Could this be down to max_trace_level?

I have read before that black dots in a translucent/reflective object 
could be caused by too low max_trace_level.

Dave.

Bruno Cabasson wrote:
> "etrask" <et7### [at] yahoocom> wrote:
> 
>>I have not checked your code, but perhaps you could try moving the glass
>>block upwards very slightly, to make sure there is not a coincident surface
>>at the bottom. This could cause the strange dots you see at the top.
>>
>>But if there is no coincident surface down there, then I am just rambling
>>for no apparent reason. =P
>>
>>I like your picture though, very neat. Only thing I could suggest is perhaps
>>to get the glyphs to stand out a bit more.
>>
>>"Burki" <nomail@nomail> wrote:
>>
>>>Hi all !
>>>
>>>
>>>A Mayan stela with the date glyphs of Dec-21-2005.
>>>
>>>
>>>
>>>Any idea how to get rid of the silly dots on top of the stela?
>>>It is an isosurface with function f_superellipsoid. But these dots also turn
>>>up when using a real superelli of the same size and material.
>>>Find the code at povray.binaries.scene-files with the same subject name.
>>>
>>>Comments welcome!
>>>
>>>
>>>Yours,
>>>Bu.
> 
> 
> I tried to follow etrask's idea and moved the stella a little upward, but
> it still persist. I also tried to reduce the accuracy of the
> isosurface down to 0.00001, but still the same. I also tried to put a white
> plane above the whole scene in case it would come from reflection of the
> black sky ...
> 
> I just notice that the bottom of the stella renders quite dark.
> 
> Maybe someone else can help you. Could it be a numerical artifact (things go
> better with a box instead of superllipsoid-like shape)? Anyway, I wonder
> why this undesirable effect...
> 
> Regards.
> 
>     Bruno.
> 
>

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I doubt raising the max_trace_level would help but you can give it a try. i
would recommend raising the max_gradient of the isosurface instead (a value
of 4 should work i guess). at least this always helped with my isosurfaces
;-)

Regards Roman

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"Roman Reiner" <lim### [at] gmxde> wrote:
> I doubt raising the max_trace_level would help but you can give it a try. i
> would recommend raising the max_gradient of the isosurface instead (a value
> of 4 should work i guess). at least this always helped with my isosurfaces
> ;-)
>
> Regards Roman

"Roman Reiner" <lim### [at] gmxde> wrote:
> I doubt raising the max_trace_level would help but you can give it a try. i
> would recommend raising the max_gradient of the isosurface instead (a value
> of 4 should work i guess). at least this always helped with my isosurfaces
> ;-)
>
> Regards Roman

I did some tests and found that max_gradient, adc_bailout, max_trace_level
all have no effect on the outcome. Neither does moving the superellipsoid
(except if you move it down so that it DOES intersect the plane).  This is
true for both the superellipsoid and isosurface versions.  The only thing
that I could change to remove the dots was reducing the ior value. In the
end, I'm not sure what causes the dots, but perhaps this will help someone
else identify the problem...

-tgq

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Thank you etrask and Bruno, David and Roman.

Before I read Bruno's post I already tested etrask's proposal and gave the
stela a translate 0.002 * y (1/1000 of its height). That already removed
most of the dots.

The carving should really a bit deeper. I have to balance the depth of the
carving with the refraction of the light in the shadow to the right.

Didn't try the accuracy trick as I feared the rendering time. BTW Bruno, you
must have spend a real long time with it, hope you didn't do it all with
photons...

The dark part at the bottom of the stela I think comes from the large ior.
Value is 1.765 - that of corundum (or of ruby what I just found out) ,
somewhere in between of glass and diamond. I will also try a lower value,
say that of emerald ( 1.575, ok emerald is hardly ever red but we're all
flexible aren't we).

To David and Roman: max_trace_level was 24. I'm asking myself if it can be
to high. But this is an the global block, not in the photons block.
max_gradient was 4. Wondering if higher values do anything

---
What I want to do further is to place a few tools that archaeologists use
around the stela. E.g. a brush, trowel, plumb bob, these kind of things.
Thinking about a background but still not sure what. Maybe a book shelf.


