This is driving me up the wall - I've been trying for hours now with no
results.

I want to render caustics "behind" a glass mesh2 object using photons.
Sounds simple. ;)

Please see the attached images. I've managed to get photon
mapping "happening" (i. e. Pov says its generating photons, and the photons
appear as listed in the final scene statistics.)

Image 1 shows what I want - i. e. the surface "under" the green plate
is "green" - exactly the effect I'm looking for. I want the green plate to
cast a green shade under it.

What I want is to have caustics appear in this green shaded area caused by
the glass object.

Now look at image 2 - the moment I turn on photon mapping, I get this black
shadowed area where the caustics caused by the glass plate are supposed to
appear - only it is a pitch black shadow, and STAYS that way whenever
photons are turned on.

How can I eliminate this black area and get caustics to show up in there?

My photons block:

photons
  {
        spacing 0.0025
        autostop 0
        jitter .4
  }

The object's photons block:

photons
 {
        target 0.05
        refraction on
        reflection on 
        collect on
 } 

The object's texture statement:

#declare GreenPlatetex = 

   material  // GreenPlatetex

   {   

      texture

      {      

         pigment

         {

            color rgbf <0.27451, 0.741176, 0.156863, 1.0>

         }      

         finish

         {

            ambient 0.2

            specular 0.4

            roughness 0.01

            conserve_energy

            reflection

            {

               0.15 , 0.25

               fresnel  off

               falloff  1.0

               exponent 1.0

               metallic 0.0

            }

         }

      }   

      interior

      {

         ior 1.5

         caustics 4.0

         dispersion 1.0

         dispersion_samples  12

      }

   }

NOTHING I've tried worked to eliminate the black shadow once I turn on
photons. This includes

- More photons (lower global density to increase number of photons shot) -
no result
- Lower and lower densities in the target object so more photons get
deposited - no result
- Material statement with "caustics" removed - no result
- Material statement with "caustics" given differing values - from 0.1 to
8 - no result
- Material statement with "dispersion" given differing values - from 0.1 to
8 - no result
- Material statement with dispersion removed (i. e. default "dispersion"
values active) - no result
- Material statement with caustics removed (i. e. default "caustics" values
active) - no result
- Material statement with transmit = 1, filter = 0 - no result
- Material statement with various other combinations of filter and
transmit - no result
- Make the object underneath the glass plate a photon target - no result
- Experiment with setting lower and higher target densities on the object
underneath the glass plate - no result
- Make sure collect is on for both of the objects / one of them at a time -
no result
- Make sure collect is off for both of the objects / one of them at a time -
no result
- The glass plate and the flat area under it is inside a sphere differenced
from another sphere. Remove the difference around them - no result.
- Make sure the difference is declared "hollow" - no result.
- Test in both Povray and Megapov (useless, but I tried anyway) - no result.
Same dark shadown in both. No surprise.

I've been at this for a few hours, with still zero progress. I must be
making some basic error or my usual stupidities are manifesting badly
today.

What am I doing wrong? How can I remove the solid black shadow as shown in
image2.jpg and have caustics show up there?

-- 
Stefan

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