Display calibration


(KV Subbaiah Setty) #1

I was refreshing my CMS (color management system) knowledge and I came across the below statement in an X-rite article which contradicts an article in “below ninedegree” site.
“It is impossible to calibrate a display without being able to manipulate the LUT in the video card.”

So what is the view of readers and color geeks here.?Any help


(Pat David) #2

If you have a link to the article I’m sure that @Elle could clarify any points for you. :slight_smile:


#3

@patdavid Perhaps it is this link that @KVSSetty refers to?


(Morgan Hardwood) #4

@KVSSetty calibration is adjusting the monitor output using physical knobs and using LUT curves in the graphics card. Don’t confuse that with profiling.
http://ninedegreesbelow.com/photography/monitor-profile-calibrate-confuse.html#calibrate-profile


(KV Subbaiah Setty) #5

@patdavid here is the link :
http://www.xrite.com/i1display-pro/support/kb3020
@Morgan_Hardwood ,I am talking about calibration of displays and I said nothing about profiling in my question.
and i am very clear on the topic that calibration and profiling are two different and independent processes.
Yes ,I also know confusion creeps in ,especially in displays because some times calibration data of displays is embedded in display profiles in vcgt tags , though we say that calibration and profiling are two different independent processes.


#6

Well, X-Rite is basically right. There is the exception of some higher-end monitors that have a configurable LUT themselves so using the video card’s isn’t required, but that doesn’t apply to most people’s systems.

Where did @Elle say that you don’t need the video card’s LUT?


(Elle Stone) #7

Here: http://ninedegreesbelow.com/photography/monitor-profile-calibrate-confuse.html#calibrate-profile

Quoting:
"When you calibrate a monitor, there are two completely separate and independent ways to alter the monitor’s display characteristics. You can:

  1. Alter the monitor itself (not every monitor has a mechanism for altering the monitor itself)
  2. Alter your video card’s LookUp Tables (LUTs)"

High end monitors do provide more calibration options than low end monitors.


(Morgan Hardwood) #8

Calibration relies on getting your monitor’s parameters into the right ballpark using (imprecise) knobs, using options in the on-screen display menu, and finally by creating (precise) calibration curves in the VCGT. All of these things fall under calibration. If you don’t have access to the VCGT but you can still twiddle knobs or use the menu, you’ve still calibrated your monitor, though not as precisely, but the quoted statement makes no claims about precision, therefore it is false.

As Elle wrote.


(KV Subbaiah Setty) #9

Thanks houz for clearing a point, let me say what i understood from your answer and just say whether my interpretation is correct or not.

“High-end monitors have inbuilt facility to store LUT which can be configured ( manipulated/altered)by the user during calibration using OSD controls and this altered LUT will be used during display to alter its signals and LUT of video card is not used or touched from its native values or default values.”

BTW,while I am writing this, I received my “i one display pro” colorimeter from Amazon, I am just excited.
Elle Stone’s belowninedegrees site is the inspiration to go for a colorimeter ,I have read almost all the articles related to CMS on that site (not once but twice and feel like reading it many more times as time permits and as i explore my i1pro device.


#10

No, these LUTs are configured from the computer when running the calibration process. Playing with OSD settings (or knobs as some call it) might be considered calibration, too, but since it is a process without feedback and thus with unknown results I wouldn’t consider that calibration at all.


#11

What I do not understand:

With analogue signalling (VGA etc.) you may be able to tweak (calibrate) the colour response with a LUT in the graphics card in a certain range almost without losing anything (maybe losing some gamut), of course only as long as a certain SNR is maintained.

With digital interfaces (HDMI etc.) you will almost always lose information by tweaking (or calibrating) the colour response in the graphics card, assuming that the bit depth is limited. If there are 8 bits available per colour channel and I use 8 bit colour input and have a LUT inbetween, I may lose several bits caused by the nonlinear response. Having a LUT in the monitor, just before the DA conversion which may have a much better resolution, makes again perfect sense.