Thanks again all !
Yours,
Bu.

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Wasn't it Trevor G Quayle who wrote:
>"Roman Reiner" <lim### [at] gmxde> wrote:
>> I doubt raising the max_trace_level would help but you can give it a try. i
>> would recommend raising the max_gradient of the isosurface instead (a value
>> of 4 should work i guess). at least this always helped with my isosurfaces
>> ;-)
>>
>> Regards Roman
>
>"Roman Reiner" <lim### [at] gmxde> wrote:
>> I doubt raising the max_trace_level would help but you can give it a try. i
>> would recommend raising the max_gradient of the isosurface instead (a value
>> of 4 should work i guess). at least this always helped with my isosurfaces
>> ;-)
>>
>> Regards Roman
>
>I did some tests and found that max_gradient, adc_bailout, max_trace_level
>all have no effect on the outcome. Neither does moving the superellipsoid
>(except if you move it down so that it DOES intersect the plane).  This is
>true for both the superellipsoid and isosurface versions.  The only thing
>that I could change to remove the dots was reducing the ior value. In the
>end, I'm not sure what causes the dots, but perhaps this will help someone
>else identify the problem...

I stripped the problem down to a fairly minimal scene, to eliminate
anything to do with isosurface parameters, coincident surfaces, photon
artefacts, etc. and the dots are still there.

My guess is that they might possibly be real effects. The particular
shape of the superellipsoid causing infinite internal reflections.

If you set the max_trace_level to 100000, then there are fewer dots, and
the ones that are left turn red. That suggests to me that some rays are
getting trapped forever inside the object, becoming slightly redder with
each internal reflection.



global_settings {
     max_trace_level 24
}

background {rgb 1}
camera {
     location  <0.0, 1.5, -4.0>
     look_at   <0.5, 0.0,  0.0>
}

light_source {
     <200, 180, -200>                  
     color rgb 1
}
       

#declare gemmat=
material {
     texture {
          pigment {
               color rgbt <1,0,0,0.9>
          }
     }
     interior {
          ior 1.765                // Corundum
     }
}     


superellipsoid {
     <0.15, 0.15> 
     scale <0.6, 1, 0.25>
     rotate 15 * y
     translate z
     material {gemmat}
}

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> My guess is that they might possibly be real effects. The particular
> shape of the superellipsoid causing infinite internal reflections.
>
> If you set the max_trace_level to 100000, then there are fewer dots, and
> the ones that are left turn red. That suggests to me that some rays are
> getting trapped forever inside the object, becoming slightly redder with
> each internal reflection.
>


I also assume that this must be the case (I am unable to do any further
checks here at work, when I try to DL the source it gets corruped.  I'll
have to check later at home).

BTW I believe that max_trace_level can be maximum 256:

3.3.3.7  Max_Trace_Level
....
Values for max_trace_level can be set up to a maximum of 256. If there is no
max_trace_level set and during rendering the default value is reached, a
warning is issued

-tgq

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Hi!

I agree with Mike:


>My guess is that they might possibly be real effects. The particular
>shape of the superellipsoid causing infinite internal reflections.

When I set
max_trace_level 10
color instead if rgbt 1 to 0.96
and in the ior: fade_power 1001
then also black dots appear:

Bu.

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"Burki" <nomail@nomail> wrote:
> Hi!
>
> I agree with Mike:
>
>
> >My guess is that they might possibly be real effects. The particular
> >shape of the superellipsoid causing infinite internal reflections.
>
> When I set
> max_trace_level 10
> color instead if rgbt 1 to 0.96
> and in the ior: fade_power 1001
> then also black dots appear:
>
> Bu.

I just made another try with simplified scene (no carving, no photons, one
single light source), scaling the isosurface <0.6, 1, 0.45> instead of
<0.6, 1, 0.25> and lowered the 'east' and 'north' parameters (curvature
radii of the superellipsoid) down to 0.005 (almost a box), and here is what
I get. Rem: to avoid any undesirable interaction with the ground, the shape
is translated vertically 0.1*y.

The bottom of the stella is dark (adding a sky_sphere makes it look better)
because it reflects the black sky.

