 200" and "samples 2000"

> One thing to try: make two nested spheres, the inner one about 4/5 the
> diameter of the outer one. Give them the same media, but give the outer
> one much more samples. Then the edge of the media (in the outer sphere)
> will be well sampled - as it needs to be to avoid this problem, but the
> inside of the media (the inner sphere) will be poorly sampled, which is ok
> since it's all dark in there anyway. If this works, it may be the fastest
> way to get the effect with accuracy. (Be sure, of course, to avoid having
> the sphere's volume's overlap (use a difference on the outer sphere to do
> this), and to avoid coincident surfaces.)

Hmm... It didn't seem to be quicker in this case, the effect of the samples
was very similar and the inner surface was visible (in fact, the inner
media didn't have much influence).

I guess it's best to have as few samples as possible, as long as there are
enough to accurately sample the media (i.e. the appearance doesn't change
much with increasing samples)

> When the media has a constant density, the first case will take 10 samples
> and the latter case will take 160 samples. Only if the aa_threshold is set
> to 0 (or if the media has a very noisy density) will the two samplings be
> equivalent.

Unfortunately, povray doesn't like "aa_threshold 0". So, it seems the
adaptive sampling refers to the possible variation in the media density,
not to the effect on the image.

-- 
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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While it is probably bad form to continue one thread in another, I'll do 
it anyway here. :)

Slime, you mentioned some method 3 default settings which you understood 
to be different from the method 1 defaults (but not listed in the docs). 
Those were intervals 1 and (min) samples 5. These values, by the way, 
are explicitly set in many of the media example scenes distributed with POV.

Anyway, I fiddled around with a couple of different settings on the 
example test scene given earlier in the thread. Going by the media 
samples listed in the render stats I get this:

method=3
intervals=default (not explicitly set)
samples=default (not explicitly set)
Media Samples: 2668281

method=3
intervals=10 (default as per the docs)
samples=1 (default as per the docs)
Media Samples: 30730560

method=3
intervals=1
samples=5
Media Samples: 1515497

method=3
intervals=1
samples=10
Media Samples: 2668281

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.


-Abe

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Jellby wrote:

> 
> 
>>| But I see the opposite,
>>| as if the influence of the samples was "averaged" (since most
>>samples are
>>| deep inside, the effect of the surface ones is weaker). I would have
>>| thought that the sampling algorithm (and colour calculation) would be
>>| smarter than that, but maybe that's not so easy...
>>|
>>Do you have a sample code that produces this effect?
> 
> 
> Yep, try this (povray 3.6.1, custom compile):
> 
> ---------------------------------------
> camera { location <0, 0, -10> look_at 0 }
> 
> light_source { <-20, 20, -20>, rgb 1 }
> 
> sphere {
>   0, 3
>   hollow
>   pigment { color transmit 1 }
>   interior {
>     media {
>       scattering { 5, rgb 3 }
>       method 3
>       intervals 1
>       samples 10 // try also with 200
>     }
>   }
> }
> ---------------------------------------
> 
> I've attached two images: "test1" has samples 10, "test2" has samples 200.
> 
> 
>>It takes some trial and error, but it should be possible to get the
>>same effect with a much faster render. Of course, it all depends on
>>whether you can afford to waste more time fiddling with the
>>parameters or simply letting the computer render in the background
>>while you're doing something else ;) To make accurate comparisons, 
>>you also need to make sure that the maximum number of samples is the
>>same in both cases (I'm not talking of the second parameter of the
>>"samples" keyword here, but of the number of potential samples which
>>is something in the order of min_samples * 2^aa_level). Then if you
>>set aa_threshold to 0 the results should be the same (and take
>>*forever* to render) so all you need to do is find a value that's low
>>enough to get the effect you want while not being exactly 0.
> 
> 
> 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?
> 
> 
>>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.
> 
> 
> 
> ------------------------------------------------------------------------
> 
> 
> ------------------------------------------------------------------------
> 

Interestingly, in this particular case, method 2 with intervals 1 and 
samples 100 (or possibly less) gets pretty close to method 3 with 
samples set to 200.

Abe

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