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Author Topic: What actually happens during Calibration?  (Read 3913 times)

Rob C

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What actually happens during Calibration?
« on: March 23, 2017, 07:35:58 AM »

Hi!

A question for you technical minds out there (or in here, as the case may be).

When one calibrates a monitor, what exactly is being done: is the monitor itself being altered in any way, or is it the interaction between the monitor and computer that's being affected? In other words, does the way the monitor itself is functioning remain unchanged (apart from age-related decay), and the way that the computer works being changed, so as to allow the monitor, in its current condition, see the computer's output in a standardized way?

In the case of two computers working with one, common monitor, does that mean that the monitor has to be calibrated twice, once for use with each different computer? If so, then it would seem to me that it's the computers that are being calibrated, not the monitor, or you'd have to recalibrate every time you switched between computers.

I have a LaCie blue eye pro calibration device that came bundled with the LaCie monitor a few years ago. It works with the old Microsoft XP computer, but won't have sex or any other relationship with my current Windows 8.1 outfit.

If, indeed, the monitor is being calibrated and not the computer, then I could always calibrate the monitor using the old computer, but I'm sure that's not going to be the answer: just too sweetly convenient and inexpensive! Of course, I've already tried this out, but can't tell whether or not anything is being changed, especially as most of my own interest is in black/white photography.

I have to say, looking at Hans Fuerer's images, as well as those of other respected snappers I love, online, the colours seem absolutely perfect on my monitor, but who really knows - including myself - what version of perfection I think that I see?

So what's actually happening during calibration?

Rob C

Mark D Segal

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Re: What actually happens during Calibration?
« Reply #1 on: March 23, 2017, 07:56:03 AM »

My understanding (which is for sure incomplete and therefore open for discussion) is that the calibration tweak's the monitor's internal responses - usually in respect of white point, brightness and contrast ratio; and that the calibration sets these responses by one time sending instructions through calibration software; that software may or may not be compatible with the computer operating system to which the monitor is connected, so the user needs to assure this before trying it.
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Mark D Segal (formerly MarkDS)
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GrahamBy

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Re: What actually happens during Calibration?
« Reply #2 on: March 23, 2017, 07:56:30 AM »

Hi Rob,
My understanding is "possibly both". A few monitors, such as some recent Benq models and those from the pricing stratosphere, allow "hardware calibration" of the monitor itself (of course it's really software, but software running in the monitor).

I've only ever used low-rent monitors, where the monitor does as it pleases other than for brightness, and the calibration is done by asking the computer to ask the video card to modify the outgoing signal appropriately. The standard calibration tools like Syder etc etc all work that way, although some of them can also be co-opted to do the hardware part of the "hardware"="in monitor" calibration.
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degrub

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Re: What actually happens during Calibration?
« Reply #3 on: March 23, 2017, 08:01:23 AM »

At a high level, what is happening is the calibration software tells the video card to output a known combination of Red, Green, and Blue ( a colour) to an area spot on the monitor. The sensor reads this and passes back to the software what it read. If they match, then nothing happens. If they do not match, then an adjustment value is entered into a reference table for that colour. The software goes through all or many of the colours it thinks should be displayable on the monitor. The final adjustment table is then used to adjust the output to the monitor each time a colour pixel is sent to the monitor.  The adjustment table can reside in the monitor or in the video card, but not both. With CRT, ie TV tube type monitors, most were dumb and only displayed what was sent to them. All adjustments were done in the video card. With LCD panels, it could be in the panel memory as there is a CPU controlling the display digitally. What colours can be output depends both on the colour space (sRGB, Adobe RGB, etc) that is assumed and what the video card and monitor are capable of. Most websites operate using the sRGB colour space since most users do not have calibrated systems nor devices capable of displaying a wider range of colours than the sRGB range.
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Mark D Segal

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Re: What actually happens during Calibration?
« Reply #4 on: March 23, 2017, 08:18:05 AM »

............With LCD panels, it could be in the panel memory as there is a CPU controlling the display digitally. ............

This is the set-up I had in mind in my response. But it's correct not all displays work this way.
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GrahamBy

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Re: What actually happens during Calibration?
« Reply #5 on: March 23, 2017, 08:35:52 AM »

PS Keith's article on the new 32" Benq talks about in-monitor profiling:

http://www.northlight-images.co.uk/benq-sw320-32inch-4k-monitor-review/

Oh, to further confuse the issue, there is also a distinction between matrix and LUT (Look Up Table) calibration.

