Re: gamma bewilderment wrt/ Argyll’s documentation
Re: gamma bewilderment wrt/ Argyll’s documentation
- Subject: Re: gamma bewilderment wrt/ Argyll’s documentation
- From: Graeme Gill <email@hidden>
- Date: Wed, 30 Mar 2016 14:02:27 +1100
Uli Zappe wrote:
In the Mac OS X 10.6 release notes
(https://developer.apple.com/library/mac/releasenotes/MacOSX/WhatsNewInOSX/Articles/MacOSX10_6.html),
Apple clearly stated that the gamma is now 2.2 (not 2.4, as the Argyll doc says):
Apple aren't very clear on what they mean by "2.2", but the assumption (based on industry
practice) is that it is a 2.2 encoding gamma, intended for display on a standard CRT
with gamma 2.4, just as in the television standards it is based on.
If you want more information about this, I recommend that you read Pointon's
"Digital Video and HDTV". BT.1886 is also pertinent.
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..239, to be exact).
The sRGB TRC is an approximate match to a 2.2 power overall.
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.
I don't say they are 2.2 though, I say that they are "approximately 2.2".
2. Why does the documentation assume that a color appearance adjustment (= contrast
expansion of gamma 1.1) is desirable by default?
Because that's how color appearance works.
Video production is a very specific case, and it’s not clear from the documentation
whether the "bright viewing conditions such as a television studio" refer to the
recording or the editing environment.
Television wasn't developed with any editing. What came out of the
camera was what was broadcast.
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.
You're confusing what was established by standard and practice at the time, to latter
developments that have to work within the established standards.
What happened was that (out of the available display technologies), CRT's were
chosen as the primary display medium. As well as other desirable qualities,
they have an (approx.) 2.4 power EOTF. The TV's were viewed in dim environments, and
the studio's had bright lighting. So the TV cameras were deliberately configured
to give good looking images on the TV's. This turned out to be an encoding
gamma of about 2.2. These two things effectively fixed the TV medium. All
subsequent developments then have to work within that. So if you are shooting
in a dimmer environment, then you should (ideally) increase the encoding gamma.
If you are viewing in a brighter environment with a brighter display, you should
decrease the display gamma.
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.
Yes. Calibrating a display in a computer environment achieves two things:
1) Makes the display better behaved, which makes profiling easier/more accurate.
2) Determines the look of non-color managed output.
In an ICC color managed workflow, any viewing condition adjustment needs to be achieved
elsewhere, i.e. at profile link time, or as a pre-determined link in a B2A table.
Graeme Gill.
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