Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale <stevekale@b...> 
wrote:
> Hi Roy
> 
> Ok so I have it up now - well done, just the L channel - cool!  That saves
> us a lot of storage space should we choose to work in LAB rather than Grey
> Gamma 2.2 and like I had said before LAB is hard for colour but easy for
> B&W.  
> 
> A couple of points:
> 
> 1.  I suspect most of us will still start either with a Gray Gamma 2.2 file
> (scanning B&W film) or an sRGB/Adobe RGB/scanner RGB file (shooting
> digitally or scanning colour film).  Once this is in grey scale we should
> CONVERT to LAB else all the tonal values will shift (where we had one shade
> of grey will now become another and if we exposed a part of the image to
> Kodak grey it will no longer be so).  Not a problem, of course, and better
> than converting to LAB because at least with this Grey LAB we strip off the
> a and b that aren't needed. Very very cool.

The new space does several different things.   For printing through QTR
there is no effect.  Remember in the Print with Preview we select Same
as Source for the print.   We still have a grayscale image so this means 
the exact pixel values are sent to QTR.
For display though -- what you see on the screen -- there's still a bunch
of color management going on.  The pixels go from the gray space (now
just L) to Lab to the RGB of the monitor.  Before they went from gamma 2.2
to Lab to RGB.   So the new way we see the difference between 95 and 100
on the screen as well as on the print.

The bottom line is that you should Assign to the new space NOT Convert to it.

> 
> (I am very intrigued as to how you did this by the way.)

With mirrors of course.  Actually I tried a few things to find out what
was in the icc file for a gray space and then matched it to the known formulas.  
Now I have to go back and have it all calculated.

> 
> 2.  Now that we have our image in LAB-lite we still have the issue of this
> space not being the same as the printer space.  For starters, an image using
> the full tonal range (such as an ordinary step wedge) has deeper blacks than
> we can print and hence we don't have WYSIWYG.)  So no "built-in" proofing
> :-(  See my last post to Keith in the other thread which I will reproduce
> here:
> 
> "Even if the printer RIP automatically spaces LAB values from
> dMin to dMax, ie 0 gets mapped to 16, 5 to 20 etc, you still end up with the
> same result: the mid-point shifts.
> 
> (By the way, take a look at this sequence of co-ordinates, and their density
> equivalents, and look how all print values are shifted.  Plot the step vs
> density figures of the two papers and overlay LAB.  Even if the RIP doesn't
> make this linear, as I believe was suggested, but curved in equal increments
> of LAB, each paper has a different gamma and non have the same value or
> curvature as LAB. Only as paper white moves closer to perfect white and ink
> dMax approaches perfect black do the curves begin to converge and have the
> same gamma.)"

You are right that absolute values will vary based on dMin, dMax, the kind
of display you use, the lighting etc...  I lean toward the everything relative
point of view though.   With color management there are the different
renderings and Perceptual is usually the one of choice.   This corresponds to
the compressing the gamut mode.  The Absolute Colormetric is more like
your view but seems rarely used.

Roy


> 
> Cheers
> 
> Steve
> 
> > From: Roy Harrington <roy@h...>
> 
> > 
> >  Alternatively you can use it
> > as a
> > proofing space.   You can also Assign the profile to your grayscale files --
> > they
> > will inherently display just as they print with QTR -- builtin proofing!
> > 
> > Roy
> >