Here is the scene file which renders a black media object under the clouds
in front of a blue background. (See p.b.I. for an example image with has a
different background). Please try if you get the same results or if it is
white in your case.

Thanks,

Marc-Hendrik


#version unofficial Megapov 0.5;
global_settings {
   assumed_gamma 1.0
   ambient_light 0
   max_trace_level 14
}

camera
{
  location  <4, 1.7, -25>//
  //direction 1.5*z
  //right     4/3*x
  //angle 35
  look_at   <0, 11.200001,  0.00001>

}
background {rgb <0.3,0.3,0.5>}

light_source
{
  0*x // light's position (translated below)
  color rgb <1, 1 , 1> // light's color
  //area_light y,z,3,3 orient
  translate <-300, 1000, -450>
}

#declare Clouds=function "ridgedMF",<1,2.25,7,-1,1>

#declare Y=0;
#declare Ivals=5; //Number of cloud planes

#while(Y<1)
//plane{y,100+(Y*20) clipped_by{box{<-1000,0,-1000>,<1000,200,1000>}}
polygon{5,<-3000,-3000>,<-3000,3000>,<3000,3000>,<3000,-3000>,<-3000,-3000>

  rotate x*90 translate y*100+(Y*20)
  translate y*100+(Y*20)
  pigment{function Clouds(x,y,z)/3.8+noise3d(x/20,y/20,z/20)/3.5+(z/20)
   scale<300,150,300> phase -.5
   color_map{[.25 rgbf 1][.4 rgbt<1/1.5, 1/1.5, 1/1.5, .01>]}
  }
  finish{diffuse 1.5 ambient .25 }
  hollow double_illuminate
}
#declare Y=Y+1/Ivals;
#end

box {<-6,0,-2>, <8, 3,0>            //Schneegestoeber
     hollow on
     pigment {rgbf 1}
     interior{
        media{
           scattering{1,0.002}
           density{ gradient y
              color_map{
                 [0.4 rgbt 1]
                 [0.5 rgb 500]
                 [0.6 rgbt 1]
              }
              translate <2,-2.6,0>
              rotate z*-15
              turbulence 0.3

              scale 6
              translate <3,-0.5,0>
           }
           method 1
           intervals 1
           samples 10, 10
           confidence 0.999999
           variance 1.0/1024
        }
     }
}

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"Marc-Hendrik Bremer" <Mar### [at] t-onlinede> wrote in message
news:39d71e43@news.povray.org...
| Here is the scene file which renders a black media object under the clouds
| in front of a blue background. (See p.b.I. for an example image with has a
| different background). Please try if you get the same results or if it is
| white in your case.

Well, this much is fairly certain.  It's the polygon primitive causing it,
or at least something about it is.  I thought perhaps odd/even number of
them at first but no, that didn't make any difference.
Using the plane instead is fine as you probably know.
Nothing I can really say definite about it.  I mean, it sure doesn't seem to
be a inside/outside hollow problem since it's not disappearing only getting
darkened.
There might be previous mention of polygon not coexisting well with media
but if there is I don't know about it.

Addendum:
Wrong!  Well, partly wrong anyhow.  I'm leaving the first part of my reply
to prove how mistaken a person can be while trying to debug their scene
files (everyone knows that, don't they...?)
I was curious why I didn't see it the same this time as before.  Because I
had used my own light and camera!  Apparently the light location has a lot
to do with it, although it's still related to polygon in some way.
If the light source is further away from the camera line of sight the media
becomes darker.  Much like media type 5 (Henyey-Greenstein) would do I
guess, where it is less or more scattering depending on light direction.
Even absorption might behave similarly, however since none of these are used
it still remains the polygon in question.
It could be something to do with the high density amount too possibly but
that's pure conjecture from me.
Hope I've shed some light (pun) on the problem anyway.

Bob

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Indeed, the planes seem to work well (I did not know it, that commented out
line was in SamuelTs code already). But that might be 'cause the clouds are
on other positions with them, so the shadows are different.
It remains odd, 'cause it was the shadows of the clouds which  led to the
dark media. Even a second light source of rgb 1/2 at the camera position was
not able to light the media enough. But a "no_shadow" at the cloud layers
gave me white media. 1 Layer of clouds made them a little bit darker, with
two of them, the effect was even stronger. But while the surrounding white
HF remained somewhat lite grey, the media turned totally black with more
layers. Not what one would suspect, is it?
The high density amount is no problem, I think. I tried to raise the
over-all scattering amount and lowering the density value which gave me the
same problems (but did not look exactly the same, because of the different
transitions to the rgbt 1-values).

Well, I'll take the planes, as long as they work. Perhaps polygons are
faster? Anyway, the clouds don't add to much rendertime to this project, so
it doesn't matter.

Thanks for the help,

Marc-Hendrik

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The polygons are cleaned ;-) In fact the planes give the same problem, if
the shadows of them hit the media. What I don't understand is, why it is a
that dramatic difference. With my current settings the media is half lit,
half shadowed - half black or dark grey and half grainy white.
Perhaps I should work with light_groups to give the media an extra light.

Any further ideas?

Marc-Hendrik

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"Marc-Hendrik Bremer" <Mar### [at] t-onlinede> wrote in message
news:39d7c117@news.povray.org...
| The polygons are cleaned ;-) In fact the planes give the same problem, if
| the shadows of them hit the media. What I don't understand is, why it is a
| that dramatic difference. With my current settings the media is half lit,
| half shadowed - half black or dark grey and half grainy white.
| Perhaps I should work with light_groups to give the media an extra light.
|
| Any further ideas?

No but light_groups does seem a good work-around, if it's okay.  It could be
difficult (impossible?) getting it right if animated clouds were used since
the shadow changes would possibly need counteracting in the light used for
that media only.
I didn't see anything wrong with using the planes in place of polygons, no
shadowing.  I just now realized the translation was being done twice for the
plane.  Hastily checking out pov script is like that.

Bob

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Hello Bob and all the others,

does media interact with shadowless light_sources??????
I tried to use light_groups with the media, but although the media went dark
when excluded from the main light_source (witch was partly shadowed from the
clouds) - it does not light up with another "medialight" which was
shadowless. Removing the "shadowless"-keyword did work. Now I wonder if this
is a known and documented feature, since I couldn't find anything about it
in the Docs.

Marc-Hendrik

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Try to put an extinction smaller than 1 in your scattering media.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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