Re: Theoretical CMYK Profile "and Lch"
Re: Theoretical CMYK Profile "and Lch"
- Subject: Re: Theoretical CMYK Profile "and Lch"
- From: Roger Breton <email@hidden>
- Date: Wed, 17 May 2006 21:46:37 -0400
Ray,
I don't mean to be controversial. You know I respect your opinion and those
of about everyone on this list.
I wrote:
>
>> Lch makes color editing a breeze because it is intuitive. Quick: tell me
>> what color is a* -40 and b* - 35? You can't.
To which you replied:
> It is a cyan. No problem. It is obvious.
Obvious for you, Ray. Not so for a lot of good folks I know. I agree that
interpreting a* and b* coordinates is not rocket science. But I contend that
it is not as *direct* an interpretion as hue and chroma is because, as you
know, "hue" and "chroma" don't require any interpretation: in and out of
themselves they *mean* something to us perceptually.
> Over and over I hear people talk about the fact that CIELab is not
> intuitive. I don't understand this.
It is not that a* and b* can't come to be meaningful perceptually. It is not
that they take years of training to become loaded with meaning. It is that,
by design, they don't *code* as much information as c and h.
Take a bunch of ISO-12647-x CIE a* and b* numbers, and try to compare them
with some other bunch of a* and b* numbers in an effort to contrast the
visual appearance of process inks. To me, there is a cognitive effort
involved in turning a* and b* into visual appearance attributes. And I
suspect it is the same thing for a lot of people in everyday color work (a
lot of people are probably not using Lab all that much in their everyday
color work but that's another story). But not so with c and h: zero cognitve
effort.
> Let me explain how I visualize and
> use Lab.
>
> First, let identify the main axis's.
>
> If a* is positive and b* = 0 then the color is magenta.
>
> If a* is negative and b* = 0 then it is green.
>
> If b* is positive and a* = 0 then it is yellow.
>
> If b* is negative and a* = 0 then it is blue.
>
> If a* and b* are negative it is cyan.
>
> If a* and b* are positive it is red.
OK. And, strictly, we know we can't say that positive increments of a* do
appear "magenta", but that's besides the point. Your account of how you
process a* and b* in your brain to extract perceptual meaning is the same as
mine, and probably the same as everybody. We all agree with this, I think.
> A lot of confusion arises when many books state that positive a* is
> red. I have measured many different magenta printing inks and they all
> lie very close to the positive a* axis. I don't understand why they
> call it red.
OK. I won't debate that.
> Now how do we apply this knowledge in everyday work in the print industry.
>
> Let's say the we are trying to adjust to get better gray tracking on a
> step wedge and we are working in a CMYK color space. Note that the
> fully saturated colors measure about 70 to 80 on the a* or b* axis. We
> specify CMYK color on a 0% to 100% range. We want to know how much of a
> CMYK adjustment to make to get closer gray tracking. We will look at
> one gray patch.
>
> Our measurement is L* = 54, a* = 3, b* = -2. This patch has a
> blue-magenta cast.
>
> The CMYK value that produced it is C = 42% M = 36% Y = 30% K = 2%
>
> If we want a* and b* to be zero then what adjustments must we make?
>
> We see from a* = 3 that the gray is three units too magenta so we are
> going to subtract magenta. We might try 80% of 3 Lab units which would
> be 2 units of magenta. So subtract 2% magenta.
>
> We see from b* = -2 that the gray is minus 2 units too blue. We will
> add 80% of 2 Lab units or (rounding) 2 units of yellow.
>
> Now we may need to add or subtract CMY values in proportion to hold the
> L* value.
>
> While this is not exact is is very close for small changes.
>
> I hope this takes some of the mystery out of Lab.
I'm sure a lot of readers have gotten a clearer understanding of Lab and let
me thank you for sharing that example with everyone, Ray.
And I won't start re-expressing your example in terms of Lch to see how many
words I would this save because that won't convince you. I'll leave it at
that. My point remains unchanged and I respect your opinion. It's a free
country (world).
> Ray
Regards,
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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