Stephen Klebs wrote:

> Bluntly put. You're making this much too complicated.

There is probably some merit both in making things
simple and making them work right. Ideally, of course,
they should be both.

> If it looks right, it IS right. 

On the other hand, the amount of time needed to get something
to look right in the first place might depend on whether the
process of rendering is realistic.

> Now what do I get? Not a straight, smooth, gradual gradient from
> light to dark but a parabolic curve of values from light to slightly
> darker but still light to abruptly skewed to black. 

I have to admit I don't understand this either. Until now I
thought that since 3.7 now uses a linear color space, 0.5 would
represent a true midgray and all the nifty gamma handling ensures
that everyone perceives that midgray on their display when viewing
the image file. So I'd also have expected the end result of your
test scene to visually yield evenly spaced brightness steps.

Looking at the png itself it seems the upper two rows use 127
for midgray while the lower uses 186. From all I heard so far
186 should appear as midgray on a calibrated gamma 2.2 display
so that would appear to be correct. But I too perceive the
upper two rows as more visually correct. It's as if my lcd
used linear color space. I wonder if the Windows 7 built-in
display calibration is somehow playing tricks on me.

You can get the old behavior with assumed_gamma 2.2 or
by replacing rgb x with rgb pow(x,0.45).

Independent of this issue: Would it be useful to have a
setting of "color_gamma" that tells POV-Ray how to interpret
literal color values specified in SDL? The default value of 1.0
would yield the current behavior while e.g. 2.2 would internally
convert a value as rgb 186/255 to rgb 0.5 without the need for
plastering your code with gamma macros.

Post a reply to this message

Christian Froeschlin <chr### [at] chrfrde> wrote:
> Stephen Klebs wrote:

>
> > Now what do I get? Not a straight, smooth, gradual gradient from
> > light to dark but a parabolic curve of values from light to slightly
> > darker but still light to abruptly skewed to black.
>
> I have to admit I don't understand this either. Until now I
> thought that since 3.7 now uses a linear color space, 0.5 would
> represent a true midgray and all the nifty gamma handling ensures
> that everyone perceives that midgray on their display when viewing
> the image file. So I'd also have expected the end result of your
> test scene to visually yield evenly spaced brightness steps.

I too had assumed the same thing. (But I haven't yet tried out the relevant 3.7
beta and its gamma change, so I don't want to say too much more, because my
assumption could be wrong.) Yet Stephen's result does seem unexpected (that is,
based on my experience with v3.6.1 using assumed_gamma 2.2, not 1.0--I *thought*
the new 3.7 change was meant to match that, at least when viewing POV-Ray's
preview image.)
>
> [clip] I too perceive the
> upper two rows as more visually correct.

Same here (viewing it on my CRT monitor, BTW.)

Post a reply to this message

Am 30.11.2010 23:58, schrieb Christian Froeschlin:
>> Now what do I get? Not a straight, smooth, gradual gradient from
>> light to dark but a parabolic curve of values from light to slightly
>> darker but still light to abruptly skewed to black.
>
> I have to admit I don't understand this either. Until now I
> thought that since 3.7 now uses a linear color space, 0.5 would
> represent a true midgray and all the nifty gamma handling ensures
> that everyone perceives that midgray on their display when viewing
> the image file. So I'd also have expected the end result of your
> test scene to visually yield evenly spaced brightness steps.
>
> Looking at the png itself it seems the upper two rows use 127
> for midgray while the lower uses 186. From all I heard so far
> 186 should appear as midgray on a calibrated gamma 2.2 display
> so that would appear to be correct. But I too perceive the
> upper two rows as more visually correct. It's as if my lcd
> used linear color space. I wonder if the Windows 7 built-in
> display calibration is somehow playing tricks on me.

No, it's your eye playing tricks on you.

Our eyes are highly calibratible optical measurement tools - they work 
as well at a brightness of 10% as at 1,000% (and then some), and get 
object colors right regardless of lighting conditions.

There's the key.

Grab a piece of 100% white cardboard and another piece of 90% gray 
cardboard.

Now dim the light to half brightness. Obviously, your white cardboard 
will now reflect only 50% of full-brightness white into your eye, and 
your gray cardboard will reflect only 45% of full-brightness white.

So the absolute difference between the two pieces of cardboard has 
diminished from 10% full brightness to 5% full brightness. But our eyes 
are designed and trained to look not for the absolute light intensity, 
but for /relative/ difference, so that we can identify the "pigment" of 
objects irrespective of lighting conditions. The /apparent/ brightness 
difference between 45% and 50% is therefore roughly the same as between 
90% and 100%.

