In article <3cf5a5f2$1@news.povray.org>, bob h wrote:

>Opaque, in this context, is probably meant to be about no transparency at
>all for the sake of POV-Ray. 

Yeah, that's what I thought it meant. But:

>Therefore, the statement should be right. 

Surely a material doesn't lose its refractive index just because it's
opaque. If I have a very thin slice of lead, and a thick block of lead,
both have the same refractive index, even though one sample absorbs more
light than the other.

>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.

I don't think the IOR could be a property of the surface only,
because it depends on the speed of light in the bulk material.
Obviously, refraction and reflection are surface phenomena only because
that's where the IOR changes.

>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. 

Yep, fair enough.

>I think it was discussed before that once you have any IOR you also
>have some sort of transparency anyhow. 

I don't think that can be true; metals are not (usually) transparent but
the Fresnel reflectance model can be applied to them. Can you point me
at that discussion, if it was in these newsgroups? My search must have 
been more fumble-fingered than I thought :-)

>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".

Is that really necessary when dealing with the Fresnel model? I can
understand the "glaze" doing the reflecting for ceramic tiles, etc, but
metals, simple plastics and the like don't have to have a glaze to
reflect. I could make an opaque material with an IOR and apply the
Fresnel model to it in POV, couldn't I? It'd render faster and be more
realistic (unless you were really dealing with ceramic tiles).

>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
>

I agree, a stray underscore.

Thanks,

Tom

Post a reply to this message