Le 18-03-18 à 16:31, Kenneth a écrit :
> clipka <ano### [at] anonymousorg> wrote:
>> Am 17.03.2018 um 15:39 schrieb Kenneth:
> 
>>> This probably still amounts to 'tweaking'-- but unless there's a remaining
>>> technical problem, it works for me.
>>
>> It remains tweaking indeed: This approach still breaks some (realistic)
>> brightness balance - namely that between radiosity and specular reflections.
> 
> :-(   As the old Yiddish saying says, "Oy Vey!"  :-P
> 
> I'm beginning to think that the only way to get a truly 'realistic' radiosity
> render is to use an HDR light probe for the sky, with no additional light
> sources... in which case, POV-Ray itself works with all the correct values(?)
> and in the correct way.
> 
> Otherwise, it seems that using a LOW-dynamic-range sky + light_source(s)
> requires *some* kind of tweaking (of one subtle thing or another).. yet which
> still works against the radiosity mechanism, if only in a technical sense.
> 
> My desire to increase the contrast of a rad scene-- if I feel that it's
> necessary-- looks like a no-win situation (except in the HDR light probe case).
> Unless I simply postpone that step, and do it later as a post-processing effect
> in another graphics app.
> 
>> Radiosity and diffuse are mutually balanced automatically (if you don't
>> tamper with radiosity brightness...
> 
> *That* is a key insight that I've been wondering about. I hope I understand it
> (as it relates to a LOW-dynamic-range sky set-up):
> 
> Given:
> radiosity{brightness 1.0}
> light_source{rgb .7}
> object{... pigment{rgb <.3,.5,.7> finish{diffuse 1.0}}
> 
> Does the 'automatic balancing' keep the object surface color at <.3,.5,.7>
> (depending of course on the angle-of-incidence of the light source hitting it)?
> Or does my diffuse setting need to be manually reduced, to keep the color from
> washing out to, say, <.6,1.0,1.2> (just as a conceptual idea, not real math). Or
> am I still clueless about what 'automatic balancing means? :-O
> 
> 
> 
> 
> 

You always can use a sky_sphere or a large sphere enclosing your scene. 
You can use emission value larger than 1 if you want, or need.
You don't need to use HDRI, or even any image format at all for your sky.

To increase the contrast, and get better shadows, you can reduce the 
error_bound value. The default is correct when you also use some 
conventional light, but it need to be lowered for pure radiosity scenes.

In the example that you provide, you need to also take into account the 
diffuse part of your finish. It's diffuse 0.7 if you don't explicitly 
change it.
Here, the colour is, for a light perpendicular to the surface and that 
highly unrealistic diffuse of 1 :
rgb<0.3, 0.5, 0.7>*<0.7, 0.7, 0.7> = <0.21, 0.35, 0.49>
Next, if you use some colour picker, there is an additional gamma 
handling to take into account that will effectively change all channels 
in a non-linear way and thus change their relative values and reduce the 
saturation.

Don't forget that using diffuse 1 is NOT realistic. A very bright 
surface may have a diffuse as high as 0.95 or even 0.98, never 1.

Using diffuse 1 is a very good way to get washed out colours.


Alain

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