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Author Topic: What defines the limits of gamut when profiling ?  (Read 6403 times)
Rhossydd
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« on: December 02, 2009, 09:11:52 AM »
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We frequently see comparisons of gamut, by volume or area, using various tools such as Color Think Pro, Gamutvision, Colorsync etc.
Can someone explain if there’s a standard deviation factor(eg + or - 1.2 DeltaE ) that defines where the gamut boundaries are ?
Is this enshrined in the ICC standards ?

Thanks

Paul
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PeterAit
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« Reply #1 on: December 02, 2009, 10:30:56 AM »
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Quote from: Rhossydd
We frequently see comparisons of gamut, by volume or area, using various tools such as Color Think Pro, Gamutvision, Colorsync etc.
Can someone explain if there’s a standard deviation factor(eg + or - 1.2 DeltaE ) that defines where the gamut boundaries are ?
Is this enshrined in the ICC standards ?

Thanks

Paul

I don't understand the question. The boundaries of a theoretical gamut (sRGB, pro-photo, etc) are defined by the gamut itself. The boundaries of an empirical gamut (printer X, paper Y) are defined by measurement. Where would SD come into play?
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Peter
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Rhossydd
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« Reply #2 on: December 02, 2009, 10:41:42 AM »
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Quote from: PeterAit
I don't understand the question. The boundaries of a theoretical gamut (sRGB, pro-photo, etc) are defined by the gamut itself.
So at what measurement does the gamut deem to stop ?

How is the limit of gamut defined ? and what is the measurement tolerance ?
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NikoJorj
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« Reply #3 on: December 02, 2009, 03:22:15 PM »
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Quote from: Rhossydd
How is the limit of gamut defined ? and what is the measurement tolerance ?
I'll try to answer from what I've understood so far.. If someone knowledgeable can correct, please do.
For a peripheral, the gamut is defined as all the colors the device can output. As long as a color is produced it is in the gamut, the limit we see on most (all?) graphs is the set of the most extreme (the most saturated for a given luminosity, or vice-versa) of these colors.
The profiling target "just" has to parse the RGB values to make the device output all the possible colors.
Instrument precision does not take part of the definition of the gamut (of course, it still has an influence on the measured one).
« Last Edit: December 02, 2009, 03:22:58 PM by NikoJorj » Logged

Nicolas from Grenoble
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Ethan_Hansen
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« Reply #4 on: December 04, 2009, 01:56:12 AM »
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For simplicity, let's consider only the two most common types of profiles: matrix and Look-up Table (LUT) based. A matrix profile is defined by a (comparatively) simple set of equations and parameters. Examples include Adobe RGB and ProPhoto RGB. The gamut boundaries of these profiles are smooth and easy to calculate. A profile of a real world device such as a printer uses a LUT. This is a table of points correlating a actual, real-world color to the corresponding color in the printer's color space. While matrix profiles are nicely behaved mathematical abstractions, LUT profiles are usually anything but.

Determining the gamut boundary of a real-world device takes measuring it. Errors occur due to measurement inaccuracies or, as is becoming increasingly common with wide gamut LCD displays and colorimeters such as the EyeOne Display, the range of the measurement device being insufficient to read the full range of the display. Any profile made under such conditions is guaranteed inaccurate, so worrying about gamut boundaries is the least of your problems.

Once you have a profile, computing the gamut boundary is a matter of number crunching. The gamut hull is computed at a finite number of slices. How these steps are performed, how coarse or fine the sampling grid, and the mesh size used to fit the gamut hull all factor in to how accurate the gamut calculation will be. A pathologically behaved printer requires a finer sampling grid to produce the same absolute accuracy of a gamut volume calculation than does a colro space such as PP RGB.

The other tradeoff is computation time. You can perform a very accurate calculation if you are willing to wait long enough. We use gamut hull calculations as part of our printer profiling software and went through many algorithm tuning iterations. We finally settled on a set of calculations that spits out a gamut volume in few seconds on our 8-core server boxes. This gives values within 1% of those produced by an hour long run. If there is interest I can cobble together a standalone program to calculate gamut volumes on a more typical hardware system with ~+/- 5% accuracy (Windows only -- sorry).
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Rhossydd
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« Reply #5 on: December 06, 2009, 10:39:21 AM »
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Maybe I'm not being clear enough with my question here. I'll try phrasing it differently;

How accurate are the gamut boundaries ? 2%, 5%, 10%...... ?

Presumably the factors involved are measuring device accuracy, device stability, rounding errors in profiling software calculations, plus is there a tolerance factor within the specification to allow for these inaccuracies.
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Jonathan Wienke
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« Reply #6 on: December 06, 2009, 02:11:55 PM »
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Quote from: Rhossydd
Maybe I'm not being clear enough with my question here. I'll try phrasing it differently;

How accurate are the gamut boundaries ? 2%, 5%, 10%...... ?

