Dan Margulis Applied Color Theory
LAB Conversions and White Point
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Date: Mon, 25 Oct 2004 15:47:36 +0100
From: Andrew Haley
Subject: Photoshop Lab
Let’s say you have a file saved as Lab.
Open it in Photoshop and render it for printing. What white
point does Photoshop assume for the source (Lab) file? I can’t
find this information anywhere.
Thanks,
Andrew.
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Date: Mon, 25 Oct 2004 09:43:32 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
on 10/25/04 8:47 AM, Andrew Haley wrote:
Let’s say you have a file saved as Lab.
Open it in Phososhop and
render it for printing. What white point
does Photoshop assume for
the source (Lab) file? I can’t find
this information anywhere.
D65.
Andrew Rodney
http://digitaldog.net/
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Date: Wed, 27 Oct 2004 16:15:41 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Does this mean that an actual D65 lab measurement taken
of a real object whose values are then used for reference in an object in a
lab document would, when passed through an accurate
profile(Absolute/RelCol), render as close to that measured color as is
possible? If that color is within the gamut of the device, then
shouldn’t it nail it? As I think you get the drift of the
question, would you kindly offer some amplification on this topic.
Henry Davis
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Date: Wed, 27 Oct 2004 16:24:31 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
D65 is a mathematical description (an Illuminant.).
These Illuminants are tied to the relative energy distribution that
corresponds to the radiation emitted by a so called black body. Therefore
D65 has a spectral energy distribution that closely matches that of a black
body at 6500K. Lab too is a totally synthetic color model. Passing this
through a color space conversion could produce any number of results based
upon the source profile.
Andrew Rodney
http://digitaldog.net/
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Date: Wed, 27 Oct 2004 17:18:26 -0600
From: Nathan Wade
Subject: Re: Photoshop Lab
On Oct 27, 2004, at 2:15 PM, Henry wrote:
Does this mean that an actual D65 lab measurement
taken of a real
object whose values are then used for reference
in an object in a lab
document would, when passed through an accurate
profile(Absolute/RelCol), render as close to that
measured color as is
possible?Ê If that color is within the gamut of
the device, then
shouldn’t it nail it?Ê As I think you get
the drift of the question,
would you kindly offer some amplification on this
topic.Ê
It has the potential to render “accurately”
but context is everything. Relationships between neighboring colors are
probably more important than an exact colorimetric match.
And, technically there is no such thing as a D65 Lab
measurement. Measurements are usually in XYZ and are converted to Lab (and
sometimes require chromatic adaptation). The point is that a D65 Lab value
is not the end-all-be-all description of the color of the original physical
object.
Trying to nail every color colorimetrically is probably
a recipe for heartbreak. If you only have a few colors you’re
concerned about, it might be possible to make a device link profile that
converts your special Lab colors to device-dependent colors as you dictate.
Nathan Wade
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Date: Thu, 28 Oct 2004 11:06:21 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Exact colorimetric matching is the goal, in this case.
This isn’t about pretty pictures. Why is it that a
profile cannot be made whose job is exact coloirmetric matching, especially
in a hypothetical case where every color is within the gamut of the output
device? Why is a device link the better option for exact matching?
Henry Davis
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Date: Thu, 28 Oct 2004 12:18:57 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Then a Lab document can be expected to convert properly
through an output profile, even though there is no “source”
profile in the Lab document?
Henry
On Oct 28, 2004, at 11:58 AM, Andrew Rodney wrote:
If you built the file in PS, then use,
it’s a direct LAB creation.
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Date: Thu, 28 Oct 2004 10:21:30 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
on 10/28/04 10:18 AM, Henry wrote:
Then a Lab document can be expected to convert
properly through an output
profile, even though there is no
“source” profile in the Lab document?
Properly? That would depend on the definition of
properly and of course how the output profile behaves. Then there are other
issues that could rear it1s ugly head (which CMM, does the CMM have a bug
etc). Use ACE.
Andrew Rodney
http://digitaldog.net
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Date: Thu, 28 Oct 2004 12:44:55 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
By properly, I mean, an output profile that produces an
exact colorimetric match when printed - when the target color is within the
gamut of the output device. Are profiles expected to misbehave with
colorimetric conversions when the target color is in gamut? If so,
and if it’s bugs, this isn’t good to hear. The definition
of a colorimetric conversion isn’t about getting unexpected results
from in-gamut colors.
Henry
On Oct 28, 2004, at 12:21 PM, Andrew Rodney wrote:
Properly? That would depend on the definition of
properly and of
course how the output profile behaves. Then there
are other issues
that could rear it’s ugly head (which CMM,
does the CMM have a bug
etc). Use ACE.
