Burki nous apporta ses lumieres en ce 2005-06-20 06:52:
> Hi all, especially Houston!
> 
> I've got a problem.
> 
> I want to set up an underwater station with spotlights pointing in +/- all
> directions. The scene I made produced some strange artefacts (not shown
> here).
> 
> So I set up a test scene containing 11 spots, media and cam only. I could
> see black circles produced at the end of the spots pointing towards -z.
> 
> What can I do to get rid of these?
> 
> The athmospheric media (medium...) is Rayleigh or Henyey-Greenstein
> scattering. System is Windows XP.
> 
> The scene (below) should render in ~20 s at medium image size.
> 
> Hope you could give me some hints!
> 
> Yours,
> Bu.
> 
> 
> 
> 
> ////////////////////////////////////
> // Persistence of Vision Ray Tracer Scene Description File
> // File: ?.pov
> // Vers: 3.6
> // Desc: Basic Scene Example
> // Date: mm/dd/yy
> // Auth: Burki
> //
> 
> // #version 3.6;
> 
> 
> global_settings {
>   assumed_gamma 1.0
>   max_trace_level 15  //         <--- Max Trace quite high
>   ambient_light 0
> }
> 
> // ---------------------- CAM ------------------
> 
> camera {
>   location  <0.0, 4.5, -12.0>
>   direction 1.2*z
>   right     x*image_width/image_height
>   look_at   <0.0, 0.0,  0.0>
> }
> 
> 
> // -------- Athmospheric Media ---------
> 
> media {
>   scattering {
>      4,                      // 4=Rayleigh; 5=Henyey-Greenstein
>      rgb <0.4, 1, 0> * 0.02  // color / 'density'
>   }
>   method 3               // adaptive sampling
> }
> 
> 
> 
> #declare spot =
> light_source {
>   0
>   color rgb 100                 //  LIGHT COLOR 100 !!!
>   spotlight
>   translate <1, 0.0, 0>         // 1 unit right
>   point_at  <2, 0.1, 0>         // slightly upward
>   radius 5
>   tightness 50
>   falloff 8
>   fade_distance 1               // fades fast
>   fade_power 2
> }
> 
> 
> 
> // number_of_spots Spots in a circle (e.g. eleven)
> #declare spot_count = 0;
> #declare number_of_spots = 11;
> #while (spot_count < number_of_spots)
>         object {
>                 spot
>                 rotate spot_count * 360/number_of_spots *y
>         }
> #declare spot_count = spot_count + 1;
> #end
> 
> 
> // no objects, just light, media and cam.
> 
> ///////////////////////////////////// END
> 
> 
> 
> 
> 
There are to few samples for your situation,
Put your media in a large hollow box as follow:
box{50,-50 pigment{rgbt 1}
	interior{
		media {
		  scattering {
		     4,                      // 4=Rayleigh; 5=Henyey-Greenstein
		     rgb <0.4, 1, 0.0> * 0.02  // color / 'density'
		  }
		  method 3   // adaptive sampling. default, don't need to specify
		  samples 15 // increase as needed, weak effect on render time
		}
	}
	hollow
}
This will limit the rays to a finite distance. I increased the number of samples, it
was also a 
cause of your artefacts.

Alain

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