Digital BW, The Print

One other thing, a transfer curve function doesn't typically allow you to
alter the two end points.  So a transfer curve can simulate a BPC-embedded
ICC profile (which would technically be non-spec) but not a non-BPC ICC
profile.  A transfer curve can of course be any shape you want in between.
The ICC profile approach measures the ACTUAL stimulus-response behaviour of
the printer and does the scaling of white point as per the spec.  Hence the
shape of the curve is defined by the spec and the behaviour of the printer.
Hope that makes sense!


> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Mon, 17 Oct 2005 19:45:43 +0100
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Conversation: [Digital BW] ICC v. Transfer Function in Epson driver
> Subject: Re: [Digital BW] ICC v. Transfer Function in Epson driver
> 
> 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