Two polished spheres with fresnel reflection (at an ior of 1.5), 
illuminated from a single light source (behind and above the camera), 
with radiosity enabled.

Quiz:

One of these spheres is physically wrong. Can you spot the problem?

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Preview of image 'fresnel_conserve.png'

clipka <ano### [at] anonymousorg> wrote:
> Two polished spheres with fresnel reflection (at an ior of 1.5),
> illuminated from a single light source (behind and above the camera),
> with radiosity enabled.
>
> Quiz:
>
> One of these spheres is physically wrong. Can you spot the problem?

I guess the image name is a hint. I'm leaning toward the left one, as it seems
to be reflecting things a bit too brightly.

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clipka <ano### [at] anonymousorg> wrote:
> Two polished spheres with fresnel reflection (at an ior of 1.5),
> illuminated from a single light source (behind and above the camera),
> with radiosity enabled.
>
> Quiz:
>
> One of these spheres is physically wrong. Can you spot the problem?

The one on the right looks like it reflects the sky more realistically.

I have long noticed that conserve_energy does not have any effect on opaque
objects, although that seemed strange to me.  Shouldn't whatever light that is
reflected by the Fresnel effect not be reflected by other means?  So I'm going
to guess that the left sphere is Fresnel reflection as of 3.7.RC6, and you're
applying conserve_energy to Fresnel reflection on the right sphere.

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Am 11.02.2013 22:15, schrieb Samuel Benge:

>> Quiz:
>>
>> One of these spheres is physically wrong. Can you spot the problem?
>
> I guess the image name is a hint. I'm leaning toward the left one, as it seems
> to be reflecting things a bit too brightly.

Dang, I should have renamed it :-)

You're leaning in the right direction (no pun intended).

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Am 12.02.2013 01:05, schrieb Cousin Ricky:

>> Quiz:
>>
>> One of these spheres is physically wrong. Can you spot the problem?
>
> The one on the right looks like it reflects the sky more realistically.
>
> I have long noticed that conserve_energy does not have any effect on opaque
> objects, although that seemed strange to me.  Shouldn't whatever light that is
> reflected by the Fresnel effect not be reflected by other means?  So I'm going
> to guess that the left sphere is Fresnel reflection as of 3.7.RC6, and you're
> applying conserve_energy to Fresnel reflection on the right sphere.

You absolutely nailed it. Not much more to say about it actually. 
(Except maybe that I'm still cheating: I'm only attenuating the 
/outgoing/ diffuse light, depending on the angle to the camera. To 
/really/ do it right, I would also have to attenuate the /incoming/ 
light, depending on the angle to the light source; but that would 
require deeper changes to the existing code.)

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clipka <ano### [at] anonymousorg> wrote:
> Am 12.02.2013 01:05, schrieb Cousin Ricky:
>
> >> Quiz:
> >>
> >> One of these spheres is physically wrong. Can you spot the problem?
> >
> > The one on the right looks like it reflects the sky more realistically.
> >
> > I have long noticed that conserve_energy does not have any effect on opaque
> > objects, although that seemed strange to me.  Shouldn't whatever light that is
> > reflected by the Fresnel effect not be reflected by other means?  So I'm going
> > to guess that the left sphere is Fresnel reflection as of 3.7.RC6, and you're
> > applying conserve_energy to Fresnel reflection on the right sphere.
>
> You absolutely nailed it. Not much more to say about it actually.
> (Except maybe that I'm still cheating: I'm only attenuating the
> /outgoing/ diffuse light, depending on the angle to the camera. To
> /really/ do it right, I would also have to attenuate the /incoming/
> light, depending on the angle to the light source; but that would
> require deeper changes to the existing code.)

you know, this is one of those occasions when you know you're so used to CG that
you are tricked into believing the CG image is the correct one.  I really
thought
a shiny ball ought to have that bright sheen outside... :p

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I would also have picked the sphere on the right as being more correct--but only
because it simply appears to be reflecting the sky 'more' than the other
(without really knowing why.) Actually, my very *first* thought was that it
looked kind of like subsurface scattering.

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Am 12.02.2013 12:05, schrieb Kenneth:
> I would also have picked the sphere on the right as being more correct--but only
> because it simply appears to be reflecting the sky 'more' than the other
> (without really knowing why.) Actually, my very *first* thought was that it
> looked kind of like subsurface scattering.

As a matter of fact, when subsurface scattering is engaged, POV-Ray 
already does the right thing. This is because the whole diffuse term is 
replaced entirely by the so-called BSSRDF term, which already takes into 
account all relevant IOR-related effects (including, but not limited to, 
the fresnel effect).

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