"Tom York" <tom### [at] compsocmanacuk> wrote in message
news:3cf51e5a$1@news.povray.org...
> A very minor point; "Variable reflection", in Section 6.7.3.4
>
> "Remember that in real life many opaque objects have a thin layer of
> transparent glaze on their surface, and it is the glaze (which
> -does- have an IOR) that is reflective."
>
> I read that as suggesting that opaque materials do not have an index of
> refraction. Maybe the segment in brackets could be removed, since I
> think the emphasis on the glaze having an IOR gives the impression that
> the object beneath the glaze does not. Or is that just me? :-)

Opaque, in this context, is probably meant to be about no transparency at
all for the sake of POV-Ray. Therefore, the statement should be right. The
point of attention is about refraction of only a surface, however I don't
know if this itself is misleading since 'fresnel' is used for bodies of
water, etc., too.
But anyway, the paragraph there is actually trying to put emphasis on the
fact you may also want fresnel reflection from the ior of a solid objects
surface. I think it was discussed before that once you have any IOR you also
have some sort of transparency anyhow. The only thing I can think of about
this matter would be that the infinitely thin transparent surface is
presumed to be a part of a finite non-transparent object, the two being
apart yet the IOR applied to all. Either to be dealt with on a per object
basis or you must create a CSG pair of objects to act as the "glaze".

While on the subject... a sentence in that last paragraph has what a person
might think could be a keyword but is not:
So with a fresnel >>reflection_type<< an interior { ior IOR } statement is
required, even with opaque pigments.

bob h

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