Digital BW, The Print

The ICC profiles are of a very specific file type and description as
outlined by the International Colour Consortium.  You can get the
specification from www.color.org.  Therefore, subject to spec version
changes, there is just one type of file and ICC profile capable applications
can use any profile that meets the spec for which they are compliant.

As far as greyscale is concerned, an ICC profile is sort of similar to what
I believe a transfer curve is in the sense that typically a greyscale ICC
profile comprises the mandatory header information, the paper white point
(wtpt tag) and a tag called kTRC.  kTRC is a curve describing the stimulus
response behaviour of the printer/ink/driver/paper combination.  The data is
scaled in XYZ to the media white point, ie it is media relative.  Therefore
the curve runs from the very top right to some point on the left y axis
equivalent to the black point of the output as scaled for white point.  If
you build into the ICC profile kTRC tag black point compensation then the
curve will hit 0,0.  Therefore one can, via a transfer curve, cause the same
sort of luminance adjustment that an ICC profile can achieve.  You just need
to know the points for the transfer curve.  There exists at the ICC site a
white paper by Adobe on how they do their BPC (which fills a gap in the ICC
spec).  In essence, it is a scaling of the stimulus-response behaviour of
the black end of the curve in XYZ similar to the media relative white point
scaling.

Therefore as I understand it, one could theoretically achieve the same thing
with either approach.  However, the transfer function in PS (and other
apps), if I recall correctly, limits the number of points.

The old QTR Create ICC app essentially created a kTRC (k for greyscale) from
the input data.  WTPT scaling was done and, depending on the version of QTR
Create ICC, BPC scaling was done.  So you can go into one of these profiles
and take a look at the kTRC tag and see the curve I am talking about.  For
colour ICC profiles one could take the curve approach and have an rTRC,
bTRC, and gTRC.  In general terms, though, the TRC (tonal response curve)
approach is deemed insufficient for most devices and lookup tables are
preferred, specifically A to B and B to A tables. There is a pair for each
rendering intent.  One for PCS (Profile Connection Space - currently can
only be XYZ or CIELab) to device conversion and one for device to PCS
conversion.  The new/current QTR Create ICC app takes advantage of this and
uses A2B0 and B2A0 (perceptual) tags.  This allows Roy to have colour
information one way - for soft proofing - but not the other way - for
controlling the luminance output of the printer.  (The only issue with the
current version of QTR Create ICC is that Roy is scaling the soft proofing
data for luminance only and not all three elements of the wtpt.  Hence paper
white isn't yet shown as monitor white without Simulate Paper White checked
in a PS soft proof and when Simulate Paper White is checked there is a hue
imperfection.)

Hope this helps

Steve

> From: Paul Roark <paul.roark@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Mon, 17 Oct 2005 10:14:39 -0700
> To: DigitalB&WPrint <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] ICC v. Transfer Function in Epson driver
> 
> Here's a question I've been curious about:
> 
>  
> 
> Photoshop's "Print with Preview" has a section just below the preview where
> one can switch between "Color Management" and "Output."
> 
>  
> 
> "Color Management" has an option to "Let Photoshop Determine Colors."  At
> that point you can enter a number of "Printer Profiles."  These appear to be
> ICCs, about which I have very limited expertise.
> 
>  
> 
> "Output" has an option  -- "Transfer" - that allows one to incorporate
> "Transfer Functions" into the normal PS printing workflow.  At this point a
> curve made in Photoshop Curves Adjustment section will also work.
> 
>  
> 
> How do these 2 parallel print control workflows - curves v. ICC - compare
> when used in the PS "Print with Preview" workflow approach?
> 
>  
> 
> What are the curves and ICC making and linearizing options?  Are ICCs a
> uniform file format or are there multiple file types such that an "ICC" made
> in one application is useless in another application?
> 
>  
> 
> Does one approach preserve more of the grayscale steps than the other?
> 
>  
> 
> I'd appreciate learning how these 2 approaches compare and might be used for
> easy fine tuning or "linearization" of the printer ramp.
> 
>  
> 
> Paul
> 
> www.PaulRoark.com <http://www.paulroark.com/>
> 
>  
>