Alf Burau wrote:
> Jim Charter wrote:
> 
> <snip>
> 
>>Thanks for replying
>>
>>So I take it if you give the finish
>>reflection { .96 fresnel on } with interior { ior 1.5 }
>>then .96 is taken as the high reflectivity value as angle of incidence
>>approaches 0 and the low reflectively value, at angle of incidence 90,
>>is derived from the ior value?
> 
> 
> Jim,
> 
> maybe this will help you:
> 
> <http://www.povray.org/documentation/view/3.6.1/348/>
> 
> Alf
Thanks Alf, yes I was familiar with that page of the manual.  It is not 
a life and death thing I am just blathering.  But you see my confusion 
right?  The manual doesn't really say what happens if both "variable 
reflection" is invoked ie. a min_value and a max_value are specified, 
and fresnel is on.  It treats treats these nore like they are exclusive 
situations.  I, meanwhile had thought they were dependent on each other. 
  My long time assumption was that fresnel was some sort of falloff 
curve that was scaled to "fit between" the two values.  So I did not 
expect to see variation when I specified only one value.

So I guess it is like this:

Always the "standard" ( presumably linear ) function is used if non zero 
reflection is specified with the single value specified taken as min = 
max = value.  If two values are there, the slope of the falloff is max/min.

If fresnel is specified it overrides the standard function except in the 
case of ior = 1, then ?  If one reflection value is specified, the 
fresnel equation which returns zero reflectivity seems to take 
precedence,  but if two values are specified, the standard function 
seems to be used to return a reflectivity value but a lower one than it 
would if fresnel is off.  I just wondered if all this is folded into 
some universal formula combining the two functions, that I am too dense 
to see.

Anyway I will research fresnel as you suggest and probably come to look 
back on this whole post with embarrassment.  And I hadn't realized that 
it related so directly to ior, I just thought it was some sort of curve 
that happened to model reflectiveity falloff with increased accuracy.

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