"Ard" <ard### [at] waikatoacnz> wrote:
> Kenneth, thanks for stirring this up again (seriously).

Thanks for your very well-considered response.  Believe me, I donned a very
thick suit of armor before deciding to post my discussion! I figured I'd be
seen as either the new Einstein of POV-dom, or else a lowly protozoa ;-)
But I LIKE a lively dicussion!

Let me apologize to everyone for originally posting such a long discourse;
it's just that I wanted to cover things thoroughly.  It DID start out even
longer!

Something you said (and which Christian Walther also discussed) is probably
at the CORE of my basic misunderstanding of the topic.  As I mentioned in
my post...

"We all assume --don't we?--that POV's
color/brightness values, as used in a typical PIGMENT block, are meant to
reproduce brightness levels such that <.5,.5,.5> represents "half as
perceptually bright as" <1,1,1>."

Your own response has given me MUCH to think about...
>
> Before we go further you should take a mathematical look at what POV
> does to a 0.5 pixel.... with assumed_gamma
> omitted or set to display_gamma, your output pixel will be exactly as
> set in the pigment.  With display_gamma/assumed_gamma at 2.0, every
> pixel value is raised to the power of 1/2.0.  0.5 grey will be raised
> above 0.70. Worse, 0.05 grey is taken to 0.22, which is why the difference between
> the darkest two bands in your test seems ridiculous.  To get a
> near-black in my scenes that will react to diffuse light but still
> look dark, I have to play with input values around 0.03.  I see your
> point: it seems daft.  It's just the way it is.
>

and Christian Walther responded...

      >No. At least I don't assume that, and I think most other people with
      >some background in physically-based color theory and computer
      >graphics don't either....Of course that means that you and I are
      >starting from different premises...

That really does open my eyes, and explain a lot. Essentially, I've been
writing POV scenes using the wrong assumption! I confess it never occured
to me that I should be choosing "non-linear" color and brightness values in
my POV scenes...which would make perfect sense when using an assumed_gamma
of 1.0.  And would make my grey-band test incorrect. (Am I putting the facts
together correctly?) If so,
then...WOW...that's a big conceptual change for me.

Of course, the question that immediately comes to my mind is, "Gee, why NOT
simple linear POV values?" But I need to assimilate this paradigm shift
before posting further...

Thanks again (everyone!)

Ken

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