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: Wed, 30 Mar 2016 10:20:26 -0700
The mention of gamma 2.4 in the display is an aside. It does not change the recommendation that the OETF have an exponent of 2.2. However, an increase in contrast is desirable for scenes (not talking about reproduction of a given object such as a printed page) not only due to surround effects, but also due to the Stevens effect, in which the apparent contrast of a scene decreases as absolute luminance of the display decreases compared to the original scene. So, if your application is optimum with a profiled display with gamma 2.2, fine. But if you are producing images of scenes for the world of viewers out there, you should expect displays with gamma 2.4, which gives a first order approximate correction to the perceptual effects of both absolute luminance and surround.
See the Stevens and Hunt effects in this exposition of appearance effects:
http://rit-mcsl.org/fairchild/PDFs/AppearanceLec.pdf
-----Original Message-----
From: colorsync-users-bounces+waynebretl=email@hidden [mailto:colorsync-users-bounces+waynebretl=email@hidden] On Behalf Of Uli Zappe
Sent: Wednesday, March 30, 2016 9:18 AM
To: Graeme Gill; ColorSync
Subject: Re: gamma bewilderment wrt/ Argyll’s documentation
Am 30.03.2016 um 05:02 schrieb Graeme Gill <email@hidden>:
> 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.
Why you are talking about television here? Conceptually, a computer has nothing to do with television.
Apple started out with using a gamma of 1.8 which reflected printing technologies rather than displays. This alone should make clear that television played no role whatsoever when Apple conceived ColorSync.
1.8 meant 1.8. When Apple switched to 2.2/sRGB because it had become the de facto web standard, they meant 2.2 just the same; you won’t find any mention anywhere in their documents that this should be used in conjunction with a 2.4 display (and certainly not a 2.4 CRT – those were completely dead when Apple switched to 2.2 in 2009).
*Actually, this is the core of what I do not understand in Argyll’s documentation:* What does it even mean in an ICC color managed system “to use gamma 2.2 with a gamma 2.4 display”? Assuming you calibrate your display to gamma 2.4 and then profile it, the monitor profile will also sport a gamma 2.4 TRC (if it’s built correctly and without color appearance adjustments). And when the system is using gamma 2.2, the CMM will match colors to gamma 2.4 as soon as they are sent to the display, so there is no difference at all vs. a display with gamma 2.2.
In short:
1. In ICC color management, the display gamma is completely irrelevant in principle (= assuming no limitations in bit depth), because changing it will not change the image display at all.
2. In ICC color management, it isn’t even possible/makes sense to “use a system gamma 2.2 with a gamma 2.4 monitor”. Whatever monitor gamma you have, you’ll have a monitor profile with the same gamma, and every differing gamma will be matched to that.
> If you want more information about this, I recommend that you read Pointon's "Digital Video and HDTV".
I read this of course, but again, it has little to do with computers. Pointon comes from the video world, which is a very different world.
> BT.1886 is also pertinent.
Certainly not.
BT.1886 is a spec for watching videos in a dim environment, using calibrated monitors instead of ICC color management (in conjunction with a color appearance model) to achieve desired color appearance adjustments. Computers, on the other hand, are typically used in a bright environment, and watching videos on them is but one of many different usage patterns, if they are used for that at all. And ICC color managed computers use (ideally individually measured) monitor profiles which mirror the hardware specs and calibration settings of the display, making these hardware specs and the values the hardware might be calibrated to irrelevant. I see no place for BT.1866 anywhere in computer usage.
>> 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".
Well, if even 1.961 still counts as “approximately 2.2”, then almost any gamma curve will be “approximately 2.2”. ;-) Gamma curves outside of the 2.2 ± 0.3 range are probably quite rare.
>> 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.
? Where is the “law of color appearance” that says “contrast expansion is always desirable”? A contrast expansion is desirable if and only if the viewing environment is dimmer than the production environment. This is typically not the case with computers.
> Television wasn't developed with any editing. What came out of the camera was what was broadcast.
Yep, but that’s history. And we’re talking about computers here. Why this constant reference to a technology that has nothing to do with computers (and is outdated, and is polluted with commercial interests (“contrast sells”))?
> 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. [...]
I’m well aware of the *history* of *television* color processing.
What I don’t understand is why *computer* users should care about that *today*.
I could understand that for a very specific group of users, those who wanted to edit video on their computers or those who want to watch movies on them, the “established standards” were something to consider as long as ICC video color management was not yet possible because of a lack of processing power.
But this is history now. Today you can color manage video just as you have been able to color manage still images for the last 20 years. So whatever historical standards existed, you can emulate in a corresponding video profile which “virtualises” the historic video hardware, so to speak. So I don’t understand why, even considering this specific group of computer users, computers should care about these standards (apart from correctly color managing video files).
> Yes. Calibrating a display in a computer environment achieves two things:
> 1) Makes the display better behaved, which makes profiling easier/more accurate.
Agreed.
> 2) Determines the look of non-color managed output.
There shouldn’t be any non-color managed output anymore in 2016 ... oh well ...
But if you agree that these are the two reasons for calibrating a display for an ICC color managed computer, aren’t you saying yourself that appearance adjustments (i.e. contrast adjustments via gamma settings) are *not* among the reasons for a specific calibration? Or does all this “gamma 2.2 vs. 2.4 vs. BT.1886” talk only refer to your point 2), i.e. non-ICC color managed output that today shouldn’t even exist anymore?
> 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.
I think the monitor profile is the only logical place to perform any output appearance adjustments, because color appearance phenomena happen at the screen → observer stage. (And the monitor profile is the only profile which is guaranteed to be used by all applications which output content to the screen. Any other place would not be used by all applications, thus leading to color inconsistencies between applications.)
Bye
Uli
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