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Of black point compensation
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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 -:).


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