Depends on the the accuracy of the measuring device. As to where the boundaries are located, that depends on the physical characteristics of the device (monitor, printer, etc.). With monitors, it depends on the CRT phosphors, LCD color filters and backlight source, or LED characteristics. With printers, it's a combination of the inks and paper.
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Rhossydd
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« Reply #7 on: December 07, 2009, 12:54:05 AM »
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Quote from: Jonathan Wienke
Depends on the the accuracy of the measuring device.
Er, yes. I did say that already and added some other possible factors. I'm starting to wonder of anyone here actually knows the answer.
It seems an interesting issue to me when so much is concluded from gamut visualisations.
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Jonathan Wienke
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« Reply #8 on: December 07, 2009, 09:41:50 AM »
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Quote from: Rhossydd
I'm starting to wonder of anyone here actually knows the answer.
For synthetic editing profiles (ProPhoto, sRGB, etc.), there is no measurement error--the gamut boundaries are an intrinsic part of the mathematical definition of the profile.

For measured profiles, you'd have to look up the accuracy specifications for the device that measured the profile data, if you know what device was used to make the measurements. If you don't even know the measuring device used, speculating about measurement accuracy is kinda pointless...
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Rhossydd
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« Reply #9 on: December 08, 2009, 12:42:28 AM »
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Quote from: Jonathan Wienke
speculating about measurement accuracy is kinda pointless...
If you take that point of view or specification any review that references gamut data without qualification (generally not possible with manufacturer's own materials) is worthless.

This comes back to the original question, how accurate are profile gamut measurements and what tolerances are deemed acceptable.
So, when comparing gamuts how much can we conclude from them ?
For example, are transmissive measurements generally more or less accurate than reflective measurements. If there were a significant difference, comparing screen profile gamuts to other gamuts might be unreliable. Should we trust figures like "107% of AdobeRGB" when reading a monitor specification if the tolerances involved in building the profile are +/- 1.5% ??
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tived
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« Reply #10 on: December 08, 2009, 05:03:09 AM »
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Rhossydd,

How long is a piece of string?

Your question is very open ended, people here have tried to accommodate your original question. Perhaps if you become a little more specific and word your question differently, as in a way that include more information as to what you really want to know.

To answer the question " Should we trust figures like 107% AdobeRGB?" Do you have access to the procedure that the device was measured under, if so, and that this procedure satisfy you, then you have good reason to believe/trust that the device covers 107% AdobeRGB. Do you trust some who says their monitor covers 50% of aRGB? when you measure it crudely it does indeed cover 95% aRGB? <where crudely is with your own color device, under uncontrolled conditions>

I use a EIZO CG241W, NEC 2690 and a DELL 2407... I think the first two is rated by their respective companies to be at or around 95% or higher in terms of coverage of the aRGB color gamut. I will profile them with two trusty iOne's and the CG is just under, the NEC is over, by a degree more then the CG is under. The Dell however, is well under, but that I would also expect, as they have not made any attempt to tell me its close.

Trust.... what is your tolerance of trust? is it equal to, is it 0,00000000000000000000000000000......00000000001% or greater precision acceptable?
 is 2-3% ok? is it binary, YES or NO? maybe 5-10% is acceptable?

Can you operate, at any of these tolerances when you work on any of these monitors, devices? can you tell the difference? Some can, maybe not with precision, but they can tell you that there is a difference. That difference may be to one, an advantage, a positive thing, but to another it is just not good enough.

So, in my own case, my latest monitor, an NEC 2690 has met and exceeded my expectations. I don't actually recall how big a gamut NEC claimed and I am too lazy to go looking now, but whatever the number was, it has given me enough confidence in the product to recommend it to you or anyone else who needs a nice Monitor, in its price range. Can it display 32bit color files NO. Did I expect it? NO Would i like one that could? YES! Can I afford one? NO So, the NEC fits the bill for me and I trust it to display enough or more of aRGB gamut for me to output or produce files for other people. Not that this has anything to do with NEC, but I have used it as an example here to illustrate my point.

I hope you find what you are looking for, and when you do I hope you are able to see it.

:-)

Henrik
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Jonathan Wienke
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« Reply #11 on: December 08, 2009, 09:07:12 AM »
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Quote from: Rhossydd
This comes back to the original question, how accurate are profile gamut measurements and what tolerances are deemed acceptable.
So, when comparing gamuts how much can we conclude from them ?
For example, are transmissive measurements generally more or less accurate than reflective measurements. If there were a significant difference, comparing screen profile gamuts to other gamuts might be unreliable. Should we trust figures like "107% of AdobeRGB" when reading a monitor specification if the tolerances involved in building the profile are +/- 1.5% ??

