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

>
> Be careful with the word "linear" - what you mean is "linear to
> perceived brightness" (right?), whereas I (and again, I guess most other
> people with some background in the field) say "linear" when I mean
> "linear to physical light intensity". That could lead to
> misunderstandings -

I've tried to use the word "linear" solely as a way of describing the
relative differences between my initial grey-band test values. That is,
.....05, .10, .15, .20, etc. changing "linearly" by .05 each time. And
"non-linear" as being deviations from that. I thought I made this clear in
my initial post, but if I've muddled the word usages since then, my
apologies. So "linear to perceived brightness" would be correct, as to my
usage.
>
> ...you may change the sphere's reflectivity properties using the
> "brilliance" keyword. With "finish { ambient 0 diffuse 1 brilliance 2 }"
> (and assumed_gamma 1), a similar image to Ard's lower one is achieved.
> (I'm too lazy now to look up what brilliance does exactly, but it may be
> that  "brilliance <display_gamma>" exactly reproduces that image.)
>
Hey, that's a  brilliant use of brilliance! ;-) But upon re-reading the POV
docs explaining that keyword, no mention is made of your particular use of
it.  Quite an important, basic use!  If brilliance was actually intended
for correcting the lighting of an object, shouldn't that have been covered
in a major way in the POV docs? Such an omission makes me wonder if
"correcting" for lighting anomolies...when using assumed_gamma of 1.0...was
ever even considered!

I'm following your overall logic quite well. Yes, we ARE approaching this
topic from two different standpoints. Yours seems to be that of the
scientist/engineer...and that's fine, of course. Wheras, I think of
POV as an artist's tool, one where <.5,.5,.5> being
"half as perceptully bright as white" has real validity. Let me illustrate
my mind-set this way: I came to POV from having used Photoshop for years.
As a graphics toolset, PS allows me to pick color/brightness values either
visually or by choosing rgb values. Numerically, it's own range is from 0
to 255. But the important thing is that, if I choose 127,127,127...right in
the middle...I do indeed get an on-screen gray that is "half as perceptully
bright as white." (Which is what I thought POV's <.5,.5,.5> should give me
as well.) Photoshop is, in effect, insulating me from the need to worry
about gamma correction. A beautiful, intuitive way of working!!  Yes, I
assumed from the get-go that POV operated like Photoshop...leading me to
use simple and intuitive "linear" color values in all my scenes (and an
assumed_gamma of 2.0 rather than 1.0, to "visually correct" for
that in the POV render preview...against the wishes of the
POV docs.) Believe me, I do now understand
that POV does NOT insulate me from gamma worries!!

So which philosophy of use is the more correct? I think it depends on the
use to which POV is put, and the mind-set of the user.  Purists and others
may disagree. I'm not trying to be "stubborn" about this. From my own
standpoint, of simply wanting to produce nice, realistic,
perceptually-pleasing images on my own system, then using assumed_gamma of
2.0 is just...easier and more intuitive...while 1.0 creates hurdles and
difficulties that have to be constantly addressed: having to specify
"non-linear" color values; having to add and tweak other POV values (like
brilliance) because of lighting anomolies; having to deal with image_map
images (created in any typical graphics program) that don't render
correctly unless they are in the .png format with an embedded gamma of 1.0.
These are time-consuming and non-intuitive drawbacks...to me, anyway.

If my use of assumed_gamma of 2.0 is robbing POV of its intended way of
working, I honestly won't shed too many tears. Not at the present time,
anyway! ;-) (If doing so created odd PROBLEMS in my scenes, I would think
otherwise; but so far I haven't seen gross manifestations of
that...not even subtle ones, for that matter...or
perhaps I've just figured out my own way around them.) I'm not closing the
door on assumed_gamma of 1.0; but so far, the cons outweigh the pros. And I
guess I'll just have to deal with POV 3.7 when it arrives (in its mature
state.)

My wish, from an artist's standpoint:  WOULDN'T IT BE NICE IF...POV allowed
an alternate way of working, one that allowed the use of "linear" color
values (as in Photoshop, and as in my initial POV gray-band test scene),
CHANGED those values internally to work in its own ideal color space of
assumed_gamma 1.0, 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? This would allow us graphics-oriented
folks to work in a way, and in an environment, that we're used to. It seems
that all the ingredients are there to do so..by a different internal use of
display_gamma and assumed_gamma, perhaps.  (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.)

Ken

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