Digital BW, The Print

Steve, once again I have to cry uncle. I just can't follow the twists
and turns of your posts and how they relate as an answer to mine. We
have somewhat of a disconnect on the written page.
We pretty much know all of the below, certainly the cook book's been
around a very long time. I just don't know what you're getting at. All
I'm saying is that the two intents of interest, Perceptual and RelCol
w/BPC, behave differently here. The difference may not be entirely
consistant, but generally. Additionally, that difference makes some
sense to me, sorry.
The exact math is of no interest to me, I can't do anything about it.
These are the options given us, they both work, they are both useful.
I am happy, prints are beautiful, life is occassionally good.
By the way, I did not say RelCol/BPC was "confined" to the nether regions.
Good luck with this-
Tyler

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale
<stevekale@b...> wrote:
>
> 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@t...>
> > 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
> >
>