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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Attachments:
Download 'image1.jpg' (11 KB) Download 'image2.jpg' (11 KB)

Preview of image 'image1.jpg'

Preview of image 'image2.jpg'

On Sun, 07 Jan 2007 21:48:22 +0200, Stefan Viljoen
<spa### [at] removethispolardcom> wrote:

>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, 
Do you realise that when a material concentrates light into a bright spot the
rest of the shadow is darker? I asked the same question in
povray.binaries.images on 12 Nov 2004 under the subject 
Loss of colour in shadow when using photons. - a02a_1xb2.png


Regards
	Stephen

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

You can get the effect you want by lowering the ior from your interior,
I'm not sure if POV ior is really one for one acurate with RL ior numbers.
(1.01 would work ok)

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Tim Attwood nous apporta ses lumieres en ce 07-01-2007 16:46:
>>> What am I doing wrong? How can I remove the solid black shadow as shown in
>>> image2.jpg and have caustics show up there?

> You can get the effect you want by lowering the ior from your interior,
> I'm not sure if POV ior is really one for one acurate with RL ior numbers.
> (1.01 would work ok)

That's a very low ior. Barely more that that of air (about 1.0001, vacuum is ior 
1), more like compressed air. A realistic ior for a glass object is around 1.5 
to 1.8. POV use the standard equations to calculate the effect of the ior: 
reproduce real life acurately.

-- 
Alain
-------------------------------------------------
How come we choose from just two people to run for President and over fifty for 
Miss America?

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Stefan Viljoen <spa### [at] removethispolardcom> wrote:
> 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.

  It seems to me that POV-Ray calculates the refraction of light (for
the photons) too accurately and realistically, and what you want is just
some fake effect which looks good, regardless of physical accuracy.

  Photon mapping is not a "magic trick" which will produce sparkling
caustics-looking effects regardless of the object and other properties
of the scene. The older fake caustics (used with the 'caustics' keyword)
is more like that, but it's of course quite limited.

  Photon mapping calculates *accurately* how light coming from a point
light reflects/refracts from objects and hits other surfaces. This means
that the shape of the refracting object is quite relevant if you want to
get sparkling beautiful caustics. Caustics are produced when refraction
concentrates light from a larger area into a smaller area, thus
intensifying its brightness in that area. Naturally the object must
have a certain shape for this to happen. For example a planar sheet of
glass will not concentrate light and thus there will be no sparkling
caustics behind it.

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

  The area is black because light is not reaching it. What other color
would you expect it to be? The object is diverging the light so that it
won't reach all the shadowed parts.

  Of course in real life light will reach those areas from elsewhere,
reflected from other surfaces around, and this can be simulated with
radiosity, but that's a whole different question altogether.

-- 
                                                          - Warp

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Warp wrote:

> Stefan Viljoen <spa### [at] removethispolardcom> wrote:
>> 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.
> 
>   It seems to me that POV-Ray calculates the refraction of light (for
> the photons) too accurately and realistically, and what you want is just
> some fake effect which looks good, regardless of physical accuracy.
> 
>   Photon mapping is not a "magic trick" which will produce sparkling
> caustics-looking effects regardless of the object and other properties
> of the scene.

Exactly what I want in here...

> The older fake caustics (used with the 'caustics' keyword) 
> is more like that, but it's of course quite limited.
> 
>   Photon mapping calculates *accurately* how light coming from a point
> light reflects/refracts from objects and hits other surfaces. This means
> that the shape of the refracting object is quite relevant if you want to
> get sparkling beautiful caustics. Caustics are produced when refraction
> concentrates light from a larger area into a smaller area, thus
> intensifying its brightness in that area. Naturally the object must
> have a certain shape for this to happen. For example a planar sheet of
> glass will not concentrate light and thus there will be no sparkling
> caustics behind it.

Ok, that's what I was looking for as regards an explanation of what was
broken in my scene. I understand what you are saying. *sigh*

Back to the drawing board...

-- 
Stefan

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Stefan Viljoen wrote:
> Ok, that's what I was looking for as regards an explanation of what was
> broken in my scene. I understand what you are saying. *sigh*
> 
> Back to the drawing board...
> 

Note that you can still adjust the effect by changing your IOR value. 
For instance, many times an item which is meant to be made of crystal 
(ior 2.5, I believe) looks much better with one closer to water (1.33 
iirc).  If you're just going for artistic, feel free to experiment.

...Chambers

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