> Hi povray people,
>
> in my code below i have camera looking at a diffuse disc. A second disc with
> specular reflectance is used to reflect light onto the diffuse disc. When I run
> this I get a strange halo around a central bright spot. If I remove the second
> disc and point the light direct at my diffuse screen I get a simple spot in the
> image with no halo. Any thoughts on what might be causing the halo, and if it
> can be removed?
>
> thanks for your help
> stu
>
>
> global_settings
> {
>     assumed_gamma 1
>
>    photons{count    100000000 max_trace_level 20 adc_bailout 1e-11 jitter 0
> gather 500,1000}
>
> }
>
> #macro PhotonTarget(Reflect, Refract, IgnorePhotons)
>   photons {
>    target
>    reflection Reflect
>    refraction Refract
>    #if(IgnorePhotons=no) collect off #end
>   }
> #end
>
> //point light at 0,0,0 or at 0,0,-50
> light_source{<0,0,-20>, 1 cylinder radius 1 falloff 1 tightness 0 point_at
> <0,0,0>}
>
> camera
> {
>     orthographic
>     location <0,0,-20>
>     angle 40
>     look_at  <0,0,-50>
> }
> //delete this disc and point light at 0,0,-50 for simple case
> disc
> {
>     <0,0,0> <0,0,1> 50
>     pigment{rgb 1}
>     finish{ambient 0 reflection{1}}
>     PhotonTarget(yes,no,no)
> }
> disc
> {
>     <0,0,-50> <0,0,1> 50
>     pigment{rgb 1}
>     finish{ambient 0 diffuse 1.0}
>     PhotonTarget(yes,yes,yes)
> }
>
>

Your gather value is to large. Without it, the halo is very small.

Uasualy, you don't need to set gather and let POV-Ray estimate the 
appropriate parameter, whitch it normaly do quite well.

That said, when using photons, you always have some spreading and 
bleeding around sharp bright spots.

Why setting adc_bailout so small? 1e-11 is prety close to the epsilon 
value (a thressold value for considering a very small value as zero).
Commenting it out have no effect on your scene.

You don't need to set the screen object as a target for the photons, it 
will collect them just as well without any photons block.

jitter default to zero and you only need to use it if you want to have 
some random jittering to smooth things out.


Alain

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