POV-Ray : Newsgroups : povray.newusers : projected_through as a light portal? : Re: projected_through as a light portal? Server Time
26 Feb 2024 06:53:19 EST (-0500)
  Re: projected_through as a light portal?  
From: Alain
Date: 28 Jul 2017 23:02:43
Message: <597bfad3@news.povray.org>
Le 17-07-28 à 02:51, Mr a écrit :
> Hello. Suddenly the question occured to me: for an outside sunlight entering an
> interior scene, can we use the projected through keyword to the window glass to
> avoid wasting rays or photons oudoors? or os this not practically efficient?

projected_through can be viewed as a kind of negative shadow. Only the 
areas that would have been in the shadow of the projected_through object 
from a normal light are illuminated. Can be used to create some coloured 

If you use projected_through, you beed to add photons{pass_through} to 
the projected through object. Otherwise, it will intercept your photons. 
At least, that's what my tests show.

If you don't see some areas, you don't shoot any rays at it. So you 
don't waste any. You shoot rays from the camera toward the scene. Then, 
you test if there is a clear path toward your light(s). That's called 
backward ray tracing.

For the photons, they are ONLY shot toward objects that have a photon 
block that set them as target. That block can look as one of the 3 that 
follow :
photons{target reflection on}
photons{target refraction on}
photons{target reflection on refraction on}

For effeciency, don't stack several objects that are set as target. The 
first object will intercept, and block, all photons targetted at the 
objects behind it.

The refraction and reflection switches apply ONLY to primary photons. 
Secondary photons from an object set as reflection on refraction off CAN 
get refracted by another object. That object DON'T need to have any 
photons block, just be transparent and have an ior.

By the way, the full syntax for the target keyword is «target Float», 
where Float specify a spacing multiplier.
target 0.5 will cause the spacing for that object to be halved, causing 
the object to receive 4 times as many photons.

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