Digital BW, The Print

I don't want to get caught up in semantics but really when we profile a
printer with say an Eye-One we aren't calibrating. We don't change anything
the printer does, we don't tweak the printer control.  Rather we _measure_
how the printer renders pixel values and hence simply profile it.  Colorsync
takes this profile and does all the mapping from one profile to the next -
altering pixel values as necessary.  When we do change how the printer acts
for a given pixel value, telling it to put down x ink, I would say we _are_
calibrating it (rather than profiling it).  That is why I used the term
calibration.

With regard to linearity, I clearly misunderstood you when you said:

"I need all those relationships to be proportionally maintained,
absolutely, and that is what I think is generally accepted as
linearization, even though that moniker may be incorrect by strict
definition."

Apologies.

Call it output density adjustment or anything else, you are defining what
density the printer will print for a given pixel value.  Typically
"linearization" refers to the last step in the process whereby the raw
instruction set developed from partitioning is refined to meet a particular
curvature in the pixel value vs density graph.  Do we agree on this?  I
think we do and this is good.

(I would suggest that the term came from the fact that for sRGB, Adobe RGB,
Apple RGB, Gray Gamma 1.8 and Gray Gamma 2.2 are all linear when log(pixel
value) is plotted against density as is any function where y=x^g g being the
gamma value.  So linearization meant fine tuning the curvature of the
printer output so that log(pixel value) plotted against density is a
straight line.)

Now we come to the question of whether or not we achieve the results we
intend. This is made up of two parts:  (a) did we get an even curvature such
that if we plotted the log of each side we would get a straight line (at
least over the dynamic range of the printer) and (b) is this the straight
line we were after.

I would suggest that when someone says I want to linearize to LAB, for
example, what they intend to achieve is a curvature of the pixel vs density
plot that matches the curvature of LAB.  What this would mean is that the
printer would reflect a change in density for a change in pixel value that
matches LAB, how the eye sees and all that good stuff.  If it doesn't then
we have failed to meet our goal.  If you say "well who cares if it doesn't"
then I would say "why reference your target to anything then, let alone
something which denotes a sense of rigour.  Pick any old number or target -
what the hell you can always soft proof the file and fix it with an edit."

So do we meet our target of linearizing to LAB?  Pick any paper and
associated curve, print the step wedge, measure the printed densities and
plot them versus those defined by LAB.  I can tell you now that they will
not be the same, they will only overlap right up near dMin, the midpoint
will not be 18% reflectance and that the chart will be different for each
ink/paper combo.  EPSG is closer than EEM but neither are "right".  Neither
will bear any resemblance to the LAB curve except they are all curved.  Each
will exhibit a lesser or greater degree of error from our target.  This
error is unnecessary.

With the methodology I am proposing they would all overlap LAB for the range
of density values able to be rendered by the printer.  And I get an added
bonus if I choose LAB: any part of my image where L=50 will always print
Kodak 18% reflectance grey.


Cheers

Steve



> From: Tyler Boley <tyler@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Wed, 08 Dec 2004 18:02:40 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] Re: Tonal range and linearization
> 
> 
> 
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale
> <stevekale@b...> wrote:
>> 
>> One other perspective which ought to make you stop and think a
> little about
>> the current methodology.  Tyler has nicely espoused the current way
> of doing
>> things:  a desire for a nice linear (straight line) progression of
> density
>> from dMin to dMax across the full range of pixel values, 0-255 in 8
> bit, 0-1
>> normalised.  So, for EEM for example, this would mean 0 maps to a
> density of
>> 1.68 and 1 maps to a density of 0.04 and all other points in between
> sit on
>> the straight line between the two.  (If I am wrong on this please
> correct
>> me.)  In so doing you are defining (or calibrating) the colour space
> of the
>> printer....
> 
> Well, if I understand you, not exactly. I've never mentioned a
> straight line. I have mentioned calibration to a selectable standard,
> a concept about which we have been discussing proper nomenclature.
> Roy has selected LAB, IJM has selected user selectable gamma 2.2 or
> 1.8 (someone correct me if I'm wrong). StudioPrint lets one select any
> dot gain along a continuous line from 0 to 49%. I suppose 0% might
> bear some relation to "linear", but as you suggest isn't very useful.
> I wouldn't call this process having much to do with color spaces as we
> think of them, but output density adjustment. Though obviosly the
> process "defines" (measures) an uncalbrated output in order to
> calibrate it, I wouldn't say we are conciously doing that the way icc
> profiling does (accept those profiles made for soft proof, once
> linearized). Calibration would be a better term, and if using a RIP
> for color, profiling is then done after "calibration".
> So again, I suspect we have a linguistics issue.
> Also, possibly of interest-
> When working with Photoshop gray scale spaces, notice that gamma and
> dot gain are interchangeable. So I, for example, could just as easily
> work in a 2.2 gamma gray space, and use an equivalent dot gain as my
> target calibration in StudioPrint, and achieve the same thing as we
> are now doing with a 20%dg space and a 20%dg linearization. Very
> similar to Roy developing a LAB based gray space to work well with LAB
> calibrated output. We chose 20% because it's a prepress standard and
> commonly used.
> Have I missed your point?
> Tyler
> 
> 
> 
> 
> 
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