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Date: Thu, 28 Oct 2004 12:17:03 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
on 10/28/04 11:20 AM, Henry wrote:
I see, thanks. Do you think there’s a
need for a profiling product that
allows the user to custom define the number of
swatches, and their values,
specifically for producing colorimetrically
accurate profiles. Such a product
would have no need for perceptual tables nor
interpolations for colors not
included in the swatch set and out of gamut
colors wouldn’t be an issue.
There are products that allow you to build custom
targets with various numbers of patches. The biggest patch sample I1ve
personally measured was over 10,500 patches and even with a very fast auto
Spectrophotometer, that took an hour. This is far short of the 16.7 million
I indicated before and the profile was 5mb in size! It was slightly better
then the standard target from this package which asks for 918 samples.
One other important point. Colorimetrically correct
data isn1t always preferable. First, profiles don1t know anything about
images, only devices. Second, I1ve seen many cases where clients and end
users prefer imagery that isn1t Colorimetrically correct compared to
imagery that is.
Andrew Rodney
http://digitaldog.net/
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Date: Thu, 28 Oct 2004 13:20:42 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
I see, thanks. Do you think there’s a need
for a profiling product that allows the user to custom define the number of
swatches, and their values, specifically for producing colorimetrically
accurate profiles. Such a product would have no need for perceptual
tables nor interpolations for colors not included in the swatch set and out
of gamut colors wouldn’t be an issue.
Henry
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Date: Thu, 28 Oct 2004 10:52:27 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
Output profiles are far from perfect
(colorimetrically). You1d literally have measure 16.7 million patches to
build a profile that used no extrapolation in conversions. You1d end up
with a profile larger then many images as well. So all kinds of
3assumptions2 are made. I1d be hard pressed to believe that a prefect
colorimetrically correct profile exists but I could be wrong.
Andrew Rodney
http://digitaldog.net/
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Date: Thu, 28 Oct 2004 14:36:44 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Yes, printing pretty pictures isn’t the goal for
this hypothetical, only the accurate reproduction of in-gamut colors.
Thanks for the input.
Henry
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Date: Thu, 28 Oct 2004 15:53:31 -0600
From: Andrew Rodney
Subject: Re: Photoshop Lab
on 10/28/04 9:06 AM, Henry wrote:
Why is a device link the better option for
exact matching?
Device links are useful for doing CMYK to CMYK
conversions because there1s no trip to LAB; a link is a one way street. A
device link is like two profiles joined at the hip. The source and
destination are fixed and specified and the conversion is direct. That1s
useful when you want to control CMYK to CMYK with control over black
generation (no trip to LAB). So instead of CMYK-LAB-CMYK, you get
CMYK-CMYK.
Andrew Rodney
http://digitaldog.net/
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Date: Thu, 28 Oct 2004 19:43:00 -0400
From: Jim Rich
Subject: Re: Photoshop Lab
Henry,
I have only seen a few reasons for colorimetric
matches. And those situations are for printing products
that require a numerical (say LAB) product color matches and pantone
colors. And there might be more than I have mentioned,
Practically speaking, most if not all
continuous tone images are that are reproduced are pretty pictures
and that the quality of the color is in the eye of the person who pays the
bill.
But that is my view of color reproduction and has been
for some 30+ years.
As for getting more accurate color matching.
Theoretically, the way to do that is to measure all of the colors you
need reproduced in to your calibration software that was created to handle
all of those measurements. That could be millions of measurements. Then use
those measurements as the basis for color matching in that
calibration system. Now if that type of system was m commercially
available, that calibration model would costly and just not
practical.
ICC profiles use a relatively small sample size
of the colors in your process. Then the profiling application
uses some science, color engineering skill and color
interpolation from the samples to try and do color matching. In the
ICC world color management software is taking an educated guess at
colors that were not measured.
Is color matching with ICC profiles perfect? Nope there
are trade-offs. But IMHO, ICC profiles generally work quite
well.
And some of todays profiling applications
do a great job and are practical and effective as a calibration
strategy.
My .02.
Jim Rich
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Date: Thu, 28 Oct 2004 16:27:24 -0700
From: Mike Russell
Subject: Re: Photoshop Lab
Henry wrote:
I see, thanks. Do you think there’s a
need for a profiling product
that allows the user to custom define the number
of swatches, and
their values, specifically for producing
colorimetrically accurate
profiles. Such a product would have no need for
perceptual tables nor
interpolations for colors not included in the
swatch set and out of
gamut colors wouldn’t be an issue.
You’ve got my curiosity going re your
application.
