clipka <ano### [at] anonymousorg> wrote:
> - Gradients have been on my agenda for quite a while already, but I 
> hadn't found the time to investigate; nor was I aware that more 
> "natural-feeling" gradients can be achieved using "poly_wave 2.2". While 
> being more cumbersome than I'd like, again it /can/ be done, and I still 
> have it on my personal agenda for a later version.

  I think that the problem with gradients not looking anywhere even close
to linear by default showcases a more fundamental problem.

  When you specify eg. a color_map, without any fine-tuning parameters,
the *default* result should be a color map that looks linear (eg. when
applied to a gradient pattern). Only if you wanted a non-linear mapping
would you specify parameters such as poly_wave with a value different
from 1.0 (the default value of "poly_wave 1.0" means linear mapping).

  The problem with the current pov3.7 is that such a gradient is not
looking even close to linear, even with monitors where 'rgb 0.5' is
truly 50% bright, as compared to a test pattern. I don't know why this
is so, but it just isn't.

  And it's not just a question of it looking slightly off. Like if you
took an image with a linear gradient created by pov3.6 and looked at it
in different CRT and LCD displays and comparing them to each other
side-by-side, you would probably see slight variations, and you could
make an estimate of which one of them looks the most linear transition,
hence making the others just slightly off. Different people would most
probably choose a different display which they thought produced the most
linear-looking gradient.

  No, it's nothing like that. The gradient produced by pov3.7 is very
*clearly* non-linear looking. It's *way* off. It looks very pronouncedly
logarithmic in nature. If you show the gradients created by pov3.6 and
pov3.7 to any random person, there will be no doubt in their choice (yes,
I have actually tested this with a friend).

  As said, I don't really know *why* this is so. As I have mentioned,
with my CRT a 'rgb 0.5' as produced by pov3.7 by default looks about
50% gray when compared to a test pattern, so *in theory* a linear
gradient produced by pov3.7 should look linear. But it just doesn't.
A linear pattern produced pov3.6 looks significantly more linear (and,
as I also said, it's not just me, as I have actually tested this with
another person, looking at the same images on the same display).

  Anyways, whatever the reason might be, it's undeniable that a problem
exists. What should be producing a linear gradient isn't doing so.

  A suggestion like "use 'poly_wave 2.2' in your color map" is not the
correct solution to the problem. It's simply counter-acting the effects
of the gamma correction. What "poly_wave 2.2" is saying is "make the
color transition pronouncedly non-linear, by a power of 2.2" (which is
quite a significant deviation from linear mapping; just compare the
functions "x" and "x^2.2", which is what this is exactly doing).
A suggestion of "use a non-linear mapping to achieve a linear-looking
gradient" is contradictory and shouldn't be the correct answer. A linear
looking gradient should be *the default*.

  And it's not just color gradients that are affected by this. It's any
transition from one color to another which should look linear. This
happens in surface lighting, shadow borders when using area lights,
and so on and so forth. (Note that surface lighting very rarely looks
literally linear because the cosine of the angle between the surface
normal and the direction of the light seldom changes linearly, but if
you devised such a surface, it *should* in that case look linear, but
with pov3.7 it won't.)

-- 
                                                          - Warp

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