POV-Ray : Newsgroups : povray.documentation.inbuilt : Finish-level Fresnel : Re: Finish-level Fresnel Server Time
28 Sep 2022 12:42:53 EDT (-0400)
  Re: Finish-level Fresnel  
From: Jim Holsenback
Date: 21 Dec 2016 12:09:51
Message: <585ab75f$1@news.povray.org>
On 12/21/2016 10:35 AM, clipka wrote:
> As for the syntax, I've decided to take it into my own hands now.

good thing i'm /going with the flow/ ... i've touched this waaaay too 
many times to justify my further involvement with the issues on this 
page. btw: look at the old page and see that i've /done/ a lot to clean 
up the one big gigantic run one sentence that this page /was/ before i 
got hold of it

> You know that something is utterly wrong with the structure of a page if
> the table of contents is preceded by an entire 2 pages worth of text.

this is a side-effect of the half-baked reference re-sectioning that i 
was opposed to at 3.7 release ... added __NOTOC__ to suppress table of 

> I think the description of the finish-level `fresnel` keyword and its
> effect is utterly misplaced there, and should instead be given a section
> of its own

disagree ... but it's in your hands now ... correct?

> (In a similar vein, I think the `use_alpha` keyword is also misplaced.
> There's not much to say about it, but it is a dedicated feature
> nonetheless, and shouldn't be in the section that deals with finish /per
> se/.)

disagree on the placement issue here as well

> The "Diffuse", "Phong" and "Specular" sections still have the "fresnel"
> keyword appearing in examples that seems to no longer have any
> connection to that keyword.
> Speaking of the "Diffuse" section, the description of the example in
> there has struck my as rather odd:
> -------------------------------------------------------
> finish { diffuse albedo 0.7 fresnel }
> Means that 70% of the light seen comes from direct illumination from
> light sources. The default value for diffuse is 0.6.
> -------------------------------------------------------
> In this form this is nonsense: "the light seen" that "comes from direct
> illumination from light sources" would also include highlights.
> I'd suggest something along the following lines:
> -------------------------------------------------------
> finish { diffuse albedo 0.7 }
> means that a white object with this finish reflects 70% of all incoming
> light /diffusely/.
> -------------------------------------------------------
> On to the "Phong" and "Specular" sections: In both of these, the
> `albedo` keyword description was inserted in a section that deals with
> `phong_size` or `roughness`, respectively, now leaving the sentence "If
> phong/specular is not specified phong_size/roughmess has no effect."
> dangling, disconnected from the section it belongs to.
> Also, the description of the reflection's `exponent` parameter is pretty
> bogus (but that's not your fault): Claiming that "POV-Ray uses a limited
> light model that cannot distinguish between objects which are simply
> brightly colored and objects which are extremely bright" is just plain
> wrong.
> I guess the text is just the result of people struggling to find an
> explanation for a phenomenon they failed to understand back then, which
> can now be named with just one word: Gamma!
> The text mentions "partially reflective surfaces". Now while most
> operations in the render engines _multiply_ colours, in this particular
> case colours are _added_ - namely the object's own colour and the
> reflected colour. And while multiplications work fine in any power-law
> gamma colour space, additions absolutely positively don't. In
> "assumed_gamma 2.2" mode, "middle and lower brightness objects typically
> look too bright" when another colour is added to them.
> So we might just as well replace the entire section with something like:

done ...

> -------------------------------------------------------
> exponent
> This property pre-dates the introduction of proper gamma handling.
> People found that it was difficult to model partially reflective
> surfaces in a realistic way, as middle and lower brightness objects
> typically looked too bright when reflected. As a means work around the
> phenomenon the optional exponent keyword was added, producing non-linear
> reflection intensities. The default value of 1.0 produces a linear
> curve. Lower values darken middle and low intensities and keep high
> intensity reflections bright. While this feature may still be used for
> artistic effects, it is strongly discouraged for renders aiming at
> realism. The original phenomenon is well understood by now, and using
> "assumed_gamma 1.0" as recommended will avoid it entirely.
> -------------------------------------------------------
> (Hold off on this for a moment though; I'll do a few experiments to
> verify that I'm right in my explanation of that old phenomenon.)

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