Christian Walther <cwa### [at] gmxch> wrote:
> Kenneth wrote:

> > Such an omission makes me wonder if
> > "correcting" for lighting anomolies...when using assumed_gamma of 1.0...was
> > ever even considered!
>
> I don't know - reading
> <http://www.povray.org/documentation/view/3.6.1/346/> in my opinion
> describes quite well what brilliance does. Isn't what's described there
> exactly "correcting the lighting"? I don't see a fundamental difference
> between those two uses...

That's true; but as I read it, POV's discusion of that keyword treats it
more as a "special effect" than as a fundamental tool for adjusting the
surface reflectance of lighting on an object (actually, it would have to be
ALL objects) in a scene. It's just surprising that such an important,
all-encompassing use is not even touched on. The interesting thing to me is
that the default value is 1.0 (the value used when it isn't specified.) I'm
guessing this is because assumed_gamma is recommended to be 1.0 as well.
And when it is, then making brilliance equal to display_gamma (or
system/monitor gamma)..as you suggested...does indeed produce "realistic"
light/object interaction (the way I like to see it, anyway.)
>
> I guess you're just the first one to see physically correct rendering of
> an ideal diffuse reflector (which is just an approximation to real-world
> surfaces, admittedly, but not that bad for many types of materials, if
> combined with highlights) as an "anomaly".

Ah, this is where we "see" things fundamentally differently. :-) Based on
how light seems (to me) to interact with objects I look at, I think that
assumed_gamma of 1.0 actually DOESN'T create "an approximation to
real-world surfaces"--without using the various SDL  tweaks so far
discussed, anyway. (And I'm not totally convinced that ALL the "anomolies"
I see can be completely eliminated this way; don't know.)  It's
strange--yet interesting!--that we have such a different take on this. But
I have to assume that (many?) others are in your camp, so to speak.  (This
is why I was hoping to have a sort of "roundtable discussion" of lots of
users' opinions of Ard's two images.) I wonder--is the POV community
equally divided on this particular issue, or am I one of the few wandering
madmen??

I do understand all that you've said about physically-correct
lighting/object interaction, using assumed_gamma of 1.0.  It all makes
sense, physically; but, used by itself and with no SDL tweaks, the overall
effect just doesn't corresponded to what I think I should see in the POV
preview render.  Alas!  To tweak or not to tweak, that is the question! :-P

>
> > Photoshop is, in effect, insulating me from the need to worry
> > about gamma correction. A beautiful, intuitive way of working!!
>
> Right, but I suspect this is more by accident than by design. 127 is
> converted by the display's response to approximately (127/255)^2.2 = 21%
> light intensity, which by the response of the human visual system is
> converted to approximately "half the perceptual brightness".

That's, I believe, a feature BY design, in order for it to be an
easy-to-use, intuitive WYSIWYG graphics program...one of its big original
selling points. (And probably the main reason I was never really cognizant
of gamma correction until I started using POV; it was all, or mostly, kept
behind the scenes.)
>
>
> > ...and then re-massaged THOSE values before spitting them back to the
> > monitor, so that they appeared perceptually correct in the user's
> > chosen gamma environment?
>
> POV-Ray already does that (if you use assumed_gamma 1).

Hmm. Again, I think (?) we're possibly coming at that from two different
perspectives...your's physically-based, mine artistically (or perceptully)
based. I disagree, using my "linear-value" grey-band test scene as a prime
example.
>
> > (Of course, my own use of assumed_gamma of 2.0 mimics that behavior!
> > But as has been pointed out, that's not the correct way of working.
> > A real conundrum.)
>
> It mimics that behavior *and* causes the internal calculations to be
> done in a physically incorrect way. That's a price you (as an artist)
> may be willing to pay, but I (as a physicist) am not...
> And getting a realistic image is usually
> my primary goal (over getting an artistically pleasing image).

Yes, I understand the difference.

A most interesting discussion!!

Ken

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