POV-Ray : Newsgroups : povray.general : Does POV-Ray's gamma-adjustment info need updating? Server Time
28 Mar 2024 21:29:28 EDT (-0400)
  Does POV-Ray's gamma-adjustment info need updating? (Message 11 to 20 of 24)  
<<< Previous 10 Messages Goto Latest 10 Messages Next 4 Messages >>>
From: Kenneth
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 17 Oct 2017 15:15:00
Message: <web.59e6557442a9f98689df8d30@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:
>
> During normal operation, the electron beam traces a zig-zag pattern
> across the screen (of which only the zig is visible, as the electron gun
> is effectively turned off during the zag)...

Right-- the 'blanking signal', to allow the elctron gun(s) to zip back to their
start positions.

> ...This would be virtually impossible to align with the aperture mask
> so no such attempt is even made.
>
> ...The mask is carefully
> aligned with the phosphor dot matrix and has only one aperture per
> phosphor dot triplet, with the angles of the beams chosen in such a
> manner that each hits its corresponding dot from the triplet as
> precisely as technically feasible.

So the aperture mask has only ONE hole per color triplet.  :-0   THANKS, Clipka;
you've cleared up my LONG-standing misconception--I've always assumed the mask
had a hole for each and every phophor! Funny thing: I've been doing some
research about this-- 'CRTs', 'color television', 'Trinitron tubes', 'aperture
masks' etc. And not ONE of the sources I looked at mentioned this
single-aperture-per-triplet fact. So now, the slight 'angle-ing' of each
electron beam makes perfect sense, as to how it hits its proper phosphors on the
screen through that single hole.

A Eureka moment!

And getting back to the subject of gamma 2.2 non-linear blending of colors: It
occured to me that POV-ray's focal_blur camera might be mistaken by some users
for actual 'blurring'-- when it's really just lots of discreet 'randomized'
camera rays, shot out in a stochastic fashion. I.e., there's no actual
*blurring* of scene colors to cause the gamma 'darkening' effect.

But that raises a question in my mind: If a focal_blur camera has it
blur_samples set at a high value (like 500, for example), do any of those
discrete samples actually overlap? IF so, then I would imagine that there's some
color 'averagaing' going on behind-the-scenes, of the overlapped rays. Does that
relate to the gamma 2.2 color-blending problem? (Hmm, probably not-- because the
'averaging'-- if done-- is done *within* POV-ray, before the resulting pixel
ever reaches the monitor.)


Post a reply to this message

From: Bald Eagle
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 17 Oct 2017 20:25:00
Message: <web.59e69f5a42a9f9865cafe28e0@news.povray.org>
Just because I always happen to serendipitously stumble upon things that are
related:

http://makeanddo4d.com/spreadsheet/
https://www.andrewt.net/megapixel/

( Gamma!  :D )


Which I discovered through
https://www.youtube.com/watch?v=UBX2QQHlQ_I


Post a reply to this message

From: clipka
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 05:45:01
Message: <web.59e7218042a9f986160105a20@news.povray.org>
"Kenneth" <kdw### [at] gmailcom> wrote:

> And getting back to the subject of gamma 2.2 non-linear blending of colors: It
> occured to me that POV-ray's focal_blur camera might be mistaken by some users
> for actual 'blurring'-- when it's really just lots of discreet 'randomized'
> camera rays, shot out in a stochastic fashion. I.e., there's no actual
> *blurring* of scene colors to cause the gamma 'darkening' effect.
>
> But that raises a question in my mind: If a focal_blur camera has it
> blur_samples set at a high value (like 500, for example), do any of those
> discrete samples actually overlap? IF so, then I would imagine that there's some
> color 'averagaing' going on behind-the-scenes, of the overlapped rays. Does that
> relate to the gamma 2.2 color-blending problem? (Hmm, probably not-- because the
> 'averaging'-- if done-- is done *within* POV-ray, before the resulting pixel
> ever reaches the monitor.)

