Hi guys

If I want caustics on a surface BELOW a photon target, must the surface that
the photon target is on also be a photon target?

I have tried many combinations but simply cannot get caustics on a surface
"behind" a photon target, even with very small spacings and lots of photons
- all my photon caustics stay confined to the glass object. I simply cannot
get caustics onto the table "behind" the glass standing on it.

Any idea what exactly is wrong? Concisely:

                        photon emitting light
                        v
            |   |       *
            |   |
____________|   |
     ^         ^ glass          
Surface

I want caustics not just inside the glass, but also here, on the "table":

caustics
    |
    |    |
    v    |
_________|

What should my scene code look like? Must the "table" be a target or not?
Must the glass "collect" photons? If I make it "collect" photons its
completely white, which I do not want.

Any help appreciated!

Thanks...
-- 
Stefan Viljoen
Software Support Technician
Polar Design Solutions

Post a reply to this message

Stefan Viljoen <rylan@ nous apporta ses lumieres ainsi en ce 2004/05/22 
09:13... :

>Hi guys
>
>If I want caustics on a surface BELOW a photon target, must the surface that
>the photon target is on also be a photon target?
>
>I have tried many combinations but simply cannot get caustics on a surface
>"behind" a photon target, even with very small spacings and lots of photons
>- all my photon caustics stay confined to the glass object. I simply cannot
>get caustics onto the table "behind" the glass standing on it.
>
>Any idea what exactly is wrong? Concisely:
>
>                        photon emitting light
>                        v
>            |   |       *
>            |   |
>____________|   |
>     ^         ^ glass          
>Surface
>
>I want caustics not just inside the glass, but also here, on the "table":
>
>caustics
>    |
>    |    |
>    v    |
>_________|
>
>What should my scene code look like? Must the "table" be a target or not?
>Must the glass "collect" photons? If I make it "collect" photons its
>completely white, which I do not want.
>
>Any help appreciated!
>
>Thanks...
>  
>
You use "target" to designate objects that actively interact with the 
photons: reflect, scater or diffract them. You need to explicitely set 
it. Default is target off.
You use "collect on" for objects on whitch the photons will show. 
Default is collect on.
You must set "refraction on" if you want the photons to bend according 
to the ior. Default is refraction off.
You must set "reflection on" if you want the photons to be reflected. 
Default is reflection off.
You MUST set an ior >1, otherwise, the photons will pass straight true. 
About 1.3 for fresh water, around 1.5 for glass. Also, you can optionaly 
set dispersion 1.01 to 1.1 to give you some chromatic dispersion.
Refraction and reflection must be set both for the light_source AND for 
the object in ther respective photons blocks.
If you want the photons to visible inside a media, you need to set 
"media on" for that media. Default is media off.

Alain

Post a reply to this message

> If I want caustics on a surface BELOW a photon target, must the surface
that
> the photon target is on also be a photon target?
>
> I have tried many combinations but simply cannot get caustics on a surface
> "behind" a photon target, even with very small spacings and lots of
photons
> - all my photon caustics stay confined to the glass object. I simply
cannot
> get caustics onto the table "behind" the glass standing on it.

Can you provide small, minimal sample scene (post it on
povray.binaries.scene-files), so that I could have a look at it? You could
also try to put both objects in a union and shoot photons at the union, but
that's just a random guess, nothing based on experience or such.

What might be happening is that POV-Ray shoots photons towards the general
direction of the "above" object, where they get stopped. Or, an idea which
has just popped up: how about raising the max_trace_level for the photons?
Perhaps you really just need to put the target onto the table, but raise the
max_trace_level of photons so that they rebounce/refract off of the
"underneath" object? Just another guess though.

Regards,
Tim
-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

Post a reply to this message

I've had a look at the file you've posted in scene-files. It isn't much help
when you post a working scene, cause that won't help me much in determining
the problem you're having with the problematic-scene.

What you might want to try though: If you don't have much experience with
photons, don't tinker with the default settings, for example: why are you
setting "radius"? Do you know what it's supposed to do and what it changes
with the new setting?

For another, I never use a modifier with the "target" keyword, but that's
more personal preferance than objective fact.

Finally: if the scene isn't working as expected, try narrowing it down.
Remove objects which you clearly know shouldn't affect the lighting (e.g.
objects which don't throw shadows on the glass-object), and tinker with the
transparancies of other objects.

Those are just general thoughts, now to some more specific stuff: "collect
off" will only stop photons from being deposited on a given object, photons
will still get refracted/reflected and they still deposit on other surfaces.
You're also using dispersion on your glass-object. Though it is a neat
effect, I've personally never achieved an effect worthwhile with it,
especially considering the amount of photons it would need to look really
good. It might be that I'm too dumb to make proper use of it, but I've just
used "normal" photons, and it works fine for me. You might want to switch
dispersion off for a few tests, just to see if that changes anything.

That's about all I can say which might help in finding the problem. Maybe
try a different camera angle, perhaps something in the scene isn't where
you're thinking it is, and thus it screws things up.

Regards,
Tim

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

Post a reply to this message