POV-Ray : Newsgroups : povray.beta-test : weird white stain with beta povray v3.8.0 (not with povray v3.7.0) : Re: weird white stain with beta povray v3.8.0 (not with povray v3.7.0) Server Time
27 Sep 2022 20:44:21 EDT (-0400)
  Re: weird white stain with beta povray v3.8.0 (not with povray v3.7.0)  
From: Thomas de Groot
Date: 21 Jan 2022 11:10:04
Message: <61eadadc$1@news.povray.org>
Op 21-1-2022 om 15:52 schreef Kenneth:
> Alain Martel <kua### [at] videotronca> wrote:
>>
>> WHY put that light so far away ? Why not use a much closer one...
> 
> I agree. And from more tests, I see that the sky-sphere's gigantic size has a
> great deal to do with those super-bright radiosity patches-- which surprised me.
> 
> I ran an animation test using my posted 'simplified' code, while changing that
> sphere scale from 8000 (which is not *too* large) down to 50. At 8000, the rad
> patches are very bright; at 50, they are MUCH less bright. Currently, I don't
> know why that should happen-- so I came up with yet another very simple test
> scene of my own(!), which I post here. No light_sources in it. It produces the
> same weird effect; change the sphere scale to see the difference. (I've also
> attached an image example; if you can't see it here, it will nevertheless appear
> in the 'image digest'.) Note that all of the scene's color values are purposely
> very low.
> 
> What this tells me is that any scene with a media object in it, surrounded by
> another larger rad-emitting object like that sphere, may show some unexpected
> rad/media interactions somewhere, depending on the *scale* of the larger object
> and the proximity of the media object to others. It's 'as if' the sphere is
> causing brighter radiosity lighting from its colors, the larger it gets. I have
> no other guess or explanation at this point. It makes me wonder if certain
> radiosity values need tweaking, depending on how large a typical rad-emitting
> 'sky' sphere is (for example.)
> 
> Of course, one of the major reasons for the bright rad patches in all of the
> posted scenes here is that the media objects are halfway-embedded in the
> 'ground'... so there's a proximity effect at work. But I wonder if subtle flaws
> might also be occurring in 'typical' media/radiosity scenes that would be
> similar to Warren's.
> 
> ----
> #version 3.8;
> #declare RAD_ON = 1;
> #declare P_start = 64 / image_width;
> #declare P_end_final = 4 / image_width;
> 
> global_settings{
> assumed_gamma 1.0
> 
> #if(RAD_ON)
> radiosity{ // mostly Warren's settings
> pretrace_start P_start
> pretrace_end   P_end_final
> error_bound 0.2
> minimum_reuse 0.15 // Ken
> maximum_reuse 0.151 // Ken
> nearest_count 9
> count 50
> recursion_limit 1
> always_sample off
> //gray_threshold 0.6
> brightness 1
> adc_bailout 0.01/2
> normal on
> media on // [If OFF, it ELIMINATES the super-bright radiosity patches.]
>           }
> #end
> }
> 
> camera {
>    perspective
>    location  <15, 70, -140>
>    look_at   <0, 0,  -10>
>    right     x*image_width/image_height
>    angle 40
> }
> 
> // the large sky-sphere
> sphere{0, 1
> pigment{rgb .1}
>      finish{ambient 0 diffuse 0 emission 1}
>      scale 8000 // change to 50
>     // hollow on // does not seem to be necessary-- the media objects and
>                  // rad patches show up anyway
>      no_image
> }
> 
> box{0,1 translate -.5 scale <100,.01,100>
> pigment{rgb <.3,.3,1>}
> }
> 
> merge{ // or union
> cylinder{-45*x,45*x,.3}
> cylinder{-45*x,45*x,.3 rotate 120*y}
> cylinder{-45*x,45*x,.3 rotate 240*y}
> hollow
> pigment{rgbt 1}
> interior{
>   media{
>    emission .003*<.1,1,.1>
>    method 3
>    intervals 1
>    samples 30
>    }
>   }
>   //translate ... *y
> }
> 

Is there a reason why the sky sphere has emission 1 ? It is a night 
scene, so I would use emission 0 or emission0.01......

-- 
Thomas


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