RE: gamma bewilderment wrt/ Argyll's documentation
RE: gamma bewilderment wrt/ Argyll's documentation
- Subject: RE: gamma bewilderment wrt/ Argyll's documentation
- From: Wayne Bretl <email@hidden>
- Date: Tue, 29 Mar 2016 09:37:02 -0700
You are correct that there is a lot of confused language here. The simple
explanation is that gamma = 1/2.2 is the Opto-electric transfer function for
sRGB and for Windows and the rest except older MAC stuff, where there was
some electro-electro fiddling in the system.
As stated, Gamma = 2.4 is the assumed typical electro-optical transfer (EOT)
function gamma value for a CRT. Non-CRT displays are made to approximate
this value even if they natively do not. Argyll apparently is calling this
the "target response curve gamma." As you noted, this terminology isn't
completely clear as to what part of the chain it refers to. I don't know
why they don't call it the "target display EOT function gamma," which would
be clearer terminology to me.
As far as wanting an overall optical-optical transfer gamma of 1.1, this
value (greater than 1.0) is desirable for both TV viewing and computer
monitor viewing because in both cases the image is typically much brighter
than the surround, and without this adjustment, the image will be seen as
being "washed out." This overall result was also desirable in the older MAC
hardware, but was obtained with different transfer characteristic fiddling
in the middle of the system. By the way, the extreme case of
wanting/needing an increase of overall gamma was the projection of film
slides in a dark room. Slide film typically has a gamma approaching 2.0
(although the effect is typically reduced somewhat by lens flare, mostly
that in the projector).
-----Original Message-----
From: colorsync-users-bounces+waynebretl=email@hidden
[mailto:colorsync-users-bounces+waynebretl=email@hidden] On
Behalf Of Uli Zappe
Sent: Tuesday, March 29, 2016 9:05 AM
To: ColorSync
Subject: gamma bewilderment wrt/ Argylls documentation
Hi,
Ive stumbled about the following passages in Argylls documentation for
dispcal (http://www.argyllcms.com/doc/dispcal.html):
> -g gamma Set the target response curve gamma. This is normally an
exponential curve (output = input ^gamma), and defaults to 2.4 on MSWindows
and Macintosh OS X 10.6 or latter and Linux/Unix (which is typical of a CRT
type displays real response), and 1.8 on a Macintosh (prior to OS X 10.6).
> [...]
> Also note that many color spaces are encoded with, and labelled as having
a gamma of approximately 2.2 (ie. sRGB, REC 709, SMPTE 240M, Macintosh OS X
10.6), but are actually intended to be displayed on a display with a typical
CRT gamma of 2.4viewed in a darkened environment. This is because this 2.2
gamma is a source gamma encoding in bright viewing conditions such as a
television studio, while typical display viewing conditions are quite dark
by comparison, and a contrast expansion of (approx.) gamma 1.1 is desirable
to make the images look as intended.
There are 3 things here which I dont understand:
1. Where do some of the default gamma values come from?
In the Mac OS X 10.6 release notes
(https://developer.apple.com/library/mac/releasenotes/MacOSX/WhatsNewInOSX/A
rticles/MacOSX10_6.html), Apple clearly stated that the gamma is now 2.2
(not 2.4, as the Argyll doc says):
> Historically, the default gamma correction for Mac OS has been a value of
1.8 (a useful value for print professionals). In recent years, television,
video, and web standards have all settled on a default gamma of 2.2. In OS X
v10.6, the Mac moves to this common standard
Later, Apple has repeatedly confirmed this.
It is true that when Apple talks about gamma 2.2, they actually mean the
sRGB TRC. You can verify this by looking at the OS X supplied, so-called
"Generic Gray Gamma 2.2 Profile" (/System/Library/ColorSync/Profiles/Generic
Gray Gamma 2.2 Profile.icc) which Apple introduced with Mac OS X 10.6 and
which actually sports an sRGB TRC.
But while it is true that the gamma curve *part* of an sRGB TRC is 2.4, you
certainly cannot say that the sRGB TRC as a whole is gamma 2.4". Its
either "sRGB" or (simplified) "gamma 2.2" (gamma 2.239, to be exact).
That is all the more confusing since the Argyll documentation correctly
reports the pre-10.6 gamma value as being 1.8, in accordance with Apple.
In the same vein, the simplified gamma TRCs of REC 709 and SMPTE 240M are
1.961 and 1.932, respectively, and certainly not gamma 2.2, as the Argyll
documentation says.
These might be trivial documentation errors, but they gain importance with
my 2 additional (following) problems.
2. Why does the documentation assume that a color appearance adjustment (=
contrast expansion of gamma 1.1) is desirable by default?
Video production is a very specific case, and its not clear from the
documentation whether the "bright viewing conditions such as a television
studio" refer to the recording or the editing environment. If the former,
this is certainly a gross generalization, since the recording environment
can be anything from a sunny beach to a dark and stormy night. If the
latter, I very much doubt that a typical TV editing environment is
*brighter* than the typical office environment in which a computer is
typically used.
But even more importantly, with a few specific exceptions, the task of color
management is to preserve the colors as set by the artistic intent of the
editor in the *output referred* color space, not the actual colors of the
source image, anyway. So the colors of the recording environment do not
really matter.
Additionally, as I already wrote, video is a very specific case for color
management on a computer. By far the most typical case will be that the
colors of some illustration or image should be preserved on their way from
the editor to the consumer. There may be all kinds of editing and viewing
environments, but I fail to see how in a statistically significant way the
typical viewing environment will be darker than the typical editing
environment. Therefore, I dont understand how a color appearance adjustment
can be assumed to be desirable by default.
3. Why should the user care about the TRC at all in a color managed
environment?
This is the point that baffles me most. The whole point of color management
is reliable reproduction of colors *independent of* the specific underlying
technical parameters in hardware and software.
This means that in a color managed environment, choosing a different gamma
value for a display calibration should ideally have no visible impact *at
all*, since (assuming a corresponding monitor profile) the CMM is supposed
to compensate for different TRCs of source and destination. (I assume ideal
hardware and software here and disregard real-world technical limitations
such as hardware specs and limited bit depth, as they are not the point in
this context). So if I calibrate my display to gamma 1.8 or gamma 2.4 should
be completely irrelevant for the visual results I get as a user, as long as
all my applications and the GUI of the operating system are color managed.
On an ICC color managed computer, this means that a) using a higher gamma
could not even achieve the desired contrast expansion described in point 2)
above and that b) there should be no reason to recommend any specific gamma
value to the user at all (whereas Argylls documentation makes a lot of
specific recommendations). In contrast, as I understand it, the only ICC
color management compatible way to achieve a color appearance adjustment (=
contrast expansion, as far as the suggestions in the Argyll doc are
concerned) is to build a color appearance adjusted monitor profile.
Does anyone share my confusion, or can dispel it?
Bye
Uli
_________________________________________________________________________
Uli Zappe, Christian-Morgenstern-Straße 16, D-65201 Wiesbaden, Germany
http://www.ritual.org
Fon: +49-700-ULIZAPPE
Fax: +49-700-ZAPPEFAX
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