Of black point compensation
Of black point compensation
- Subject: Of black point compensation
- From: Henrik Holmegaard <email@hidden>
- Date: Tue, 26 Jun 2001 11:45:02 +0200
There is a lot of confusion over black point compensation, so here's
the logic of how it works:
a. the Perceptual intent is white point and black point relative,
b. the Relative Colorimetric intent is white point relative and black
point absolute,
c. the Absolute Colorimetric intent is white point and black point absolute.
It isn't more complex than that, really. Except when we look at actual gamuts:
A source RGB working space like Adobe RGB has a white point of L 100
and a black point of L 0. A production space like COMMPS3
(lightweight coated magazine stock for positive process) has a paper
white of L 89 and a maximum black of L 15. An HP DesignJet 5000 dye
ink with HP photo imaging stock has a white point of L 93 and a black
point of L 4. The measured white of the monitor is L 100 and the
black is L 4. Let's look at how it makes sense to match between these
lightness ranges.
In quality reproduction the danger has always been in the shadows.
This is where we need to watch out. To follow what I'm saying, create
an Lab wedge in Pshop 6 from L 100 a 0 b 0 to L 0 a 0 b 0 in 5%
steps, and use the Type tool to number the steps for easy reference.
The default approach is to go from RGB / Lab to production CMYK space
with Perceptual. This gives perfect shadow detail with a quality
print production profile (: PS6 > Color Settings > Advanced Mode >
Intent: Perceptual > Black Point Compensation on or off doesn't
matter, BPC doesn't apply to Perceptual). Now look at the wedge (:
PS6 > View menu > Proof Setup > Custom > Ink Black on). It's all
there, every detail.
The default approach is to go from production CMYK space to soft
proof space and proof print space with Relative Colorimetric using
the normal absolute black point matching. This is as it should be
because,
d. the monitor has a much deeper black than the production CMYK
space, so if we match the black point relatively, then the light
offset black appears ten steps darker on the monitor than it really
is (!), which is why it is not a good idea to soft proof with
relative colorimetric set to a relative black point (PS6 > View
menu > Proof Colors, the default in earlier versions of Pshop) but
using relative colorimetric with absolute black point matching (:
PS6 > View menu > Proof Setup > Custom > Ink Black),
e. the inkjet proofer black is much deeper black than the production
CMYK black, so if you match the black point relatively, there are
problems here, too (: this is a confusing aspect of the PS6 UI as in
the Print dialog the UI says it will carry over the settings in the
View menu, and we never want one of those settings, namely Proof
Setup without Ink Black enabled = Proof Colors = Relative
Colorimetric with relative black point rather than absolute black
point).
Then there is an argument that says, If the RGB working space is
small, ColorMatch RGB or Apple RGB for instance, and the actual image
colors aren't even outside the gamut of the offset CMYK space, then
why separate with Perceptual when Relative Colorimetric maintains
more of the image colors?
This is why Pshop 6 defauls to Relative Colorimetric with Black Point
Compensation for the conversion from RGB working space to offset CMYK
space. But the results are not as good as with Perceptual, using the
same quality profile. If Black Point Compensation is
Pshop 6 does not default to Relative Colorimetric with absolute black
point matching to the monitor, so this has to be manually set.
Should RelCol with BPC be used to separate? For a large gamut
presentation printer with deep deep blacks and bright paper, RelCol
is fine. For a magazine stock it is less fine, depending on the
image. Some images are evenly and well lit, with no areas of black
for pop and contrast. Some images have just one or two deep black
areas for pop, but no deep shadow detail otherwise. Some images
(portraits) have rich shadow detail. Relative Colorimetric with Black
Point Compensation doesn't preserve detail as well as Perceptual, and
Relative Colorimetric without Black Point Compensation should
definitely not be used. Also Relative Colorimetric with or without
BPC may not render skin tones as well as Perceptual, again depending
on the profile.
To my mind, the PS6 RelCol with BPC setting is not a helpful default.
I'd rather see Perceptual as default for RGB working space to offset
CMYK and Ink Black from offset CMYK to monitor RGB.
Of course, if you load an L 82 paper into the DesignJet with the
intention of proofing an L 96 glossy offset, or turn off the monitor
with the intention of soft proofing, then nobody can help -:).