Crippso wrote:

> Thanks for the reply - but i notice you say that when the photons are
> perpendicular to the surface the fresnel reflection will be precisely 0. Is this
> really true in povray? In the real world the fresnel reflection would be 2 times
> ((1.5-1)/(1.5+1))^2 = 0.08.

good point. The documentation does not mention the details, and
since RC5 is broken its difficult to experiment with 3.7 behavior.
It's possible that even with fresnel the results are interpolated
between the min and max reflection values for artistic license. At
least in 3.6 changing the min value from 0 to 0.1 had an effect.
Depending on the implementation it could also be that reflections
for photons behave differently from normal tracing.

Apart from that as I already posted I found that the actual first
reflection seemed to be 0 up to some very large angle (3.6 also showed
this behavior so it's not related to the RC5 bug). It seems to me like
that may be a bug in the fresnel handling for photons ... basically
the following shows a hard fresnel reflection cutoff in 3.6 near


#default {finish {ambient 0 diffuse 0}}

global_settings
{
   assumed_gamma 1
   max_trace_level 10
   photons{count 500000}
}


camera
{
   orthographic
   location <0,0,1>
   look_at  <0,0,-1>
   angle 90
}

light_source{<10,0,-1>, 10}

box
{
   <-0.5,-0.5,3> <0.5,0.5,4>
   pigment{rgb 1}
   finish
   {
     reflection{0,1 fresnel}
   }
   interior{ior 1.5}
   photons
   {
     target
     reflection on
     refraction on
     collect off
   }
   translate 5*x
}

disc
{
   <0,0,-1> <0,0,1> 1
   pigment{rgb 1}
   finish{diffuse 1.0}
}

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