Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale <stevekale@b...> 
wrote:
>
> Hi Roy
> 
> You know this stuff much better than I do but as I understand it there are 4
> parts to any A2B0 (or other like tag):
> 
> 1. The identity matrix - a given for Lab PCS
> 2. The 1D input table (curve)
> 3. The multidimensional CLUT
> 4. The 1D output table (curve)
> 
> You've used 4. to generate the transform rather than 3. If PS normally goes
> looking into the CLUT for bpc generation etc then what will it find?
> Colorsync portrays the CLUT contents as perfect lab.
> 
> Thinking about it a little more, I guess PS would most likely pass data
> through the entire transform 1-4 but I am just speculating.  If that's the
> case, for a single channel situation it doesn't matter where in 2-4 the info
> is kept.  

Yes, this is the idea.  With multidimensional input and output there's a lot of
space saving by having multiple steps.  The CLUT is the one that takes up all 
the room.  Typically 25 points are used giving a 3in-3out size of 25*25*25*3*2 
bytes.  With only 25 points per dimension, linear interpolation is done for all
the midpoints.  The InCurves and OutCurves can afford all 256 values and these 
make the linear interpolation in the CLUT as accurate as possible.

So bottom line is the whole tag can be thought of as one transfer function that
is usually n-inputs to m-outputs but if our case only one input is used.  
The B2A0 goes from the file's L* value to the QTR driver's K value.  The A2B0
goes from a QTR driver K value to the L,a,b values of the soft proof.   
Using just the OutCurves fits the needs the best.  All the other pieces are identity
functions (pass the data through unmodified).


But I am puzzled though by a comment you made some time ago
> (probably offline) that PS bpc did not work with the new profiles and so you
> had to build it in.  It suggests PS is not finding the data it needs.  (Of
> course it all works in the end because you have done the bpc for PS.)

The difference is that in the TRC method there's only one curve which is setup
most like a A2B0 curve.  The CMS takes the one curve and calculates the two
directions so both directions come from the same data.  BPC could be used to
fix a curve that didn't go through (0,0).

With the A2B and B2A tags you specify exactly you want to happen in each
direction.  In B2A0 L=0 must naturally map to dMax or K=100.  In fact you
have to map all the L's into reasonable K's and this is the BPC-like scaling.
In the A2B0 K=100 maps into say L=16, and if you have Ink Black on it displays
as L=16 but the default for softproofs is for PS to do BPC and map it to L=0.
So PS is actually doing the BPC for the proof.

My comment in the past about "doing the BPC myself" was for the B2A0 tag.

> 
> 
> > From: Roy Harrington <roy@h...>
> 
> > 
> > 
> > Only the OutCurve is used since both directions only have one input channel.
> > You can have 3 sets of curves for the different intents but the "0" curve will
> > be used for all if the others don't exist.
> 
> Yes. My question is whether you use the output table or the CLUT.
> 
> >The softproof is based on the
> > A2B0 curve and although potentially
> >there could be 3 different soft-proof curves
> > there's no way to select which one is used for the reverse direction, nor can
> > you select BPC on/off for the reverse.  So you must decide where the black is
> > mapped before making the profile.
> 
> 
> I'm not sure I follow you here.  Presumably PS would normally look to the
> correct intent tag as per Bob Frost's earlier post.  That is there wouldn't
> be "3 different soft proof curves", per se, but rather PS would look to the
> relevant intent depending on the settings checked in the soft proof.

I'm pretty sure the Intent you are selecting in the soft-proof dialog is the
intent to be used in the forward direction not the reverse.  According to Bob's
post it's the PaperWhite which selects the different reverse intent, and that's 
for Absolute which doesn't have its own tag.

Roy

> (I don't see any other profiles using 2 or 4 above.)  We just have one intent
> so each time it looks it comes to the same place.  I guess I need to think
> this through a bit more. Hmmm.
> 
> Steve
>