I’m imagining it’s something like matching
a finite set of hues for a clothing catalog.
Mike Russell
www.curvemeister.com
www.geigy.2y.net
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Date: Fri, 29 Oct 2004 12:03:01 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Right. For trade and product specific colors, it
would be nice to have product that shortcuts the trial and error process of
finding the correct recipe for a color (that is in-gamut). Output
profiling target files are composed of either of RGB or CMYK values rather
than Lab values, but their associated reference files have Lab values, and
this is where I’m a bit confused. In order to obtain the
reference file Lab values, I’m guessing that they could be made from
actual measurements, or, they could be assumed values based on the Lab
values from a particular RGB or CMYK model. Earlier in this tread,
the particular Lab that Photoshop uses was in question, and Andrew said
that it is D65.
That there is interpolation involved in profiling and
conversion is understood. However, it seems to be almost too easy to
ask why a Lab reference value of say 76, -6, -20 (which is a light blue and
is in gamut for the press), can’t be located without interpolation
from a custom profile of the press. The press target sheet that, in
this case has maybe 1000 blue patches, is measured and listed in Lab
values. The job is done when either the desired value is found in the
list (Bingo), or the closest value found is identified.
For input profiles, I’ve made my own reference
file from a Macbeth color chart (which works a tad bit better than the one
supplied because of the target’s age), but the printer targets are
either RGB or CMYK. Where do those reference file values come from?
Suppose you had a target Lab value, as with an input
profile used for scanners and digital cameras, but it was obtained from
your fabric or plastic part. You also have good gamut description of
the press. Is it too much to ask that a profile
conversion(colorimetric), or a profiling package if need be, should provide
the exact values needed to render that color?
This would be terrific for dealing with the new batch
of fabrics or “gotta match” projects that are real world print
challenges. Heck, they do it at the paint store, and they use a lot
more colorants than 4. I’ll almost bet that they use some
colorimetric profiling that is not intended to reproduce nice pictures, but
rather accurate individual colors.
Henry Davis
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Date: Fri, 29 Oct 2004 12:05:14 -0400
From: Henry Davis
Subject: Re: Photoshop Lab
Thanks for the input. How do they do it at the
paint store?
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Date: Fri, 29 Oct 2004 18:18:22 -0400
From: Jim Rich
Subject: Re: Photoshop Lab
Henry,
It is interesting that you ask that.
The short answer is they have a paint mixing machine
and a guy with experience on that machine.
I just visited my paint store with a paint swatch and I
was prepared to give them the LAB value of the color so they could put that
in their computer and paint mixing system that mixes the paint.
While I was there it was clear that the paint
manufacturer had built a paint mixing system that allows the guy behind the
counter to measure the paint swatch and have the paint machine mix the
colors based on the measurement. It all seemed quite simple but I am
sure it took years and lots of money to develop a tool that a hardware
store person could work and get the color mixed properly. Kind of
like the ICC system. After you spend some time and money working with the
ICC tools and dtp apps it becomes relatively easy.
Jim Rich
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Date: Mon, 1 Nov 2004 11:00:00 -0500
From: Henry Davis
Subject: Re: Photoshop Lab
Jim,
From this thread, I have gathered that exact
colorimetric matches is not quite so in the ICC arena, yet I have had
several amazingly accurate matches at different paint stores. It
didn’t appear to take a great deal of understanding or hands on
science on the part of the employee, either. The sample was measured,
the formula was followed, and the color matched.
Henry Davis
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Date: Mon, 1 Nov 2004 10:14:39 -0800
From: Mike Russell
Subject: Re: Photoshop Lab
Henry wrote:
From this thread, I have gathered that exact
colorimetric matches is
not quite so in the ICC arena, yet I have had
several amazingly
accurate matches at different paint stores.
It didn’t appear to take
a great deal of understanding or hands on science
on the part of the
employee, either. The sample was measured,
the formula was followed,
and the color matched.
My guess is they used curves and not profiles :-)
Mike Russell
www.curvemeister.com
www.geigy.2y.net
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Date: Mon, 01 Nov 2004 19:10:51 -0500
From: Jim Rich
Subject: Re: Photoshop Lab
Henry,
Hmmm. My posts were meant to point out that if a
company ( or an individual) were to take the time to measure all of the
colors possible with a set of colorants and put them into an automated
color matching system that they would stand a chance at
getting exact color matches without to much pain and suffering after the
initial development.
But as I pointed out that strategy is not practical.
As for icc profiles not being as good as a theoretical
system that I proposed, well.....that is a little too much extrapolation.
