RE: CMYK to RGB to CMYK
RE: CMYK to RGB to CMYK
- Subject: RE: CMYK to RGB to CMYK
- From: Scott Olswold <email@hidden>
- Date: Fri, 2 Nov 2001 10:46:16 -0500
Dave,
You'd think that since most CMYK gamuts are within most RGB gamuts, then
going A -> B -> A would end up with pretty much the same color (within
itty-bitty percents). It seems logical enough...but it's inaccurate.
Ultimately, it depends on a couple of different factors:
A. Profiles - are they being used, and are they good enough for the job?
B. Rendering Intents - Are you using a rendering intent that scales all
data, or just data out of gamut?
If your profiles aren't very good, then expect shifting, particularly in the
extreme parts of the gamut and in gray areas.
Any rendering intent that doesn't end in the word "colorimetric" is going to
move every color in your image somewhere else. In the case of Perceptual,
your colors will maintain some semblance of a visual relationship, and if
you use Saturation, your colors will maintain some semblance of
richness/vibrancy...but at the risk of alienating color relationships.
So, I did some playing and worked this together for you:
In Photoshop 6, I created a CMYK document using (since this is what I
support) the Canon ColorPASS Z5000 profile (the canned EFI profiles are
excellent profiles as long as your copier is calibrated). Inside this
document I created 7 color swatches (each value is C/M/Y/K:
1. 100/0/0/0 (Cyan)
2. 0/100/0/0 (Magenta)
3. 0/0/100/0 (Yellow)
4. 0/0/0/100 (Black)
5. 0/100/100/0 (Red)
6. 100/0/100/0 (Green)
7. 100/100/0/0 (Reflex Blue)
I converted this to DonRGB using a Relative Colorimetric intent (I know, I
know...Absolute Colorimetric would be better, but it's my legacy Photoshop
background) and then back to the Z5000 CMYK using Profile to Profile (no
BPC). I get the following CMYK at the end:
1. 100/11/3/0
2. 5/100/5/0
3. 1/1/100/0
4. 63/59/51/100
5. 3/100/100/0
6. 76/4/100/0
7. 100/100/4/0
Then, I went back to my original CMYK, and converted first to DonRGB and
then to Z5000 CMYK, but this time in Profile to Profile I used the
Perceptual Intent (again, no BPC). I get the following CMYK:
1. 79/27/1/0
2. 8/95/18/0
3. 2/4/70/0
4. 79/66/53/85
5. 14/95/94/0
6. 75/7/99/0
7. 98/95/18/9
As you can see, there's a big change using Perceptual rendering.
Scott Olswold
Senior Systems Support Engineer
MCSE, CNA 5, A+, Adobe Expert User (Photoshop and PageMaker)
Danka Office Imaging