Let me first say that I'm appreciative of your input. Thank you. I'm beginning to understand the problem at hand. I hope you don't mind if I ask a few more questions just to gain a clearer understanding: 1) You said,
If you are running a video stream through a video card to display, it may or may not sidestep the VLUT data. It is common for this to happen under Windows, but rare on the Mac.
Who or what determines whether the VLUT data is sidestepped: is it the OS, the application, the profile in use or something else? Is it possible to determine which applications are using the VLUT? 2) You said,
Using a profile with no VLUT data is the answer, so that no changes occur to the raw RGB data you wish to send out.
What's the name of the tag(s) that would indicate the presence of a LUT? I'm guessing it may be 'vcgt', but could you confirm? 3) You said,
Selecting a generic profile such as sRGB will assure that there is no LUT data being applied at the video card.
But what about changes made at the OS-level? I suspect you're trying to tell me that QuickTime, DVD Player, VLC etc. aren't ICC profile-aware, and therefore aren't affected by OS-level colour management. Am I right? If I'm wrong, then why is sRGB the best choice? Hopefully, answers to these should set me free to make some informed decisions on the matter for the foreseeable future. Thanks :) PS: One last question: can I assume that video card LUTs are output dependent (ie. my Thunderbolt and HDMI outputs each have separate LUTs)?
On Jan 4, 2013, at 1:52 PM, SQ <sirquijote@lavabit.com> wrote:
So, first question: am I right in assuming that a display profile is
always applied? OS X seems to assign a generic profile no matter what
I do, but maybe it simply acts as a passthrough? Adjustments from the actual ICC profile are applied by applications which choose to do so, but not globally. Aha, I think I understand my mistake now When I select a colour profile for my display (say, a Cinema Display) it simply acts as a reference for colour-managed applications. That is, an application (eg. Pixelmator) can refer to my colour profile settings for a given display and if there's a profile assigned it'll modify its rendering accordingly. I think what was confusing me was that the Finder and the UI are colour-managed, and when I saw the desktop picture, menu bar and other UI elements changing with selection of a colour profile it's because the underlying software was altering its rendering, not because the OS was modifying ALL output to the display. Am I right?
No, the Finder is not color managed, but it is effected by the flashing of the LUTs, which is what you are seeing. So yes, you need to eliminate that, but no, its not technically the profile application you need to avoid, but rather the LUT calibration correction table being applied.
Second question: if I'm right and OS X always assigns a modifying display profile to any given display, is there any way of disabling it, even temporarily? I believe you may be confusing Video Look Up Table data flashed to the video card (which can be stored in a special tag in an ICC profile, and can be applied with the profile, as the Mac OS will do, if you choose a profile that has LUT data included) with the profile itself. What you need to do is be sure you are using a display profile with no LUT data, so that the data won't be adjusted at the video card level by the LUTs on its way through. But this only applies to data running through a video card, as it does to a computer display. Selecting a generic profile such as sRGB will assure that there is no LUT data being applied at the video card. Aha, I get this too. So, wouldn't I be best choosing HD 709-A for the HDTV, as this is what I'm attempting to calibrate the screen to? Or am I simply entirely wrong in thinking that any video playback software (eg. DVD Player, QuickTime) in OS X is ICC-profile aware? In which case, as you say, so long as the graphic card's LUT is untouched I've got nothing to worry about in the first place?
If you are running a video stream through a video card to display, it may or may not sidestep the VLUT data. It is common for this to happen under Windows, but rare on the Mac. But using Rec 709 is not really the answer. Using a profile with no VLUT data is the answer, so that no changes occur to the raw RGB data you wish to send out.
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