I'm an absolute beginner, so please bear with me.

I'm trying to create a snow globe, based on a hollow glass sphere. This
works as expected. However, when I place an additional object (sphere)
inside the globe, it turns out black, as if it's not lit. When I give the
additional sphere a reflective finish, it does come out as expected.
I'm using no lights in my scene, all light comes from HDRI lighting. When I
do place some lights, the second sphere does show its color, but still too
dark.

BTW, increasing max_trace_level doesn't help. The code for both objects is
stated below.


//glass globe
object{
 difference{
  sphere {
   <0, 0, 0> // center of sphere <X Y Z>
   500/65    // radius of sphere
  }
  sphere {
   <0, 0, 0> // center of sphere <X Y Z>
   (500/65)-0.15    // radius of sphere
  }
 }
 material{
    texture{
      pigment { color rgbf <0.98, 0.98, 0.98, 0.925> }
     finish { F_Glass3 }
    normal{bumps .1 scale 4}
   finish{reflection{.01,.9 fresnel} conserve_energy}
   }
  interior{I_Glass}
  }
}

//additional sphere
sphere {
  <0, 0, 0>, 2
  texture {
  pigment { color rgb <1,0,0>}
  finish {
   ambient 0
   diffuse 1
  }
 }
}

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> I'm using no lights in my scene, all light comes from HDRI lighting. When 
> I
> do place some lights, the second sphere does show its color, but still too
> dark.

Do you have a radiosity block?
With radiosity ambient 0 means an object doesn't emit light,
do you have other objects with higher abmbient as a light source,
with an HDRI image map?
Background and sky_sphere are an infinite distance so you'd
be better off using a large inverted sphere to provide a light.

> BTW, increasing max_trace_level doesn't help. The code for both objects is
> stated below.

max_trace_level 5 should be fine for this.

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> Do you have a radiosity block?

Yes.

> With radiosity ambient 0 means an object doesn't emit light,

Ambient 0 is recommended when using radiosity, no?

> do you have other objects with higher abmbient as a light source,
> with an HDRI image map?
> Background and sky_sphere are an infinite distance so you'd
> be better off using a large inverted sphere to provide a light.

I'm already using a large sphere with an HDRI image map instead of
sky_sphere. This shpere has ambient >0, hence it functions as a light
source.

BWT, I'm using MegaPov, since Povray 3.6 doesn't support HDRI.

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Tim Attwood <tim### [at] comcastnet> wrote:
> With radiosity ambient 0 means an object doesn't emit light

  The reflection of light on surfaces is controlled by the diffuse term
(as it should). The ambient component can be bumped up to artificially
make a surface emit more light than it normally would, but ambient is
not necessary, and in fact for "realistic" results should be turned off.
(After all, the ambient component is only a cheap radiosity substitute.)

-- 
                                                          - Warp

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"Arno" <nomail@nomail> wrote:
> I'm already using a large sphere with an HDRI image map instead of
> sky_sphere. This shpere has ambient >0, hence it functions as a light
> source.
>
> BWT, I'm using MegaPov, since Povray 3.6 doesn't support HDRI.

Aha. Try adding the no_radiosity keyword to your glass dome.

Bill

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"Bill Pragnell" <bil### [at] hotmailcom> wrote:
> "Arno" <nomail@nomail> wrote:
> > I'm already using a large sphere with an HDRI image map instead of
> > sky_sphere. This shpere has ambient >0, hence it functions as a light
> > source.
> >
> > BWT, I'm using MegaPov, since Povray 3.6 doesn't support HDRI.
>
> Aha. Try adding the no_radiosity keyword to your glass dome.
>
> Bill

Thanks! That did the trick.

Can anybody explain why this is needed here? I don't understand it.

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"Arno" <nomail@nomail> wrote:
> > Aha. Try adding the no_radiosity keyword to your glass dome.
> Thanks! That did the trick.
>
> Can anybody explain why this is needed here? I don't understand it.

I think it's just because the radiosity sampling ignores transparency.
Presumably, diffuse and ambient contributions to the illumination are taken
from the first object encountered along the sampling ray regardless of other
surface characteristics; as a side effect, transparent objects block
radiosity lighting. I'm sure others can expand on this if I have it wrong!

I'm not sure how one would reliably avoid this issue in standard POV-Ray
versions though - I think no_radiosity is indigenous to MegaPOV.

Bill

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Bill Pragnell nous apporta ses lumieres en ce 09-02-2007 07:32:
> "Arno" <nomail@nomail> wrote:
>>> Aha. Try adding the no_radiosity keyword to your glass dome.
>> Thanks! That did the trick.

>> Can anybody explain why this is needed here? I don't understand it.

> I think it's just because the radiosity sampling ignores transparency.
It get's thru transparent objects in standard POV-Ray.
> Presumably, diffuse and ambient contributions to the illumination are taken
> from the first object encountered along the sampling ray regardless of other
> surface characteristics; as a side effect, transparent objects block
> radiosity lighting. I'm sure others can expand on this if I have it wrong!

> I'm not sure how one would reliably avoid this issue in standard POV-Ray
> versions though - I think no_radiosity is indigenous to MegaPOV.

> Bill


Radiosity do work trough transparent objects, you can even have a lense or globe 
project an image on a surface. That image becomes crude if the focal lenght gets 
to long, but it's there.

-- 
Alain
-------------------------------------------------
WARNING: the crumsumpten of alcohol may Mack you tink you kan tpye reel gode

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Alain <ele### [at] netscapenet> wrote:
> Radiosity do work trough transparent objects, you can even have a lense or globe
> project an image on a surface. That image becomes crude if the focal lenght gets
> to long, but it's there.

Yes, I guessed it should be possible. Is there anything I can do to 'fix'
this problem except for adding the no_radiosity keyword?

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> Yes, I guessed it should be possible. Is there anything I can do to 'fix'
> this problem except for adding the no_radiosity keyword?

Render it as a 2 frame animation and use save_file to save the
radiosity data witout the glass, then load the data back in and
do a final frame with the glass.

global_settings {
  assumed_gamma 1.0
  max_trace_level 10
#switch (frame_number)
#case (1)
   radiosity {
      save_file "bidirectional.dat"
   }
   }
#break
#case (2)
   radiosity {
      load_file "bidirectional.dat"
   }
   }
   object {globe}
#break
#end

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