Am 16.03.2018 um 20:17 schrieb Kenneth:
> clipka <ano### [at] anonymousorg> wrote:
>>
>> Actually, the safest bet is: Don't set radiosity brightness to
>> /anything/ other than 1.0 in the first place! It wrecks realism anyway.
>>
>> (A reasonable case can be made for brightness values slightly _above_
>> 1.0, when recursion_limit is set particularly low (2 or even 1), to
>> compensate for the fact that this reduces the overall brightness of the
>> scene a bit. But brightness values below 1.0 are of the devil. Don't go
>> near them.)
> 
> Thinking about this further: I assume you mean, when using a HDR light probe for
> the sky and no other lights in the scene? A brightness value of 1.0 would make
> sense.
> 
> Whereas, when using a typical LDR sky photo, and a (single) light in the scene
> for the 'Sun', a rad brightness value of 1.0 could be too bright, for various
> reasons.

No, I mean, literally, NEVER EVER set radiosity brightness below 1.0 (if
you want realistic renders).

The reason is that the radiosity brightness parameter is /not/ applied
to the end result of radiosity computations, but /at each level of depth/.

Consider a scene where one area (A) receives direct light from a classic
light source; another area (B) is placed such that it does not receive
any direct light, but diffuse light from (A); and yet another area (C)
is placed such that it does not receive any direct light nor diffuse
light from (A), but diffuse light from (B).

In such a scene, POV-Ray will scale the brightness of (B) by the
radiosity brightness setting, but it will scale the brightness of (C) by
the /square/ of the radiosity brightness setting. Thus, tampering with
the radiosity brightness setting will skew the relative brightness of
areas B and C (and also areas A and B).

The algorithm is designed such that it gives most realistic results at a
radiosity brightness setting of 1.0 (if it wasn't for recursion_limit
and adc_bailout, which both cause the algorithm to underestimate the
diffuse illumination, so to compensate, radiosity brightness should be
slightly higher than 1.0).


If you really think your scene gets too bright with radiosity enabled,
it's your light sources (light_source and sky sphere) which are to blame.

> Given that the objects in the scene have  finish {diffuse 1.0 emission 0} --
> with ambient turned off automatically-- and a light_source of rgb 1 (or more
> likely less than that), it would *seem* natural to turn down rad's brightness
> below 1.0. So that any object surfaces never exceed a surface brightness of 1.0
> max; otherwise, there's a danger of washing out the object colors. It would all
> depend on how bright the single light_source is: if it's set to rgb 0.6, then
> rad brightness should not exceed 0.4, according to my theory.

I see your point, but you're mistaken: Radiosity isn't an /alternative/
way of lighting things - it accounts for paths of illumination that
would otherwise be /missing/ from classic light source computations.

So in a sense it's not /activating/ radiosity that screws up your scene
brightness - it's /de-activaing/ radiosity that does.


If you're worried about the balance between direct and indirect
illumination and/or the total brightness of your scene, the proper knobs
to tweak are the brightness levels of the light sources and the sky sphere.


> However, I do see a major problem with my argument: The parts of the scene in
> *shadow* are only receiving the rad lighting-- and such shadow illumination
> would *vary* depending on my light_source brightness. Not very logical, I admit.
> (Or more specifically, not 'realistic.')  So I guess such a lighting scenario
> is, by default, not 'realistic'  at all, but just an attempt to make the scene
> 'pleasing to the eye.'  I.e., balancing 'Sunlight' brightness against shadow
> brightness, just to prevent any colors from being washed out.

You /can/ use the radiosity brightness setting for artistic purposes, to
emphasize shadows; however, my point remains: If you want /realistic/
renders, you should never ever set it below 1.0.

The problem with that kind of artistic tweaking is that you're depriving
yourself of any means to design your materials and light sources based
on physical principles, which means you're stuck with tweaking and
re-tweaking.

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