Doesn’t that mean that calibration is counterproductive for all 8 bits per channel interfaces when done in the graphics card with digital output, by not only losing gamut but also losing colour resolution? Where is my error?


#12

You do lose color precision with a video card LUT.

I guess theoretically you’d be better off having the editor perform the LUT in floating point before sending it in original precision to the monitor…


#13

There is no error. You just discovered the reason why many people don’t calibrate their monitor but profile it, using the native color space.


#14

But, why does this feature still exist on digital output graphics cards, it makes no sense at all? OK, maybe with 30 bit modes we are back in the game, but can 10 bit per colour data be delivered with DVI/HDMI? And does every display speak 30 bit? Or is it just the graphics card access? And what about notebooks, there the manufacturer can put the LUT into the DA conversion, but is this done? How can I find out?

Next, what about the practical part? Again, how can I find out what my setup allows and where the bottleneck is? The different concurrent systems and programs to store, retrieve and manipulate colour data are not easy to understand. The colour manager in the ubuntu system settings does set a VCGT tag, but my computer is a notebook, so how can I know if the LUT is implemented in the graphics card or in the display, and if the implementation makes sense?

So many questions …


#15

Calibrating to (say) sRGB makes sense when you are using applications that are not color managed, like many video players.

At least mpv does support monitor profiles though which is nice.


(KV Subbaiah Setty) #16

@houz , I don’t understand why you don’t consider adjusting the OSD controls on the monitor is a basic step and most important step while calibrating a monitor.
Following are the Quotes from a article on DisplaCAL site:

Calibration is done by adjusting the monitor controls, and the output of the graphics card (via calibration curves, also sometimes called video LUT curves—please don’t confuse these with LUT profiles, to get as close as possible to the chosen target.

To meet the user-defined target characteristics, it is generally advisable to get as far as possible by using the monitor controls, and only thereafter by manipulating the output of the video card via calibration curves, which are loaded into the video card gamma table, to get the best results.


#17

I never said or implied that. It’s a valid part of calibration, but on its own it’s bullshit. It’s like looking at those test images you can find on the Internet to manually calibrate your monitor to show correct colors: it solely relies on your gut feelings that the colors might be right while giving you not the slightest confidence that they are not worse than before you started.


#18

It does not (somehow): Since a modern computer is a multi purpose device, the user should have the opportunity to have both, calibrated output for non-colour managed applications and uncalibrated output for colour managed apps. And that in a transparent and automatic but configurable manner. As far as I understand, when using a profiling software, I will either get a icc without or with vcgt tag, but the profile with tag will always be based on the calibrated monitor. That does not make sense. The default should be that colour managed apps are able to circumvent the LUT and the profile with tag should be based on an uncalibrated monitor therefore. For special purposes (e.g. LUT in display DAC), of course this should be configurable, both for the profiling/calibration software and for the colour managed apps. Everything else sounds like a big system misconception :neutral_face:.

Of course I can switch between two profiles, one with and one without tag, but what about watching a movie on my left display while editing photos in darktable on the right display?

Why not having a colour managed desktop?


(KV Subbaiah Setty) #19

Hello to all,
Today morning I had some free time ,Just unpacked my new i1Display pro and calibrated and profiled one of my old samsung lcd monitor using DiaplayCAL, and here is the two images of the monitor gamut compared with sRGB gamut.
legend : colored line my monitor ,dotted gray sRGB

Few things I noticed:
# took nearly 30mints to calibrate and profile
# my monitor covers 83.66% of sRGB
# LCD monitor space is bigger than sRGB by volume wise
# first fig on lab space with relative colorimetric intent
# second fig on xyY space with absolute intent
# few more things but no time to emumerate…


(Morgan Hardwood) #20

We’re generally on the same boat, except for the misunderstanding in what “calibration” means.

You can calibrate using knobs and nothing more. In most cases you don’t. In most cases you would calibrate using your colorimeter attached to the screen for instant and precise quantified feedback. Nothing to do with gut feeling.