Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Olivier" <odesmais@...> 
wrote:
>
> 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.)
> > 
> 
> Well, Steve the question as I can best formulate it would be : would 
> relcol simply scale L* from media white to BPCted black in a 
> linearmanner (the way you linearise the printer output e.g. evenly 
> spaced L* intervals) while the perceptual one would include a gamma-
> like formula to the restricted L* (L* being scaled only from media 
> white to BPCted black instead of 0-100)thus preserving human 
> perception and contrast.
> 
> Sorry I can't think in XYZ, and all the above probably sounds very 
> naive...
> 
> 
> Olivier
>

Olivier,

Both Steve and C. David have said a lot, maybe I can add a little.

XYZ is really pretty simple as far as B&W is concerned.  All the B&W info is
in the Y component.  It's called Luminance and it's basically a measure of the
total amount of light -- there's no "color" in it.   The measure is the real "physics"
measure of light.  I.e. you can think of it as "how many photons" or "how much
energy is reflected".   It's normalized to 0 to 1, pure black would be 0 i.e. nothing
is reflected, pure white would be 1 i.e. all the incident light would be reflected.
In real life you never get to either endpoint -- there is no pure black nor pure white.

Scaling calculations using Y (XYZ) is used thoughout all the ICC documents and
when you think of it, it makes a lot of sense.  Think of a print illuminated with a
specific light.  There's a number of photons reflected off various parts of the print
based on the densities.  If you reduce the illuminating light by half (.5 x photons)
all the reflections will also be reduced by half.  This is basically linear scaling
in Y units.  Print illumination ought to be invariant so linear scaling should be
invariant too.   This basically goes with the media-white-relative definitions in ICC
profiles i.e. media white is Y=1. 

Back to profiles, with B&W the given (I think) is that black has to map to dMax or
black ink and white to map to dMin or paper white.  Anything different will be worse.
Because of the above, scaling Y to paper white is almost a given.  What to do with
black however is not nearly as obvious.  In fact the ICC specs don't address it.
Adobe came up with a concept they have called Black Point Compensation.
Mathematically it turns out to be just the same linear Y scaling at the black end
that was already used at the white end.  Mathematics aside the issue is: does this
give good results?

When I first started QTR, "linearizing" from dMax to dMin was an obvious need.
But what is linear is not at all clear.   There's a function called L* which takes the
Y luminance above and makes it "perceptually" linear.  I.e L values match human
perception.   So for QTR, linear L-values are how all the linearization is done.
This might seem the best since we take the grayscale values and spread them out
with the best perceptual separation.  However in practice there are two issues.
First, on the file side there are grayscale embedded profiles that are all different
so it's impossible to match them all.  Second, even if you make a even L-value
embedded profile (that's the Gray Lab profile) when you look at the screen
versus the prints it just doesn't match that well.  This is mostly an issue with 
matte papers where dMax is not very dense.  The prints always looked lighter 
than the screen.

This is where trying to figure out mapping all the values between dMax and dMin
starts.   Implementing the BPC that Adobe defined turns out to match screen and
print a whole lot better than before.  And putting it all in an ICC profile solves
the issue (1) above.  It's probably not perfect since the blacks on the screen and 
paper are so different.

So this is where all the mapping originates.  I think the name "perceptual" seems
to fit the issue.  Because of color, ICC profiles have 3 intents but I don't think
there's any real difference as far as B&W, but coincidentally one of the intents 
has the name perceptual.  The intents sort of sneak into the issue in soft-proofing
because of the conversions into the screen profile.

Olivier, back to this question:
> Well, Steve the question as I can best formulate it would be : would 
> relcol simply scale L* from media white to BPCted black in a 
> linearmanner (the way you linearise the printer output e.g. evenly 
> spaced L* intervals) 
It would seem to be a reasonable idea, but that's basically how the QTR linearization
works and the screen to print match doesn't work as well.

Roy