The focal blur feature /is/ potentially problematic with respect to gamma; but
as it is performed well within POV-Ray's render engine, it is fine as long as
`assumed_gamma 1.0` is used.

As a matter of fact the potentially problematic operations aren't limited to
blurring, but encompass all operations where brightness values are added,
because the equation

    a = b + c

is /not/ equivalent to

    a^G = b^G + c^G

(except for a few special cases).

Blurring is problematic because it is an averaging operation, which in turn is
essentially an addition combined with a constant multiplication.


Post a reply to this message

From: Kenneth
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 08:30:01
Message: <web.59e747ea42a9f98689df8d30@news.povray.org>
SO... getting back to POV-Ray's built-in gamma set-up chart...

As that chart was made many eons ago, I had a desire to see if it is still
'accurate' in the modern world ;-)  Leaving no stone unturned, I copied that
image and took it into both my (older) Photoshop, and a recent version of GIMP--
 to check not the thin horizontal bars, but the gray 'gamma' swatches that they
are compared against.  (BTW, both of my apps are working in sRGB color/gamma
space.) Using the 'eyedropper' tools there, I measured the gamma 2.2 swatch. Its
brightness value in both apps reads as 186/255, or 0.72941    And 0.72941^2.2 =
0.49950, or almost exactly 0.5

I can't say that I know exactly what I'm doing with this experiment-- ha!-- but
is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it is, of
course, but I wanted an expert's opinion.)


Post a reply to this message

From: Ive
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 09:07:50
Message: <59e75226$1@news.povray.org>
Am 10/18/2017 um 14:24 schrieb Kenneth:
> 
> I can't say that I know exactly what I'm doing with this experiment-- ha!-- but
> is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it is, of
> course, but I wanted an expert's opinion.)
> 
Calculate for yourself:  0.5 ^ (1 / 2.2) = ?
Also note that sRGB is NOT equal to gamma 2.2.

-Ive


Post a reply to this message

From: Kenneth
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 10:35:00
Message: <web.59e765f442a9f98689df8d30@news.povray.org>
Ive <ive### [at] lilysoftorg> wrote:
> Am 10/18/2017 um 14:24 schrieb Kenneth:
> >
> > I can't say that I know exactly what I'm doing with this experiment-- ha!--
> > but is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it
> > is, of course, but I wanted an expert's opinion.)
> >
> Calculate for yourself:  0.5 ^ (1 / 2.2) = ?

0.72941 !  (or almost)
But I was somehow thinking (in a fuzzy way) that the gray swatch for gamma 2.2
should perhaps measure as 0.5 instead-- with 0.5^2.2 resulting in .217, the
'gamma corrected' equivalent of 'half brightness' (0.5).  In any case, it was
just some 'idle philosophical thought' ;-) But I wanted to make sure that the
gray swatches themselves had been well thought-out, all those eons ago.

> Also note that sRGB is NOT equal to gamma 2.2.

Agreed; I simply think of 2.2 and sRGB as 'functionl equivalents', more or less,
practically speaking.


Post a reply to this message

From: Ive
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 10:59:10
Message: <59e76c3e$1@news.povray.org>
Am 10/18/2017 um 16:32 schrieb Kenneth:
> Ive <ive### [at] lilysoftorg> wrote:
>> Am 10/18/2017 um 14:24 schrieb Kenneth:
>>>
>>> I can't say that I know exactly what I'm doing with this experiment-- ha!--
>>> but is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it
>>> is, of course, but I wanted an expert's opinion.)
>>>
>> Calculate for yourself:  0.5 ^ (1 / 2.2) = ?
> 
> 0.72941 !  (or almost)
> But I was somehow thinking (in a fuzzy way) that the gray swatch for gamma 2.2
> should perhaps measure as 0.5 instead-- with 0.5^2.2 resulting in .217, the
> 'gamma corrected' equivalent of 'half brightness' (0.5).  