As a result, a truly linear gradient doesn't /appear/ linear to us: The 
"distance" between 10% and 20%, for instance, is percieved as roughly 
equal to that between 50% and 100%. Go figure.

(Theoretically this would imply that a /logarithmical/ gradient would 
appear "linear" to us; however, that's not exactly the case either, as 
human vision is a bit more complicated than that. My point here is that 
perception of brightness is surprisingly non-linear.)


> You can get the old behavior with assumed_gamma 2.2 or
> by replacing rgb x with rgb pow(x,0.45).

... or by "rgb x gamma 2.2".


> Independent of this issue: Would it be useful to have a
> setting of "color_gamma" that tells POV-Ray how to interpret
> literal color values specified in SDL? The default value of 1.0
> would yield the current behavior while e.g. 2.2 would internally
> convert a value as rgb 186/255 to rgb 0.5 without the need for
> plastering your code with gamma macros.

While the idea is compelling, there's a nasty catch to it: With 
POV-Ray's SDL being as powerful as it is, while lacking a clear 
distinction between colors and vectors, it is pretty difficult - if not 
impossible - to draw a clear boundary between "gamma land" and "linear 
country" that is both safe, sane and self-consistent.

Post a reply to this message

Am 01.12.2010 00:55, schrieb Kenneth:

>> [clip] I too perceive the
>> upper two rows as more visually correct.
>
> Same here (viewing it on my CRT monitor, BTW.)

Perfectly normal, and perfectly wrong (from a physical viewpoint that is).

Post a reply to this message

This whole issue of "realistic" is somewhat unrealistic. Photo-realism or
whatever one calls it is as much an artificial convention as any other means of
artistic expression. But this gets us into a relativistic riddle about light and
perception and pictures and how the brain processes visual sensations and how we
see things that's well beyond this issue. Even when we assume that we want to
make a picture that "looks just like" say a sunset over a lake we are in effect
not reproducing how we would see such a sunset but how we think we see it. We
are making a picture of it.

Lecture follow so you can skip to step w: The history of visual representation
has only recently adopted the notion that we see in pictures. As if we could see
ourselves seeing. Like a little observer standing behind our retina looking out
as if viewing a motion picture or a computer screen. As the state of perception
theory stands now the model of a "realistic" picture on the retina is being
replaced by the idea of the retina as just a converter box, while it's the
visual cortex that post-processes those visual sensations in ways that alter
their purely physical or mathematical input. Note, for example the banding on
the gradient example. That scalloping effect that enhances edge, is not in the
physical input but in the way the brain processes patterns of light and one
could say distorts the purely, as Steve Jobs would say, "retinal display". There
are several artificial notions in POV that do not exist in the real perceptual
world. Like ambient. Nothing has color without light unless it's a source of
light. It's just adjust control Sorry for the lecture here but I've been
fascinated with how we see for a long time. How the "human machine" sees is
truly a mystery.

With regard the new gamma model that's being applied to POV, two points: POV is
first a description language not like Photoshop were you just slide sliders back
and forth until things look right. There is nothing wrong and even wonderful
about this approach as long as you have immediate visual feedback. In POV you do
not. It's like taking photographs in the pre-digital age. You use an educated
guess to adjust aperture and f-stop or whatever and only in the darkroom do you
finally see what develops. Add to this add the fact that rendering and parsing
do not take a few minutes but sometimes days or even weeks. So as a description
language you have to learn techniques to anticipate what you want to come out or
at least to get something that after a week is not going to look like crap. So
when you tell the program draw a straight line from point A to point B, you
don't want to see at the end of the process something that not only looks like a
curve but is a curve. So to get to the point of the gradient example. It tells
POV that we want a linear succession values over a certain distance what it does
is give an exponential curve. That's fine but not what I intended and, if you
can't depend on 1 plus 1 plus 1 etc coming out a 3 or 4 or what ever, how do you
use a language that's going to be retranslated to mean something totally
different than what you expected it to say.

In 3.6 the evenly ramped gradient looked right. The values reported in Photoshop
or whatever said yes indeed 1 plus 1 is two. I've used POV in every possible
way, on Macs and PCs, to make images that looked fine in IE and FireFox or
Chrome or in print, and have in fact never encountered problems as serious as
this major revision seems to solve. I would like to see a concrete scene we
could render in both versions where it is demonstrated the need for the change.
But now there seems no way of predicting but more trial and error. I've used POV
for a long time because I love it like an addict, it's surprises and limitless
control. Now everything feels even more unpredictable and confusing.