As described by degrub, it comes down to modifying the triplet of RGB signals sent from the computer to a corrected triplet. That can be done very efficient with a matrix of 9 values: each "new" value of R, G and B is a weighted combination of the old triplet. That works only if the required calibration is linear. An alternative is to have a large table of values in which you "look up" the computer RGB values, and read off the modified set to send to the screen. Since you can't do that for all possible values, there is a some linear interpolation (ie, my given RGB is halfway between two table entries, so I take the average of the two).

Conceptually then, a bigger LUT means less interpolation and more precision, so people sometime boast about how big their LUT is.
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daicehawk

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Re: What actually happens during Calibration?
« Reply #6 on: March 23, 2017, 10:35:30 AM »

When a hardware calibration (the one using some software like ProfileMaker, Argyll etc and a colorimetric device like i1pro) is done, basically three "equalising" curves (for R, G and B channels)  are calculated based on the calibration target values (white point, black point, gamma) and the measured RGB response along the gray axis. The calibration curves are then embedded into the color profile and are loaded into the RAM or in the LUT of the monitor.
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aaron125

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Re: What actually happens during Calibration?
« Reply #7 on: March 23, 2017, 01:54:07 PM »

Actually, calibration doesn't modify anything other than the display itself. By the very definition of the word, it is the act of putting the display into a specific state (in our case, that state is the starting point for characterisation/profiling).

Many people get confused between calibration and characterisation. The former is simply putting the display into a known state, a particular condition from which one can then go ahead and perform the latter process, namely characterising their display. Calibration is a physical process or procedure, done in hardware, not software. Software may control and greatly assist with setting the hardware, such as in Eizo CG displays whereby one tells the software what WP one would like, their desired brightness, how dark they want their BP to be set to, their chosen gamma and so on. But none of that has anything to do with profiling or characterising the display.

If one considers a set of digital scales, for example, the kind used in many science classes, which might be accurate to, say, 1/10,000 of a gram or 0.0001g, they would require frequent calibration as a rise in temperature of under 10 degrees centigrade would make the previous calibration invalid. Therefore, one would get the trusty 1.0g weight, place it on the platen to allow the scales to be calibrated to the new ambient temperature.

It's a very similar situation which many of us would be familiar with when one has to place their i1 spectro onto the supplied white tile to calibrate the device. This is simply allowing the spectro's firmware to account for temperature variations, any changes in the lamp's colour or intensity and so on. But none of that has anything to do with the process of characterisation. I think many people get somewhat confused by the two terms because for us, calibration and characterisation are most often so very closely linked.

So, just remember, calibration is putting the display into a known starting condition - characterisation is the profiling procedure and where LUTs and so forth come into play. But they are two completely different and seperate processes that don't necessarily have to be linked in any way. It's just that often the same software application will perform both operations.

FYI: back in the old days, one had to alter the RGB guns of a crt display by twiddling some knobs at the back of the display. And the same action also adjusted the brightness. These were all physical changes performed to get the display to the same starting condition as the last time on chose to build a profile or to ensure that a soft proof was indeed correct because without calibrating the display, how could one have any idea of what they were viewing was even close to what the final output would look like.

Hope that doesn't add too much more confusion for you. Don't worry though, there are a number of excellent books on these processes which explain everything step by step and in detail.


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Doug Gray

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Re: What actually happens during Calibration?
« Reply #8 on: March 23, 2017, 03:22:26 PM »

Here's what happens with an EIZO using Colornavigator. It combines characterization and calibration in one process.

The user selects their desired brightness (cd/m^2) and white point. The white point can be selected by either CIE xy coordinates, temperature, or D (like D50, D55, D65).

The user can also select the RGB xy coordinates if they prefer a slightly different white point tint. Some do this to match a slight tint on the printer viewing station.

Also, one can select either the native R, G, and B emission colors (in CIE xy coordinates)  but it is usually best to use "native" which offers the widest possible color gamut. These settings have no affect at all on any image that is within their gamut. Essentially, it determines the position of the three, triangular edges of the display gamut. While one can select xy coordinates that are outside those of the physical monitor, the colors will be clipped at the native edges.

The software then generates a profile that matches those selections and programs the monitor to produce the desired colors, gamma, and intensity of that profile.