Well, the gamma corrected equivalent of linear 'half brightness' isn't 
the power of 2.2 but the inverse, therefor 0.5 to the power of 1/2.2

-Ive


Post a reply to this message

From: clipka
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 18 Oct 2017 11:50:53
Message: <59e7785d$1@news.povray.org>
Am 18.10.2017 um 14:24 schrieb Kenneth:
> SO... getting back to POV-Ray's built-in gamma set-up chart...
> 
> As that chart was made many eons ago, I had a desire to see if it is still
> 'accurate' in the modern world ;-)  Leaving no stone unturned, I copied that
> image and took it into both my (older) Photoshop, and a recent version of GIMP--
>  to check not the thin horizontal bars, but the gray 'gamma' swatches that they
> are compared against.  (BTW, both of my apps are working in sRGB color/gamma
> space.) Using the 'eyedropper' tools there, I measured the gamma 2.2 swatch. Its
> brightness value in both apps reads as 186/255, or 0.72941    And 0.72941^2.2 =
> 0.49950, or almost exactly 0.5
> 
> I can't say that I know exactly what I'm doing with this experiment-- ha!-- but
> is  0.72941  the correct value for the gamma 2.2 swatch? (I assume it is, of
> course, but I wanted an expert's opinion.)

IF your display subsystem has a gamma of 2.2, THEN the corresponding
swatch, when displayed, should result in a physical brightness of 0.5.

In other words, the swatch should have a brightness value of N such that:

    (N/255)^2.2 = 0.5

The (N/255) part is the stored value normalized to the [0..1] range, and
the ^2.2 part is the effect of the display's non-linearity.


BTW, the swatch-based assessment has some more limitations than just the
zoom/interpolation issue:

- It relies on the image being displayed "as is", without any active
gamma correction, just the "native" effect of the display subsystem. If
for example your display is calibrated to the Adobe RGB colour space,
and the displaying software is aware of this fact and presumes the image
data to be stored in accordance to the sRGB standard, then this approach
will give nonsense results because the display software will introduce
another non-linear conversion that is not part of the native display.

- It naively presumes the display subsystem's native transfer function
to match the form f(x) = x^gamma. For display subsystems calibrated to
e.g. the sRGB, ITU-R BT.709 or ITU-R BT.2020 standards this is
specifically not the case (for the modern ITU-R BT.2100 standard, aka
HDR TV, it is probably not even close), and you'll get a gamma value
that is only valid for 50% physical brightness.


Post a reply to this message

From: Kenneth
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 24 Oct 2017 19:45:01
Message: <web.59efcf9542a9f98689df8d30@news.povray.org>
clipka <ano### [at] anonymousorg> wrote:

>
> BTW, the swatch-based assessment has some more limitations than just the
> zoom/interpolation issue:
[snip]
> - It naively presumes the display subsystem's native transfer function
> to match the form f(x) = x^gamma. For display subsystems calibrated to
> e.g. the sRGB, ITU-R BT.709 or ITU-R BT.2020 standards this is
> specifically not the case (for the modern ITU-R BT.2100 standard, aka
> HDR TV, it is probably not even close), and you'll get a gamma value
> that is only valid for 50% physical brightness.

*That's* interesting-- because, after all of this info, I'm *still* having
trouble getting my new monitor to look really accurate! (Not due to any problems
with POV-Ray's gamma set-up tests).  My monitor is a consumer-level VIZIO, a
so-called 'LED' type (which seems to mean only that the 'edge lighting' is from
LEDs-- otherwise, it's an LCD monitor.) It has only typical 'consumer-friendly'
controls-- brightness, color, contrast, tint.. and 'backlight', which looks to
simply brighten or dim the entire image. Otherwise, nothing fancy. Trouble is,
if I set it for a correct gamma of 2.2-- as best I can-- then the overall screen
appearance just doesn't look decent (tonal values and colors.) Depending on the
particular digital-photo image I view on the web, any *slight* off-kilter color
cast is accentuated.  Quite frustrating!

