Digital BW, The Print

It's simple: in every description I've read of the two (perceptual vs
relcol+bpc) including that which I quoted below there is no stated
difference with regard to the greyscale axis.  You claim, by virtue of
observation only, that there is a difference with regard to the greyscale
axis but can't adequately explain why.  You dismissed my suggestion that
possibly a profile builder would want, when given the freedom the perceptual
intent gives him/her to do things any way he wants, to tackle the issue of
mapping black point in a manner differently to Adobe's BPC algorithm simply
because you "don't think they've ever been the same nor should be".

I'd like to understand how and why.

The reason for this is QTR Create ICC currently deploys the relcol+bpc
approach (as best we understand it).  If a greyscale perceptual-intent
profile builder were, given freedom, to prefer a different approach (and,
yes, with different maths) then I'd like to understand the differences.  The
latter may well provide a better initial basis for a transform of B&W
content from file to print.

Steve


> From: Tyler Boley <tyler@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Wed, 02 Nov 2005 00:59:33 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] ICC Soft Proofing
> 
> 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
>