Digital BW, The Print

From Olivier:

"David,

The way I understand relative is that it somehow does a clipping of
out-gamut values without re-evaluating in-gamut values oppositely to
perceptual that compresses them all.

Extrapolating this to L*, while it still runs from media white to
media black (with BPC) in both case, L* would be re-scaled in
perceptive and not in relative ?

I know it doesn't add anything to anyone's workflow, I just can help
thinking about it without finding out and get obsessed...

As said, I'm new to this so thank for being patient...

Olivier"

This is why BPC fills a gap in the ICC spec.  The greyscale axis is scaled
for media white and then for ink black.  Without BPC, blacks would get
clipped. Out of gamut colours that do not sit on the greyscale axis are then
amended to the closest possible value.  So there is a "shrinking of the
gamut" as far as greyscale is concerned - the whole scale is compressed to
fit.  The question for the perceptual intent is whether Adobe BPC and simple
media white point scaling are the most visually optimal methods of shrinking
the gamut to fit?  (There then exists another much more complex question as
to whether this would remain constant for "greyscales" of varying hue -
warm, cool, sepia etc.)


> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Mon, 27 Feb 2006 16:09:03 +0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Conversation: [Digital BW] QTR Create ICC tranforms:  was Questions about
> Using Color Settings and Workspace Profiles for Grayscale Printing
> Subject: Re: [Digital BW] QTR Create ICC tranforms:  was Questions about Using
> Color Settings and Workspace Profiles for Grayscale Printing
> 
> More accurately, Roy and people like myself who were prodding him along
> weren't able to get particularly clear answers as to how the various intents
> - well relcol and perceptual in particular - OUGHT to be applied in the B&W
> case.  So Roy had to piecemeal together information from scattered sources.
> For example, some may recall the initial difficulty Roy had with clipping
> highlights.  We latter "discovered" all input data had to be scaled for
> media relativity, ie media white point, a transform in CIE-XYZ.  Roy found
> an article by Adobe which described their black point compensation at a very
> high level.  Again another scaling in XYZ space.  BPC plugs a gap in the ICC
> spec.  So then you've got (media) relative colormetric with BPC.  The
> perceptual intent allows a free hand to the profile engineer.  However,
> whenever we attempted to ask the question:  "how would you, given the
> freedom of the perceptual intent, apply such freedom to a simple B&W
> scenario?" we didn't get many, if any, answers.  Roy will correct me if I am
> wrong, but he has essentially applied relcol+bpc and slotted it in the
> perceptual tags.  It was slotted in the Perceptual tags because the
> intention from the beginning was to develop an appropriate perceptual
> transform.  In a later iteration that he mentioned in an earlier post, he
> filled out the other intent tags with the same transform because of some
> uncertainty as to how PS used these others tags when establishing soft
> proofs.  Perceptual intent formulae tend to be closely guarded secrets:  the
> secret sauce that distinguishes one profiling package from another.
> Nonetheless, since none attend to the needs of the B&W photographer (not
> using a colour workflow) we would greatly appreciate the expertise that
> perhaps someone like David could provide. I think it's fair to say that we
> are very happy with the transform currently embedded in a QTR ICC profile,
> particularly given that it's affect can be soft proofed, but would like to
> find out more about whether or not alternative transforms may be useful or
> more "scientifically correct".