You can get a rough idea of the accuracy of a profiling/measurement device by using it to profile your monitor and then visually comparing a Color Checker reference image file displayed on-screen to an actual Color Checker. The manufacturers of colorimeters and spectrophotometers do usually post measurement accuracy data, though it may take some digging to find it.

The other thing to consider is that converting DeltaE figures (which is what manufacturers usually quote, since that is the best indicator of how closely profiled colors match original colors) to percentages involves some pretty advanced math. Human vision is better at distinguishing between some subtle shades of color than others, and DeltaE takes this into account. If you want a specific percentage difference, you'd have to choose a particular shade of green, for example, and then choose similar, but slightly different green values and run them through the DeltaE formula and compare the percentage of change to DeltaE. If you do the same exercise for shades of orange, the results would NOT be the same, by design. And that's a good thing. Get over your fixation on percentages; percentages will not answer your questions in a meaningful way. A 2% change in blue is not the same as a 2% change in green is not the same as a 2% change in orange, etc, etc, etc.

Transmissive and reflective measurements are equally accurate if made with the same measuring device. There's no reason for there to be any difference. Why would there be?

The ultimate test of color management is this: Photograph something, say a garden gnome figurine. Display the photo of the gnome on your monitor, and print the photo with several different printers on a variety of papers. How closely do the colors of the original gnome figurine, the monitor image, and the various prints match? My experience with the EyeOne spectro is that you can get everything pretty darn close if you profile everything properly; close enough that a side-by-side comparison is necessary to distinguish subtle differences. DeltaE is a fairly good quantification of the magnitude of the differences you will see, but it has a non-constant relationship to the actual margin of error expressed as a percentage. My experience is that the manufacturer's claims of color accuracy (at least from GretagMacbeth/X-Rite) are trustworthy and translate well to real-world results.

The bottom line is that profile comparisons and other color management claims are generally reliable, trustworthy, and correlate reasonably well to real-world results.
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Rhossydd
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« Reply #12 on: December 09, 2009, 03:35:52 AM »
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Quote from: Jonathan Wienke
You can get a rough idea of the accuracy of a profiling/measurement device by using it to profile your monitor and then visually comparing a Color Checker reference image file displayed on-screen to an actual Color Checker.
You're missing the point here. It's not about one system's accuracy it's about comparative accuracy.
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The other thing to consider is that converting DeltaE figures........A 2% change in blue.... etc,
I was using the term percentage only as a general concept to try to help people understand what I'm trying to find out. Ultimately digital colour meters will only output numbers anyway, so a percentage error can be significant way of looking at it. How any percentage error is then handled when processed in software adds another potential tolerance factor.
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The ultimate test of color management is this:... How closely do the colors of the original .., the monitor image, and the various prints match?.
No, not really. The real test is taking an image from an outside source and being confident on how it will be reproduced on another output device, eg on a magazine cover. This is what colour management was implemented to handle.

This comes back to how comparatively accurate profiles are.
Yes, I know that without knowing what device was used to create the profile there's an element of trust involved, but is there really no specified tolerances of accuracy overall within the ICC specification ? If not, how are we to trust this information at all ?

I'd rather hoped some of the experts here might have been able to shed some light on this, but maybe it's all taken on trust and not understood too well.
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PeterAit
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« Reply #13 on: December 09, 2009, 07:39:39 AM »
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Quote from: Rhossydd
No, not really. The real test is taking an image from an outside source and being confident on how it will be reproduced on another output device, eg on a magazine cover. This is what colour management was implemented to handle.

Yes, really, at least for most of us on this forum. We are not involved in magazine production but rather in taking photos and wanting to create prints that match our vision. If the profiles I use let me do this (which they do), then academic discussions about "accuracy" become so much mental masturbation and a complete waste of time. If I was a carpenter and had a saw that let me cut wood in all the ways I wanted, I would hardly be concerned if the blade were actually 1/2 inch shorter than its design specs.

