Re: Theoretical CMYK Profile "and Lch"
Re: Theoretical CMYK Profile "and Lch"
- Subject: Re: Theoretical CMYK Profile "and Lch"
- From: Ray Maxwell <email@hidden>
- Date: Sun, 14 May 2006 08:54:18 -0700
Roger Breton 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.
It is a cyan. No problem. It is obvious.
Over and over I hear people talk about the fact that CIELab is not
intuitive. I don't understand this. 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.
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.
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.
Ray
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