Post a reply to this message

>
> No, it's your eye playing tricks on you.
>
> Our eyes are highly calibratible optical measurement tools - they work
> as well at a brightness of 10% as at 1,000% (and then some), and get
> object colors right regardless of lighting conditions.
>
>

This is not correct. You only understand the physics of the optical system, but
not the physiology nor the psychology. This is not how we see. You are talking
as if you could see yourself seeing and you can not. What you are seeing is a
model of how you think things appear. This is a very subtle and complex mystery.
The eye does not see. The lens, the retina or whatever. The are merely
transmission and conversion mechanisms. It's the brain and, to be more exact,
it's the visual cortex that perceives things. And it is nothing like a camera --
a long time but antiquated model for perception. It is not a camera but a
complex post-processing system, that enhances edges, exaggerates movements, etc.
that are not part of the optical or sensory input. And even to call it a machine
is not accurate. I suggest you read some more current literature on the
physiology of perception, how the visual cortex works, and in the artist realm a
book like Gombrich's Art and Illusion. Your are right about the so-called
"constancies" but this is not a matter of optics but of psychology and much of
that psychology comes to the conclusion that what we see less about how we see
"reality" but how we see what we think we see.

Post a reply to this message

> I wonder if the Windows 7 built-in
> display calibration is somehow playing tricks on me.
>
Actually Windows 7 is the first version of Windows that is not completely
color-blind. The color management and monitor calibration tools in Color
Management in the Control Panel are really as sophisticated as those long
existing in Mac. There you can choose to adjust your color model to match
several possible viewing  situations: monitors, offset, web or print

Post a reply to this message

> Grab a piece of 100% white cardboard and another piece of 90% gray
> cardboard.
>
> Now dim the light to half brightness. Obviously, your white cardboard
> will now reflect only 50% of full-brightness white into your eye, and
> your gray cardboard will reflect only 45% of full-brightness white.
>
> So the absolute difference between the two pieces of cardboard has
> diminished from 10% full brightness to 5% full brightness. But our eyes
> are designed and trained to look not for the absolute light intensity,
> but for /relative/ difference, so that we can identify the "pigment" of
> objects irrespective of lighting conditions. The /apparent/ brightness
> difference between 45% and 50% is therefore roughly the same as between
> 90% and 100%.
>
> As a result, a truly linear gradient doesn't /appear/ linear to us: The
> "distance" between 10% and 20%, for instance, is percieved as roughly
> equal to that between 50% and 100%. Go figure.
>
What you are missing here is that this ability for the brain to retain a
constant relation between widely varying changes in size, shape, colors or
perspective is not something you need to correct in the picture. The brain does
it. It works in a photograph as well, which is just a neutral recording device.
The Greeks made this error in their temples, for example. They thought by
flaring out the upper proportions in a trapezoid they were "correcting" the
dimension to make them look right.

Post a reply to this message

Am 01.12.2010 02:32, schrieb Stephen Klebs:
>>
>> No, it's your eye playing tricks on you.
>>
>> Our eyes are highly calibratible optical measurement tools - they work
>> as well at a brightness of 10% as at 1,000% (and then some), and get
>> object colors right regardless of lighting conditions.
>
> This is not correct. You only understand the physics of the optical system, but
> not the physiology nor the psychology. This is not how we see. You are talking
> as if you could see yourself seeing and you can not. What you are seeing is a
> model of how you think things appear. This is a very subtle and complex mystery.

I'm well aware that I'm simplifying here, but I'm not holding a lecture 
in physiology nor psychology here - I'm merely making a point that 
visual perception of brightness does not match physical brightness when 
it comes to gradients.

I didn't know you had background knowledge in those areas; it really 
didn't show in your approach to the gradients issue.

Post a reply to this message

Christian Froeschlin <chr### [at] chrfrde> wrote:
> Independent of this issue: Would it be useful to have a
> setting of "color_gamma" that tells POV-Ray how to interpret
> literal color values specified in SDL? The default value of 1.0
> would yield the current behavior while e.g. 2.2 would internally
> convert a value as rgb 186/255 to rgb 0.5 without the need for
> plastering your code with gamma macros.

Or a global_setting to toggle like default settings that just stipulate: for
every pigment past this point assume every color raised to the power 2.2 or 1.8
or whatever but then we're back to assumed_gamma again. The trouble is too that
ambience and light intensities and a whole host of other variables
are also playing their part. At this point, I would just like to see 3.6 ported
with multi-processor support.

Post a reply to this message