Friday, August 14, 2009

Black is black (II)

The second black thing has nothing to do with black point compensation, but is still black-fashioned. That are the black preserving intents. No, this does not belong to normal ICC workflow. ICC has tried to address such need but still there is nothing in the spec.

Let's see what the issue is. Suppose we work in press. Press are very tied to standards, US press uses SWOP and European folks are more toward FOGRA. Japanese people uses other standards like TOYO, for example. Each standard is very well detailed and presses are setup to faithfully emulate any of these standards.

Ok, let's imagine you got an image ad, looking like that. This is a very usual flier, now just imagine instead of getting it in PDF, you get it as a raster file. Say in a CMYK TIFF, ready for a SWOP press. And you want to print in on a FOGRA27!!

Maybe you see no issue over here, but take a look on this:

LittleCMS ColorSpace conversion calculator - 4.0 [LittleCMS 2.00]

Enter values, 'q' to quit
C (0..100)? 0
M (0..100)? 0
Y (0..100)? 0
K (0..100)? 50

C=48.21 M=38.71 Y=34.53 K=1.09

That means, if I convert from SWOP to FOGRA27, the ICC profiles totally mess up the K channel, so a portion of the picture that originally is using only black ink, after the conversion, gets Cyan, Magenta, Yellow and well, a little bit of black as well. Now please realize what happens on all the text in the Flier.

Ugly. I guess the vendor who pays the bill will not be very pleased, right?

So I've added two different modes to deal with that: Black-ink-only preservation and black-plane preservation. The first is simple and effective: do all the colorimetric transforms but keep only K (preserving L*) where the source image is only black. The second mode is fair more complex ans tries to preserve the WHOLE K plane. I'm still checking this latter. If you want to give it a try, please download the new snapshot and build the TIFFICC utility. It already implements those new intents.

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