|
 |
> Hi all
>
> thanks for the help so far with Fresnel reflection and Photons. I have got some
> accurate results for some simple tests.
>
> I am trying now to get Povray to calculate the reflectance from a stack of two
> materials stuck together with no air gap between them. One material has ior =
> 3.2, the other ior=1.5. Using Fresnel equations I estimate reflectance of this
> structure to be ~36%.
>
> Can povray handle fresnel reflection from this sort of structure, and if so, how
> is it best to describe the two layers (I have tried union& merge but both do
> not seem to give the results I expected).
>
> Thanks
> Stu
>
>
You have a coincident surface problem. Whenever 1 surfaces are at the
same location, you can't reliably estimate the order in whitch they must
be evaluated.
The end result is that for a given ray, you may evaluate the surface of
the first object then of the second, but, for the next one, you may find
yourself evaluating the surfaces in the oposite order. It's prety random
and depends on floating point precision and rounding errors.
There are some attempt at finding a workable solution for that case, but
it's not yet done.
The usual solution is to have a very thin gap between the surfaces. In
this case, the finish of both surfaces need to be adjusted to have a
lower reflection to compensate for having two reflecting surfaces.
Some will instead mage the two objects overlap by a very small amount.
The gap or overlaping is usualy set to less than 0.00001 unit.
Another possibility is to have only one surface from one object having
an ior set to the difference between the two materials.
In your case, you can use ior = 1.7 for the high index material and
remove the low index object. It may not be suitable as your ray may need
to travel from the air, the low index material, the high index material
then finaly back to the air.
Alain
Post a reply to this message
|
|
|
|
Am 10.04.2012 19:16, schrieb Alain:
> The usual solution is to have a very thin gap between the surfaces. In
> this case, the finish of both surfaces need to be adjusted to have a
> lower reflection to compensate for having two reflecting surfaces.
> Some will instead mage the two objects overlap by a very small amount.
> The gap or overlaping is usualy set to less than 0.00001 unit.
Both solutions will add surplus interfaces between materials, leading to
extra reflections that don't occur in reality when the distance is small
enough.
It might be possible to work around this problem by using a small
overlapping area, and use a texture mapping approach for one of the
objects, using a fesnel reflective finish at the air interface side but
a non-reflective (and non-fresnel) finish at the side interfacing to the
other object.
Post a reply to this message
|
|
|
|
thanks guys, that was very helpful. I have taken on board your points and now
have a structure that avoids coincident interfaces by overlapping layers. I dont
need to start in air but I do need to end in it. I get around that problem by
adding strong absorption to a final air layer which stops the unwanted final
reflection.
My code is below and I think it is giving me reasonable results. However it is a
bit 'noisy'. Any ideas which settings could improve the uniformity of my result?
#default {finish {ambient 0 diffuse 0}}
global_settings
{
assumed_gamma 1
photons{count 9000000 max_trace_level 15 adc_bailout 0}
}
#macro PhotonTarget(Reflect, Refract, IgnorePhotons)
photons {
target
reflection Reflect
refraction Refract
#if(IgnorePhotons) collect off #end
}
#end
camera
{
orthographic
location <0,0,1>
look_at <0,0,-1>
angle 90
}
light_source{<0,0,2.5>, 1 cylinder radius 10 falloff 10 tightness 0 point_at
<0,0,4> parallel}
object{
box
{
<-100,-100,-2> <100,100,50>
pigment{rgbt 1}
finish
{
reflection{0,1 fresnel}
conserve_energy
}
interior{ior 3.2 fade_distance 1000 fade_power 1000}
PhotonTarget(yes, yes, no)
}
}
object{
box
{
<-100,-100,8> <100,100,10.5>
pigment{rgbt 1}
finish
{
reflection{0,1 fresnel}
conserve_energy
}
interior{ior 3.2 fade_distance 25 fade_power 1000}
PhotonTarget(yes, yes, no)
}
}
object{
box
{
<-100,-100,13> <100,100,14.5>
pigment{rgbt 1}
finish
{
reflection{0,1 fresnel}
conserve_energy
}
interior{ior 2.68 fade_distance 1000 fade_power 1000}
PhotonTarget(yes, yes, no)
}
}
object{
box
{
<-100,-100,14> <100,100,15>
pigment{rgbt 1}
finish
{
reflection{0,1 fresnel}
conserve_energy
}
interior{ior 1 fade_distance .01000 fade_power 1000}
PhotonTarget(yes, yes, no)
}
}
disc
{
<0,0,-1> <0,0,1> 100
pigment{rgb 1}
finish{diffuse 1.0}
}
Post a reply to this message
|
|
