Digital BW, The Print

I hear you but if you read up on the literature there is little detail on
why there's a difference.  In terms of goal definition they discuss the same
thing with respect to black point.  Adobe BPC, for example, which is
applicable to RelCol/BPC, scales the entire luminance with the matrix we
have discussed in other threads - it's not confined to an adjustment in just
the darker regions (at least according to my understanding of their white
paper).   Roy could perhaps comment here.

GM put out an interesting pdf called "Cook Book: Soft Proofing Settings,
Adobe Software".  It's not quite on point because it is specifically
discussing soft proofing and I need to read the whole thing again but here's
a couple of quotes:

[pg11] "Perceptual scales the white point and black point of the color space
you are converting from to the full lightness range of the color space you
are converting into.....

"Relative Colormetric scales the white point of the color space you are
converting from relatively to the maximum lightness of the color space you
are converting into, but scales the black point of the color space you are
converting from 1:1 absolutely to the shadow end of the lightness range in
the color space you are converting into."

So far so good - recol clips the blacks, perceptual doesn't....

[earlier on pg11] "Black point compensation is as of the time of writing a
conversion specific to Adobe application software, and not yet implemented
in all Adobe software. Black point compensation is an optional modification
of relative colormetric conversions, but because it is currently applied to
all steps from source space to destination space, it also affects the black
point of the proof.

In principle limited to scaling the L channel, bpc slides the black point of
the space you are converting from up or down to match the black point of the
color space you are converting into which avoids clipping of shadow details.
Because this may expand the simulated lightness range in the destination
space, you should disable it for soft proofing and print-proofing."

Not sure why they felt the need for the cautious "in principle" and I'm not
sure about the tale end of each of those paragraphs...

[pg25] "black point compensation, a modification of Relative Colormetric
conversion, makes the black point relative just like the white point,
matching the source black L end point to the simulation black L end point.
This conversion is an alternative to the Perceptual default, if you uncheck
Black Point Compensation before proofing."

All very confusing but the drift I get from this is that relcol with bpc
scales the white point to match and (in the L channel only) "slides" the
black point to match.  I have trouble seeing the difference between this and
the scaling of both white and black point done for perceptual.

Probably a question for the guys over on the Colorsync list.

Cheers

Steve


> From: Tyler Boley <tyler@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Tue, 01 Nov 2005 21:22:55 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] ICC Soft Proofing
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale
> <stevekale@b...> wrote:
> ...
>> We are really talking about the same thing.  A straight Relcol has
> no black
>> point mapping so Adobe introduced BPC to plug the gap.  Perceptual
> requires
>> black point mapping but how you do it is not specified.  In a colour
> world
>> there are very real differences between these two intents because of how
>> non-greyscale out-of-gamut colours are managed (ie shrink the entire
> gamut
>> to fit, perceptual, or render the next best in-gamut colour,
> relcol).  In a
>> greyscale world, though, there aren't any out-of-gamut colours remaining
>> once you have mapped the black and white points.  So if you were
> approaching
>> the issue purely from a greyscale world perspective then you'd be
> facing an
>> identical technical issue.
> 
> I don't think so, and that was the point of my first reply.
> The whole point of RelCol is to map in gamut colors as closely as
> possible to the LAB equivalent in the destination space. In gray,
> everything is IN GAMUT except the extremes, white point is taken care
> of, then everything down to media/ink dmax is in gamut.
> Therefore in the majority of the scale there is little compression,
> and Adobe then came up with some math to compress shadows
> porportionally more, to bring back shadow detail.
> 
> In fact, this is why one would select that rendering over perceptual
> if apropriate. It's how most of the profiles I have here work, from
> various creators.
> Put up a gray step wedge, do a perceptual vrs relcol conversion- in
> perceptual everything lightens, in RelCol it does not, but the blacks
> clip. Now with Relcol hit BPC, mid tones lighten little, but shadow
> detail returns.
> 
> That is how I would expect it to perform, based on the definitions of
> the intents and BPC, and that is indeed how it seems to perform here.
> Of course it's not 100% consistant because of the lack of relevant
> standards, but mostly.
> Tyler
>