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
26 Sep 2022 01:39:32 EDT (-0400)
  Re: weird white stain with beta povray v3.8.0 (not with povray v3.7.0)  
From: Kenneth
Date: 21 Jan 2022 09:55:00
Message: <web.61eac7cac82cc78e4cef624e6e066e29@news.povray.org>
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;

assumed_gamma 1.0

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.]

camera {
  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

box{0,1 translate -.5 scale <100,.01,100>
pigment{rgb <.3,.3,1>}

merge{ // or union
cylinder{-45*x,45*x,.3 rotate 120*y}
cylinder{-45*x,45*x,.3 rotate 240*y}
pigment{rgbt 1}
  emission .003*<.1,1,.1>
  method 3
  intervals 1
  samples 30
 //translate ... *y

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