Some EIZOs can use 30 bits instead of 24 which can reduce subtle banding in gradients. But the monitor and display controller as well as the application program have to support it. If one doesn't it reverts to 24 bits (8 bits per channel).

You can have as many monitor setups as you wish and just right click the Colornavigator icon and select the one you want. Very handy for switching between, say, sRGB, Adobe RGB, and D50 (for print matching) setups.
« Last Edit: March 23, 2017, 03:26:20 PM by Doug Gray »
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Rob C

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Re: What actually happens during Calibration?
« Reply #9 on: March 23, 2017, 03:33:21 PM »

All clear now, Rob?

;-)

First good giggle of my day!

But seriously, Yes and No:

Yes: yes, I shall probably have to buy a new calibration device after all;

No: no, I am not bright enough to get into deep discussions on matters digital or electronic!

On the latter point, it reflects why I sometimes wonder if I'd even have thought about becoming a photographer had film not been the route into it; had I been faced with only digital, it would have put me right off without trying. Like building my own tv set, for example.

That's not to say that I currently don't enjoy digital photography, but that I do it at all is because I was very familiar with film, and so the learning curve was relegated to processing the files, not how to get them in the first place.

I could never be a geek. Or a hacker.

However, thanks to all of you who have stepped up to bat!

;-)

Rob

Peter McLennan

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Re: What actually happens during Calibration?
« Reply #10 on: March 23, 2017, 07:34:17 PM »

First good giggle of my day!

Mine, too.  Thanks, Klaban!  :)

Rob 'n me, all we had to do was look at the thermometer and make sure the Rodinal was at 20C.
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BobShaw

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Re: What actually happens during Calibration?
« Reply #11 on: March 23, 2017, 10:15:21 PM »

When one calibrates a monitor, what exactly is being done:
Well, I don't think that you are calibrating the monitor. That was pretty much done when it left the factory. Things may have been different in the CRT days.

Using the scales analogy, you calibrate a set of scales using a standard weight. You put the say 1kg weight on the scales and if the value is wrong then you adjust the scales to read 1Kg. You don't adjust the weight so the scales read 1Kg. If however the maximum weight that the scales take is only 500g, then no amount of calibration will fix it, you have to adjust the input weight to be 500g or less.

However when you "calibrate" a monitor, to me you are adjusting the input the make the appearance correct, as measured by the Spyder etc. If it is too far out then you may need to adjust the brightness, but the colours themselves are just adjusted by making a profile of the monitors response to certain inputs. So to me all that you are doing in calibrating a monitor is making a new profile for the monitor.

At the end of which it usually says, "your new profile is stored at Library/Colorsync/Profiles/newprofile.icc"
« Last Edit: March 23, 2017, 11:42:04 PM by BobShaw »
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Mark D Segal

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Re: What actually happens during Calibration?
« Reply #12 on: March 24, 2017, 10:00:00 AM »

Well, I don't think that you are calibrating the monitor. That was pretty much done when it left the factory. Things may have been different in the CRT days.

Using the scales analogy, you calibrate a set of scales using a standard weight. You put the say 1kg weight on the scales and if the value is wrong then you adjust the scales to read 1Kg. You don't adjust the weight so the scales read 1Kg. If however the maximum weight that the scales take is only 500g, then no amount of calibration will fix it, you have to adjust the input weight to be 500g or less.

However when you "calibrate" a monitor, to me you are adjusting the input the make the appearance correct, as measured by the Spyder etc. If it is too far out then you may need to adjust the brightness, but the colours themselves are just adjusted by making a profile of the monitors response to certain inputs. So to me all that you are doing in calibrating a monitor is making a new profile for the monitor.