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Peter
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Rhossydd
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« Reply #14 on: December 09, 2009, 08:22:31 AM »
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Yes, really, at least for most of us on this forum.
Statisticaly, I'm sure you're correct. Most people aren't interested in this subject, but this forum is one of the few web forums where one can ask difficult, possibly academic, questions about digital photography and stand some chance of getting an informed answer.
If you're not interested in the more esoteric discussions here, just ignore them, but don't try to 'dumb down' the forum just because you're not interested or don't understand the issues underlying a particular thread.
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We are not involved in magazine production
No, but a greater proportion than most here DO need to consider the wider issues of colour management. Maybe not for magazines, as such, but consider the increasing number of photographers self publishing through companies like Blurb, Lulu, Snapfish etc.  A look at Blurb's support forums suggests there's a lot confusion and misunderstanding about colour management when photographers starting to work with something more involved than their desktop printer.
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Jonathan Wienke
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« Reply #15 on: December 11, 2009, 01:44:51 PM »
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Quote from: Rhossydd
No, not really. The real test is taking an image from an outside source and being confident on how it will be reproduced on another output device, eg on a magazine cover. This is what colour management was implemented to handle.

WTF do you think I'm talking about? If I can color-manage my workflow such that I can shoot a garden gnome figurine, and print images of the garden gnome that are essentially indistinguishable from the original on 5 different printers and 10 different paper stocks, then color management is doing its job and I can be confident that my colors will be correct when I print, regardless of whether it's a magazine cover or whatever, as long as I follow best practices for profiling, etc.

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This comes back to how comparatively accurate profiles are.
Yes, I know that without knowing what device was used to create the profile there's an element of trust involved, but is there really no specified tolerances of accuracy overall within the ICC specification ? If not, how are we to trust this information at all ?

Let me repeat myself...Manufacturers DO specify accuracy tolerances, but in DeltaE instead of percentages.

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I'd rather hoped some of the experts here might have been able to shed some light on this, but maybe it's all taken on trust and not understood too well.

It would help if you actually read and paid attention to what you've been told here, as most of your questions have already been answered...
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Rhossydd
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« Reply #16 on: December 11, 2009, 01:47:54 PM »
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Quote from: Jonathan Wienke
as most of your questions have already been answered...
Not at all.
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waynebretl
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« Reply #17 on: February 10, 2010, 06:51:14 PM »
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I think this thread got inexcusably nasty, just because the original poster didn't phrase his question clearly.

"We frequently see comparisons of gamut, by volume or area, using various tools such as Color Think Pro, Gamutvision, Colorsync etc.
Can someone explain if there’s a standard deviation factor(eg + or - 1.2 DeltaE ) that defines where the gamut boundaries are ?
Is this enshrined in the ICC standards ?

Thanks

Paul"

I think what Paul intended to ask was "Is there a specified tolerance for stating color gamuts somewhere in a standards document?" The implication is "Are stated gamuts of tight enough tolerance to be useful, and how picky should I be in comparing published gamuts?"  

If these are the intended questions (Paul, please correct me if I'm wrong), the answers are:
1) there is no standard for how precise color gamut measurements should be
2) small differences in gamut on the order of a few Delta E are not significant anyway.

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Jonathan Wienke
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« Reply #18 on: February 10, 2010, 07:00:18 PM »
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Quote from: waynebretl
I think what Paul intended to ask was "Is there a specified tolerance for stating color gamuts somewhere in a standards document?" The implication is "Are stated gamuts of tight enough tolerance to be useful, and how picky should I be in comparing published gamuts?"  

If these are the intended questions (Paul, please correct me if I'm wrong), the answers are:
1) there is no standard for how precise color gamut measurements should be
2) small differences in gamut on the order of a few Delta E are not significant anyway.

I answered both of those questions in my previous posts. If you want to know measurement accuracy, you have to know the measurement tool and its accuracy specs.
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Rhossydd
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« Reply #19 on: February 11, 2010, 03:25:56 AM »
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Quote from: waynebretl
I think what Paul intended to ask was "Is there a specified tolerance for stating color gamuts somewhere in a standards document?" The implication is "Are stated gamuts of tight enough tolerance to be useful, and how picky should I be in comparing published gamuts?"  

If these are the intended questions (Paul, please correct me if I'm wrong), the answers are:
1) there is no standard for how precise color gamut measurements should be
2) small differences in gamut on the order of a few Delta E are not significant anyway.
Thanks Wayne. A great first post here, welcome.
Yes, you've understood what I was getting at almost completely. My original question was loosely(poorly in hindsight) worded to also cover the possibility that different profile creation tools might be working to different tolerances if a specific tolerance isn't specified in the ICC standards.
1) Right thanks. I'll get round to actually reading these standards one day when my colour knowledge is better than it is now.
2) True, but manufacturers are promoting products on the basis of percentages of standard colourspaces. If errors in measurement are greater than the implied accuracy, those figures become rather futile.

The other issue is that gamut comparisons are often made via visualisation tools where DeltaE isn't used. I wonder if it's better to consider the gamut volume of devices more as soft edged 'clouds' rather than the tightly defined wireframes. If that makes sense, what would be useful to know is how fluffy the clouds are.
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