Re: U.S. Web Coated (SWOP) v2 always produces blues that
Re: U.S. Web Coated (SWOP) v2 always produces blues that
- Subject: Re: U.S. Web Coated (SWOP) v2 always produces blues that
- From: Harold Boll <email@hidden>
- Date: Sat, 30 Jun 2007 09:51:24 -0700
In Colorsync-users Digest, Vol 4, Issue 237, Message 4
Andrew Rodney wrote:
> Thanks, I needed a reality check and some ammo to
> post to a list where at least one pundit basically said that
> "everyone agrees that the U.S. Web Coated (SWOP) v2 profile
> always produces a color shift with blues".
Some more color bullets for your colourful gun.
Using Matlab I did a comparative analysis of how each profile
(USSheetfedCoated.icc & USWebCoatedSWOP.icc) handles the sRGB
blue: Lab = [32.3 62.5 -107.4]. This color is quite far out
of gamut and so the final inking/color will provide some insight
on how gamut mapping was performed.
Putting sRGB blue through the Relative Colorimetric transform
(B2A1) yields the following inkings:
C M Y K
0.89 0.70 0.00 0.00 USSheetfedCoated
0.89 0.78 0.00 0.00 USWebCoatedSWOP
Note that Cyan is greater than Magenta. This strongly suggests
that in each case these inks when printed will appear as Blue
and not Purple.
The 'colors' (Lab) of the above inkings as determined by the
companion A2B1 transform are: (Hue in units of radians)
a* b* L* Hue Chroma
35.4 9.83 -40.1 4.95 41.3 USSheetfedCoated
39.5 10.4 -44.1 4.94 45.3 USWebCoatedSWOP
Note that the Hue angles of each color are identical. The
agreement is not coincidental. I can personally attest to
the fact that USsheetfed will print sRGB Blue as blue in
appearance when those inks hit paper under calibrated
conditions. The alignment of Hue values strongly suggests
that USWebCoatedSWOP will do so as well.
As I alluded to in my earlier email, doing Hue preserving gamut
mapping in the blue region of CIELAB causes blues to go purple.
To illustrate this point, I gamut mapped sRGB Blue along a line
of constant Hue for each profile, i.e. along line points toward
the neutral axis.
C M Y K
0.77 0.79 0.00 0.00 USSheetfedCoated
0.75 0.85 0.00 0.00 USWebCoatedSWOP
Note that for both profiles, doing Hue preserving gamut mapping
results in Magenta now being greater than Cyan which will
indeed print as purple.
The Adobe creators of these profiles no doubt understood the
weakness of CIELab in this regard and purposefully crafted
their gamut mapping to compensate for this effect.
USSheetfedCoated does the right thing and this analysis
strongly suggests that USWebCoatedSWOP is doing the right
thing as well.
Harold
TOSHIBA AMERICA BUSINESS SOLUTIONS 36 Grayson Rd, Winchester, MA
01890
Harold Boll | Color Scientist | 1.781.856.5174 |
email@hidden
" The sunlights differ, but there is only one darkness. "
Ursula K. Leguin
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