Digital BW, The Print

I need to correct myself re my end points comment. PS allows a transfer
function that shifts the end points.

If you are thinking of linearising a printer or adjusting image luminance
via the transfer curve this ought to be possible.  The trick will be in
computing the relevant coordinates.  Think unadjusted image file (input) on
the y axis, trace out to the curve and then down to the adjusted file value
(output) on the x axis. The printer is then sent this new value which of
course is printed differently from the old value. (Confirm that a curve with
imperfect black and no black point compensation clips non-reproducable
blacks to the darkest black. This is obviously undesirable and hence you
would incorporate BPC into your calculations.)




> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Mon, 17 Oct 2005 19:56:59 +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
> 
> 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
> 
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