You can do color matching anyway you want but... if you
take a look at what it takes to get an imaging system calibrated and keep
it calibrated you would see that ICC profiles are not only cost effective
but they do a good if not very good job of color matching. Will the
results always be a colorimetric match? Probably not. Will it look
good? Probably.
How far will the colors be away from a colorimetric
match? Probably not too
far, but it wont be exact as you were
asking for.
Jim Rich
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Date: Tue, 2 Nov 2004 11:30:50 -0500
From: Henry Davis
Subject: Re: Photoshop Lab
Jim,
I don’t want to be misunderstood as being overly
critical of profiling. I have read discussions about how to improve
upon the current solution, and where do we go from here talk, but in all of
that, all I hear is the limitations etc., etc. In this thread, I was
presenting a scenario that seemed plausible, but I have been surprised to
read that many experts in this field consider the topic a technical dead
end. Your comment that it isn’t practical does sound
reasonable, and at least a bit more open minded, but the paint companies
certainly believed it was practical. If there is to be an evolving future
for profiling and color management, I would be disappointed if exact
colorimetric matching was already off of the table.
Is there an evolving future ahead for profiling, or is
this as far as the technology can go?
Henry Davis
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Date: Tue, 2 Nov 2004 13:44:03 -0600
From: Jim Bean
Subject: Re: Photoshop Lab
hello jim & henry,
I may not be understanding this thread.. but certainly
there is a huge difference between matching one opaque color of paint,
utilizing unlimited resources and matching a large number of transparent
colors produced from a limited pallet and output simultaneously on a
variety of media. jim bean
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Date: Tue, 2 Nov 2004 15:21:32 -0500
From: Henry Davis
Subject: Re: Photoshop Lab
Jim,
These are good points. Since there are more than
4 colorants available to the paint store, I wonder if this complicates or
simplifies the process of matching a particular color? At any rate,
my participation in this thread was born out of ignorance. I thought
that predicting the output values for an exact colorimetric match of an
in-gamut color might be one of the future goals of profiling.
Henry Davis
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Date: Wed, 3 Nov 2004 11:31:08 EST
From: Dan Margulis
Subject: Re: Photoshop Lab
Henry writes,
These are good points. Since there are more than
4 colorants
available to the paint store, I wonder if this
complicates or
simplifies the process of matching a particular color?
It doesn’t matter. Mixing paints is an entirely
different subject. Paint has no detail, no variation in darkness or color,
so all you have to worry about is one type of measurement, which can be
accomplished in a variety of ways. Plus, you can’t mix up a
paint that’s out of the gamut of paints that can be mixed. And you
view it in a normal environment next to other things that are unrestricted
as to their colors. It’s possible, therefore, to come up with a test
that determines absolutely whether the paint is close enough to the desired
color to declare it a “match”.
If a CMYK picture contains an item that theoretically
is of a known color, no such agreement is possible. First of all, even
assuming that a machine would measure it as a certain color, there’s
no particular reason to think that a human would agree. For example, if
there’s a reasonably bright blue, within the CMYK gamut, and we try
to match it “colorimetrically”, the viewer will think
it’s *too* blue, because he’s unaccustomed to really bright
blues in CMYK pictures.
Second, there wouldn’t be any way to measure it
because there wouldn’t be agreement on what point to measure. If a
car is supposed to be PMS 357, it’s only going to be that color in
areas in which we consider the lighting to be normal. Everywhere else,
it’s going to be lighter or darker, even if the hue doesn’t
change (which it can). Two different people are likely to pick two
different points for the “normal” point to measure the color.
Third, even assuming that the basic color is within
CMYK gamut, it’s fairly common for it to go out of gamut as it gets
lighter. Therefore, we have a situation where half the object is in gamut
and half isn’t. The only way to get a natural-looking result is to
tone down the parts that *are* in gamut, deliberately not matching them
colorimetrically.
At any rate, my participation in this thread was born
out of ignorance. I
thought that predicting the output values for an
exact colorimetric match of
an in-gamut color might be one of the future goals of
profiling.
No. That’s one of the *past* goals. What you are
speaking of is what is known as “calibrationism”, the idea that
there’s a perfect conversion method that works in all cases.
When you’re trying to make a critical match, some human
intervention is usually necessary in CMYK. The point of profiling should be
to try to reduce the need for the intervention, not to eliminate it.
Dan Margulis
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Date: Wed, 3 Nov 2004 12:21:44 -0500
From: Henry Davis
Subject: Re: Photoshop Lab
Thanks, Dan. Then the approach is still one of
noodling until satisfied with the match.
And, if a profile correctly calculates the closest
starting values, this could be a time saver.
Henry Davis
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