POV-Ray : Newsgroups : povray.general : Radiosity and emitting media : Re: Radiosity and emitting media Server Time
27 Mar 2023 07:18:22 EDT (-0400)
  Re: Radiosity and emitting media  
From: Alain Martel
Date: 30 Nov 2022 12:54:18
Message: <638798ca$1@news.povray.org>
Le 2022-11-30 à 09:39, Chris R a écrit :
> I am working on a scene with a Christmas tree with tiny white lights all over
> it.  Since there are too many individual lights to model them as light sources,
> I am using media emission and radiosity.
> When I crank up the emission brightness enough so that the lights light up the
> surrounding tree branches realistically, they also create large bright spots on
> the walls surrounding the tree, which is not very realistic.  If I were modeling
> them as light sources, I would just add fading to solve the problem, but it
> doesn't look like there is a way to do that with radiosity and emissive media.
> I've played with max_sample and adc_bailout, but they uniformly reduce the
> effect of the bright objects, so I lose the illumination of the tree branches.
> I was thinking maybe the fade_distance and fade_power in the interior statement
> might work, but they seem to apply to light passing through the object, not
> emissions from the media in the object.
> Am I missing something, or is there just no current support for fading and
> radiosity from emissive objects?
> -- Chris R.

You are chasing the wrong things, using the wrong setings.
What you need to do is to increase the count. You may need to increase 
it to over 2000.
Please, use version 3.8, as it offers the importance feature.

In your global_settings radiosity block, crank up the count. Use the two 
values format : count 10000, 111111
Then, add a #default statement :
#default{ radiosity{importance 10000/75} }
That'll ensure that on average, you'll use about 75 samples.
I often use #declare for that.

#declare MaxCount = 1e6; // Yes, count can now get that big.
#declare AverageCount = 98;
#default {radiosity { importance AverageCount / MaxCount } }
		count MaxCount, MaxCount * 1.11+1
		//any other radiosity settings

THEN, add this radiosity block to your radiosity lights :
radiosity{ importance 1 }

There is no tweak for the fading of the radiosity effect of emissive 
objects. There IS fading as a side effect of how radiosity works. The 
fading do follow the inverse square law, but is also affected by the 
actual surface of the emissive object. If the emissive object is a plane 
or very big sphere surrounding the scene, then, the illumination is 
independent from the distance.

Rule of thumb :
If the object is small relative to the distance, it's contribution is as 
the inverse square law. Distance is at lease 20 times the size of the 
object. Should mostly be your case.
If the object is large relative to the distance, it's contribution will 
gradually change to straight inverse of the distance. Distance similar 
to the size up to a tenth of the size.
If the object is very large relative to the distance, then, the distance 
stop being a factor. Size of the object is at least 100 times greater 
than the distance.

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