Jim Charter wrote:

> While developing the attached images I was manipulating different
> lighting and finish effects to get a certain design result rather than
> absolute photo realism.
> 
> Working with shadowless objects I was balancing the diffuse with the
> reflection values in the finish of the reflecting plane.  When I turned
> fresnel "on" I got a result that both surprised and pleased me. It is
> demonstrated in the second inmage. It surprised me because I thought the
> fresnel function required variable reflection to be specified with a max
> *and* a min value, but I was using just one value. Turns out the inverse
> seems to be true, you can turn off all variable reflection by specifying
> both  max and min values with max = min.
> 
> This lead to running a few more tests which can be seen here:
> 
> http://www21.brinkster.com/jrcsurvey/fresnel/
> 
> Surprising to me also is the effect of ior = 1, now reflection seems to
> be cancelled unless different mas and min values are put in explicitely.
> 
> Anyway, any preferences when comparing the attached images?
> Can anyone help explain some of the effects I am seeing?

Fresnel reflection describes the reflection of light from a boundary between
two materials, one of which is probably air in your case. The index of
refraction (ior) equals 1 for air and e.g. about 1.5 for glass. ior=1 means
a boundary between air and air which means there isn't any. From a
non-existent boundary there is no reflected light. That's what you
observed.

The effect of reflection observed in you second image is the following. The
Fresnel equations describe the variation of reflectivity with the angle of
incidence of the light. Reflectivity is close to 1 for grazing incident
light and becomes less the steeper the angle of incidence is. Therefore,
the plane supporting your shoe seems bright close to the horizont and
darker close to the observer.

I'd suggest, you'd take an optics book and go through the basic chapters
again...

Alf

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