Gamma adjustment is a 'non-linear' thing, of course; but my  monitor seems to
have a NON-LINEAR non-linear response(!)-- at least regarding color intensities.
Green looks a bit too intense... but decreasing the 'green' amount only makes
images look worse.  I do know that our eyes are 'attuned' to seeing shades of
green better than other colors...but something just doesn't look right.  The
monitor also shows BIG visual differences re: the vertical viewing angle. (The
POV-Ray gamma chart's horizontal lines are an 'acid test' for this particular
problem.) And those black/white lines have a slight yellowish color-cast (when
'eye-blurred')-- i.e., not true gray.

Windows 7's built-in 'monitor calibration' feature seems to be quite useless,
BTW. The example *image* there for setting contrast is a guy in a white wrinkled
shirt, along with a black "X" seen against a black(er) background. But it's so
prone to an 'opinion' of correctness that it stinks. But I used it anyway.

Then, returning to POV-ray's older gamma set-up chart at "3.2.3.2 Setting your
Display Gamma", the monitor gamma actually looks decent... more or less. I'm
surprised!

In the newer docs section "2.3.4.1 Setting Up Your Display", with the colored
spheres, the main example image there looks correct as well, gamma-wise, as seen
within the documentation. So far, so good! At this point, my monitor is at the
best settings I can manage to get :-/  (And yes, it's set to its native
resolution -- 1920 X 1080)

As a final test,  I rendered the 'gamma_showcase' file, at both of its
recommended command-line settings. The docs say, "At 100% zoom, both images
should look identical in your viewing software." Is that true?? I ask because
the two PNG images look *radically* different from each other, in all of my
viewing apps including Windows' own image viewer...

.....EXCEPT in Ive's IC/Lilysoft app! There, they both look identical... which is
quite interesting, regarding our previous discussions about PNG gamma and
viewing apps ;-)  And it raises an obvious question, concerning either the
documentation OR the method of how that sphere-test was created:  Is IC the
'target app' for viewing the rendered images? If so, that leaves out lots of
other 'typical' image-viewer apps-- possibly an unintended consequence of the
test scene. As gamma is an important issue, the documentaion needs to mention
this (or the test scene itself needs reworking somehow, to be more inline with
the majority of 'wrong-PNG-gamma' viewing apps??)

Hmm, this smells like a conundrum  :-0


Post a reply to this message

From: Ive
Subject: Re: Does POV-Ray's gamma-adjustment info need updating?
Date: 25 Oct 2017 04:50:53
Message: <59f0506d$1@news.povray.org>
Am 10/25/2017 um 1:44 schrieb Kenneth:
> ......EXCEPT in Ive's IC/Lilysoft app! There, they both look identical... which is
> quite interesting, regarding our previous discussions about PNG gamma and
> viewing apps ;-)  And it raises an obvious question, concerning either the
> documentation OR the method of how that sphere-test was created:  Is IC the
> 'target app' for viewing the rendered images? If so, that leaves out lots of
> other 'typical' image-viewer apps-- possibly an unintended consequence of the
> test scene. As gamma is an important issue, the documentaion needs to mention
> this (or the test scene itself needs reworking somehow, to be more inline with
> the majority of 'wrong-PNG-gamma' viewing apps??)
> 

IC is not related to POV-Ray in any way and it is not a 'target app' for 
anything.
It just happens that it is accurate for the image formats it supports 
because it's original back-end-library was designed for the use in the 
medical sector where this might even be a matter of life and death.

Which reminds me that I'm using a much improved version of IC by myself 
and really should make it available for download - but as I did already 
point out somewhere else: I'm lazy!
But I did post a test-pattern for you to p.b.i.

-Ive


Post a reply to this message

<<< Previous 10 Messages Goto Latest 10 Messages Next 4 Messages >>>

Copyright 2003-2023 Persistence of Vision Raytracer Pty. Ltd.