There's a problem with scattering media in pov3.7beta4.

  Try the following scene with pov3.6. The shadow of the cylinder
is basically perfect.
  Try it with pov3.7beta4. The shadow is full of sampling artifacts.
Also the media looks darker.

-------8<-------8<-------8<-------8<-------8<-------8<-------8<-------
background { rgb 1 }
camera { location <3,4,-5>*.8 look_at 0 angle 35 }
light_source { <20,40,10>, 1 }
box
{ <-1.5,-1.01,-1.5>, <1.5,-1.2,1.5>
  pigment { checker rgb .75, rgb .25 scale .2 }
}

box
{ -1,1 pigment { rgbt 1 } hollow
  interior
  { media
    { scattering { 1, .5 }
    }
  }
}
cylinder
{ <.9,-1,.7>, <.9,.9,.7>, .5
  pigment { rgb <1,.9,.7> }
}
-------8<-------8<-------8<-------8<-------8<-------8<-------8<-------


  The documentation gives some default values for scattering media.
I was wondering if it was the source of the problem (ie. pov3.7beta
using different default values than pov3.6). The mentioned default
values are the following:

      samples 1,1
      intervals 10
      aa_level 4 aa_threshold 0.1
      variance 1/128 confidence .9
      method 3

  However, it soon became clear that those are *not* the default values
for scattering media in pov3.6. If I add them to the media block in
the example code above, the rendering time (with pov3.6 at 640x480 +a +am2)
increases from 13s to 1m 21s.
  If I render the same scene (ie. with those values explicitly added) with
pov3.7beta4 the rendering time is equivalent and the image looks about
equal (except that the media is a bit darker).

  The actual default values used by pov3.6 are excellent because they
give high-quality scattering media very fast. However, the documentation
lists at least some of them wrongly. I think that pov3.7 should use these
same default values and the documentation should be updated.

  Making some tests it seems that the documentation has mixed the
default intervals value with the default min and max samples value.
If specify these values, the rendering time with pov3.6 keeps the
same (ie. about 13s) and the image looks the same:

      samples 10,10
      intervals 1
      aa_level 4 aa_threshold 0.1
      variance 1/128 confidence .9
      method 3

  However, if I render that with pov3.7beta4 the artifacts are still
there (and the render time is 24s).

  So it may be that pov3.7beta4 is using the same default values
as pov3.6 but the artifacts are caused by something else.

-- 
                                                          - Warp

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Warp wrote:
>   The documentation gives some default values for scattering media.
> I was wondering if it was the source of the problem (ie. pov3.7beta
> using different default values than pov3.6).

No, I repeat *no* default values changed!  If there is a difference, you 
found a *bug*, nothing more, nothing less.

	Thorsten

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> >   The documentation gives some default values for scattering media.
> > I was wondering if it was the source of the problem (ie. pov3.7beta
> > using different default values than pov3.6).
>
> No, I repeat *no* default values changed!  If there is a difference, you
> found a *bug*, nothing more, nothing less.

The documentation lists the default settings for method 1, and then says
that method 3 is the default.

See "default media setting inconsistancies (?)" in povray.general
(12/27/04), and then the discussion with Mike Raiford in "Media, Intervals,
samples and getting results" in povray.general (1/03/05). Sorry, I'd link if
I knew how. =)

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

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Thorsten Froehlich <tho### [at] trfde> wrote:
> No, I repeat *no* default values changed!  If there is a difference, you 
> found a *bug*, nothing more, nothing less.

  Then it's a bug.

  However, regardless of that, the documentation should be updated
to reflect that the default value of intervals is 1 (not 10) and
the default values for samples are 10,10 (or whatever they are exactly).

-- 
                                                          - Warp

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Warp wrote:
>   However, regardless of that, the documentation should be updated
> to reflect that the default value of intervals is 1 (not 10) and
> the default values for samples are 10,10 (or whatever they are exactly).

According to the 3.6 code, the defaults are as follows:

  Intervals:     10
  Min_Samples:    1
  Max_Samples:    1
  Confidence:     0.9
  Ratio:          0.9
  Variance:       1/128
  Method:         1
  AA_Threshold:   0.1
  AA_Level:       3

We use these same values in 3.7, so unless the values are being changed
somewhere else in the code, this isn't the issue.

I do agree however that the docs need to be corrected, the defaults for AA
level and method are both incorrectly listed.

-- Chris

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Warp wrote:
>   There's a problem with scattering media in pov3.7beta4.
> 
>   Try the following scene with pov3.6. The shadow of the cylinder
> is basically perfect.
>   Try it with pov3.7beta4. The shadow is full of sampling artifacts.
> Also the media looks darker.

Both of these issues are now fixed.

thanks,

-- Chris

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Chris Cason <nos### [at] deletethispovrayorg> wrote:
> According to the 3.6 code, the defaults are as follows:

>   Intervals:     10
>   Min_Samples:    1
>   Max_Samples:    1

  There's something odd going on because if you set those values in
the scene file the rendering time increases by a factor of 10 or so.
  If you specify intervals 1 and samples 10,10 you get an equal time
and resulting image.

-- 
                                                          - Warp

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>   Method:         1


That's definitely not the default method for 3.6; that would be method 3.

The other values you listed (such as intervals 10) are defaults only for
method 1, but have different defaults when method 3 is used. (See the other
threads I mentioned which have render statistics to support this.)

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

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Slime wrote:
> The other values you listed (such as intervals 10) are defaults only for
> method 1, but have different defaults when method 3 is used. (See the other
> threads I mentioned which have render statistics to support this.)

Well, as I pointed out, it could be being changed somewhere else in the code,
and in fact this is so, now that I look at it. It depends on language
version; if it's 3.5 or later you get the values Warp listed.

-- Chris

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