"Lonnie" <lon### [at] yahoocom> wrote:
> Alain <ele### [at] netscapenet> wrote:
> > Lonnie nous apporta ses lumieres en ce 2005-06-02 18:42:
> >
> > > I am about to tear out my hair over the same problem only with refracted
> > > light.  It seems that photons and media exclude each other.  My scene is a
> > > glass model of a Greek temple, totally transparent material, with ior and
> > > dispersion, photons enabled (target) and illiminated by a spotlight.  It is
> > > surrounded by a hollow, transparent cylinder filled with scattering media.
> > > Interior dispersion works fine, BUT:
> > >
> > > 1.  When both media and photons are enabled where the media is shadowed by
> > > the (transparent) material the media vanishes.  Caustics completely
> > > diappear leaving black shadows.
> > >
> > > 2.  When media is diabled and photons are enabled the caustics work fine and
> > > the shadows are correctly rendered in shades of grey.
> > >
> > > 3.  When photons are diabled and media is enabled the media works fine,
> > > showing up even in the shadows.
> > >
> > > I have spent many hours going over the documentation and I can find nothing
> > > about the cause of this.  Here is the code:
> > >
> > > global_settings {
> > >   assumed_gamma 1.0
> > >   max_trace_level 15
> > >
> > >    photons {
> > >      count 20000
> > >      jitter .4
> > >      media 300
> > >      max_trace_level 35
> > // Add this
> >  media 10
> > // Add this
> > >    }
> > > }
> > >
> > You need to add: media <max_steps> to switch media photons on. The default is
media 0 means that NO
> > photons are deposited in any media. In that case, direct illumination will light
the media, but
> > transmited and reflected light (photons) will ignore your media.
> > The mandatory parameter "max_steps" indicate how many photons to deposit in a
given interval.
> > Try the test value given. You will probably need to increase it, maybe around 1000
or more to get
> > optimum results.
> >
> > Alain
>
> Thanks Alain, but I tried that and my machine is still not buying.  Indeed,
> whenever I enable media it generates NO photons at all.  I tried saving the
> photon map and got a message "Could not save photon map.  No photons!"  I
> wonder if a have a bad install of 3.6?

Ah Ha!  At last all is revealed.  You are quite right about a media
statement in the global block, but max_steps 100 is quite sufficient for
most purposes.  The key to getting full refraction and reflection effects
to show up in the media is putting a photon block in the media container
itself.  Like this:

cylinder{
  <0,-.01,0>,<0,19,0>,17
  texture{
    pigment{ color rgbt <1,1,1,1>}
    finish{
      ambient 0
      specular 0
      diffuse 0
      reflection 0
    }
  }
  interior{
    media{
      scattering { 4, rgb 0.007}
      intervals 1
      method 1
    }
  }
  photons{
    target
    reflection off
    refraction off
    pass_through
  }
  hollow
}

Thanks for the help - I DID need to read the docs closer. Now everything
works as expected.  OF course, I may die of old age before the render
finishes :o)

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