With higher values of the radii than my 0.005 (original 0.25 value for
example), the top seems to reflect that darkness and so on. The probem is
that these dark areas are irregular. If they were more regular, I would
think it is a normal and natural effect. So what? Numerical artifacts?

With east=north = 1, we get an ovoid that looks quite OK. With a box, it is
perfect. So, I think the problem comes from the superellisoid function
which maybe implements epsilons or whatsoever that produce those artifacts.

A last try with a sphere, and we have the same aspect than the
superellipsoid with radii=1.0.

My last remark for that post is: in the original scene, it would be cautious
to raise the shape enough along y-axis in order to make sure no point of
the ellipsoid touches the ground because the sides are also curved!

   See you soon

       Bruno

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"Burki" <nomail@nomail> wrote:

>
> Any idea how to get rid of the silly dots on top of the stela?

This is quite a thorny little problem!

Spent several hours with your code, trying just about every conceivable
combination of things, eliminating this, tweaking that, moving the object
here and there, yet the problem wouldn't go away.

Moving the camera around, I could see that the dark areas were confined to
the rounded edges of the shape and nowhere else (but visually distorted by
the high IOR to appear as odd blobs and speckles, at odd
locations...depending on where I placed the camera.)

So I added a sky_sphere, with a single gray rgb .5 value, and that DID
eliminate most of the problems (as has already been noted.)  Well, the
major dark areas were now "filled in" with gray (filtered by the cube's
redness, of course), so they blended better with the rest of the shape's
colors. But SOME actual dark areas still remained, at the object's "edge
centers."

I began to think that the problem had something to do with your scaling of
the object--that its IOR was also being scaled in some weird way,
non-uniformly.  But reading the POV docs, IOR isn't scaled (nor is anything
else in an INTERIOR block.) Eliminating your interior's light fading didn't
help either.

In frustration, I replaced your isosurface object with a regular, non-scaled
superellipsoid, as you did...which also didn't help.

Thinking your plane's marble texture was contributing something odd,  I
changed it to a simple rgb .5 value.  STILL no good.  Eliminating the
object's shadow didn't help either. MADDENING!

(In my frustration, I even began to imagine that the problem might be
lightwave interference inside the object...one wave canceling out another
and creating dark areas. But POV doesn't use "lightwaves".)

Since the MAX max_trace_level of 256 was now being exceeded, I was ready to
give up, and blame the problem on that.  (BTW, if you plug in a really high
max_trace_level value, POV simply limits it to 256... something I didn't
know until now. So 1000000 or whatever is the same as 256.)

Then it dawned on me: Your scene's lights are spotlights...which means,
somewhere outside the scene's visible area, the plane was going to
near-black, outside the spotlight cones. So I changed the plane's finish to
ambient 1 diffuse 0 to eliminate the lights' effects, and VOILA!!  The
remaining "problems" disappeared!! Your original object was just picking up
those remaining dark areas and distorting them into unrecognizable shapes.
And even though max_trace_level was being exceeded (with my "simpler"
superellipsoid), I couldn't see any trace of that.

Of course, now my superellipsoid justs looks like a big red cube!  Nothing
of visual interest outside it to give its IOR something to work with. So it
does needs *something* darker or brighter outside, to show its optical
effects...which may end up looking like artifacts.

This was a fun little scene to play around with and learn from.

Ken

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Hi all !

First thank you all for your contributions! Especially Bruno and Ken for
their experiments. They are very interesting.

Now returned from my computer-free Christmas holidays I applied Ken's idea
of adding ambience to the floor plate (but 0.3, not 1.0) plus a sky_sphere.

And: I gave the max_trace level a last change: In LOWERING it !

Since the dots may result from multiple reflections on the bent surface
inside the stela, I cut them down by lowering the max_trace value.
Bruno showed this reflections won't show in a more regular object (as in the
cube). I got the impression that all this reflections in a slightly curved
object cause this dots.

The black areas however are still there but I don't feel that they are a
problem. In fact I think they are a result of the high ior and have to be
there.

So now I present the outcome of some lower max_trace levels. The depth of
the carving is not changed though and will be my next PTS (*)




Yours,
Bu.


(*) PTS = problem to solve :-)

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