At the end of which it usually says, "your new profile is stored at Library/Colorsync/Profiles/newprofile.icc"

aaron125 got it right. In my original response I was of course referring to calibration - as Aaron says, putting the monitor into a known state before profiling it, which for photographic purposes will almost always differ from the calibration done in the factory. Profiling it creates an information set (based on that calibration) giving the colour management system in your computer the data it needs to correct differences between the monitor response relative to the reference values in the profiling application (those patches it throws up and your colorimeter reads when making the profile). Both processes, whether combined in one operation or separate, are necessary for reliable colour management. Where exactly they operate is hardware dependent, but one thing for sure: the monitor, the video card, the profiling application and the colorimeter all need to be compatible with each other for these processes to be feasible and reliable.
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scyth

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Re: What actually happens during Calibration?
« Reply #13 on: March 24, 2017, 11:40:40 AM »

Calibration is a physical process or procedure, done in hardware, not software.

the mere fact that firmware and data (LUTs that for example make the "working" gamut of your output device to sRGB or AdobeRGB) reside in a chip in your monitor does not actually make it "done in hardware" as in some actually physical operation dialing a knob on some analog rheostat by hand

---

http://argyllcms.com/doc/calvschar.html

.
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Rob C

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Re: What actually happens during Calibration?
« Reply #14 on: March 24, 2017, 04:37:39 PM »

Pandora had nothing on this in her box of many tricks!

Now some may understand why digital photography would never have had that inital appeal to me of the film version. Digital is too science-minded, technically based when compared with the visceral simplicity of film and wet darkroom work. You can't escape it with digital, but with film it wasn't one's place to be a chemist: all you had to do was know about two films intimately and, perhaps, a basic soup like D76 and you were on your way, needing little more. D163 for prints, and you had it all.

Times change, and with them, people.

;-)

Rob

elolaugesen

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Re: What actually happens during Calibration?
« Reply #15 on: March 24, 2017, 04:48:01 PM »

just upgraded to an i1 Display Pro

I have two EIZO monitors one is a Color Edge CS240 the other an Color Edge CG222W

Eizo provides Color Navigator software and xrite iProfiler software

Which tool should I use?
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opgr

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Re: What actually happens during Calibration?
« Reply #16 on: March 24, 2017, 05:11:39 PM »

All clear now, Rob?

;-)

Haha, might as well add to the confusion then:

The calibration software creates a small file called an ICC profile. It resides on your computer and at startup the computer will read the file and set up the monitor for you.

Theoretically you should be able to copy that file to your other computer, newer system but same monitor, and when properly selected that ICC profile should give you the same monitor experience.

Unfortunately, the "copy & properly selected" part usually is harder than simply recalibrating on the newer system IF your software would work there.

Since your software doesn't seem very cooperative, your choice is either pay your neighbour's 11 year old 30 bucks to tell you how to properly copy and select, or you pay 300 bucks for new software...
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Oscar

Doug Gray

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Re: What actually happens during Calibration?
« Reply #17 on: March 24, 2017, 05:23:33 PM »

just upgraded to an i1 Display Pro

I have two EIZO monitors one is a Color Edge CS240 the other an Color Edge CG222W

Eizo provides Color Navigator software and xrite iProfiler software

Which tool should I use?

I also have two EIZO CGs, a 301 and 318.  I use Colornavigator and disabled the X-Rite monitor profile management. I also run 30 bits on the 318.
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Mark D Segal

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Re: What actually happens during Calibration?
« Reply #18 on: March 24, 2017, 05:26:26 PM »

Haha, might as well add to the confusion then:

The calibration software creates a small file called an ICC profile.

You are perhaps adding to the confusion. The ICC profile comes from the characterization, not the calibration, though the same software handles both.
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aaron125

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Re: What actually happens during Calibration?
« Reply #19 on: March 24, 2017, 05:37:05 PM »

I think it's important to stress the independence of the calibration state and the subsequent characterisation - also known as the ICC profile. They are not one and the same, and if the calibration is different then the profile is essentially invalid. Hence forth, it should (hopefully?) appear obvious as to why a single profile will not be suitable for a display calibrated to both 5000K, g1.8 and also 6500K, g2.2. If the calibration is different then so to will be the character of the display. This is what's meant by characterisation - one is telling the profiling software about the physical character and properties of the display. Only then can a valid profile start to be built.

But there's absolutely no reason why one couldn't place a single profile in the correct place on two separate systems, as long as EVERYTHING on those two systems is identical. And that goes right down to the way the OS, drivers and various software packages are installed. Any differences between the two systems can lead to your outcome of paying an 11 year old to fix something or wasting much more money on something else.

It's a very familiar situation, that if the essential basics of a particular process are not thoroughly understood then why would one hope that said process would work as intended?

And I'd also stick with ColorNavigator as I can absolutely guarantee that Eizo knows their displays better than X-Rite or anyone else, so therefore the best performance should happen when using their software to control their own hardware.


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