Digital BW, The Print

Wrong forum.  Try this one:
http://groups.yahoo.com/group/digital-fineart/

On Oct 18, 2005, at 3:33 PM, Gary Brown wrote:

> Yikes!
>
> Where did the art go?
>
> Gary
> ----- Original Message -----
> From: "Steve Kale" <stevekale@...>
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Sent: Tuesday, October 18, 2005 2:36 AM
> Subject: Re: [Digital BW] ICC v. Transfer Function in Epson driver
>
>
> Paul et al
>
> I thought it might be useful to talk about some of the pluses and  
> minuses
> (in relation to this discussion) of transfer curves vs custom dot  
> gain ICC
> profiles vs PS curves vs QTR Create ICC profiles.  When Roy first  
> delved
> into this following a long discussion we had where I was saying that
> linearising to L* was not enough I immediately saw the potential of  
> the
> approach. Having made two "generic" ICC profiles, Roy then put it  
> off to one
> side for a while.  I wanted to take it further and be able to  
> create ICC
> profiles from actual rather than hypothetical data (as the generics  
> did).  I
> don't have Roy's programming skills so I was looking for another  
> method of
> creating them.  I had a bunch of conversations with Phil Green -  
> yes the
> "ask Phil" guy you see on www.color.org.  He kept saying "I don't  
> see why
> you don't just use a PS curve to put the right transform in  
> place".  Bruce
> Fraser alerted me to the fact that PS can generate an ICC profile  
> from a
> custom dot gain.  So I've been around the various options here but  
> only as
> my understanding of the topic was growing. Thankfully Roy became  
> interested
> in the subject again and really produced a great product.
>
> "Custom Dot Gain ICC Profile Generation"
>
> A very cool part of PS is that it can create a simple greyscale ICC  
> profile
> from Custom Dot Gain information.  This profile can (and is  
> intended to)
> then be used for printing.
>
> I struggled with this initially because I thought I needed to be  
> able to
> move the end points in order to reflect paper white and ink black.   
> Later I
> understood that the inputs into an ICC profile are media relative  
> and so I
> didn't need to be able to move the white point.  But you do still  
> need to
> get the white point info into the profile.  The inability to move  
> the black
> point (bottom left) is a problem. In order to do a soft proof of  
> ink black
> properly you can't give PS a profile that indicates that the  
> printer can
> achieve perfect black.  Checking Simulate Ink Black will merely  
> leave the
> deep blacks at monitor black and you miss one of the key points of  
> soft
> proofing B&W.  So that's a big negative for using the Custom Dot Gain
> approach.
>
> Of course the other issue is you still need to know the stimulus- 
> response
> behaviour of the printer (ie to be able to measure the luminance of  
> your
> step wedge) and then do the scaling calculations for media relativity
> (adjusting for white point) and also BPC (because the inability to  
> shift the
> black point forces you to "embed" BPC).
>
> The last issue is of course the limited number of observations that  
> a Custom
> Dot Gain can accommodate.
>
> "PS Curves"
>
> PS Curves are useful in two ways.  You can create a curve to use as  
> a soft
> proof of the printer behaviour (eg reduced black and dull white).   
> You can
> also create a curve to modify the luminance of the file data so  
> that it is
> scaled for paper white (media relativity) and ink black (BPC).  But  
> the two
> curves are very different and you don't want the soft proof curve  
> to still
> be there when you send the file to print.  Also you don't want the  
> scaling
> curve to be left in the file because it is only relevant for a  
> particular
> media/printer setup. It's all a bit messy. Furthermore, you still  
> need to be
> able to do all the measuring and scaling calculations to get the  
> curve(s)
> right.  So the complexity of the task has not changed except for  
> the fact
> that once you have the calculations done you do have a mechanism  
> for putting
> them into effect - better than nothing.
>
> "Transfer Curves"
>
> A transfer curve is simply the second of the two curves I mentioned  
> above.
> It has the one great advantage of not being embedded in the file.   
> It is a
> print only curve.  But you can't soft proof with a transfer curve.   
> In order
> to soft proof even just luminance, you need to create a soft proof  
> curve as
> mentioned above.  So net-net Transfer Curves don't get you much  
> further than
> PS curves.
>
> "ICC Profiles"
>
> The cool thing about ICC output profiles is that they don't need to
> "embedded" in the file by a conversion (ie a change of data info).   
> They can
> be invoked only at printing (an on-the-fly conversion) and soft  
> proofing.
> Done properly they can provide a soft proof for ink black (ie no  
> embedded
> BPC).  [I looked at Colorshop X's Greyscalebuilder utility but it  
> is clumsy
> and can't be used to automate the input of actual data.  (It did,  
> however,
> greatly help in my understanding of media relativity - at first I  
> couldn't
> understand why when I changed the media white point the curve did  
> fall from
> the top right corner!)]
>
> I thought the programming of an ICC builder that read actual
> stimulus-response behaviour into a kTRC (grey tonal response curve)  
> to be
> daunting enough.  But having got this under control Roy quickly  
> turned his
> attention to the alternative method of using A2B0/B2A0 lookup  
> tables.  When
> Roy mentioned to me that he was getting to grips with how these tables
> worked I said that would be awesome because he could record colour
> information for the soft proof direction and still leave the printer
> direction managing luminance only.  What we have now is a first  
> draft of
> exactly that and is an awesome achievement.
>
> (One has to remember that it is very difficult to ask colour engineers
> questions about this stuff.  They think and breathe colour and  
> getting them
> to help in the constrained B&W world can be difficult.  There is  
> also a lot
> of info at the ICC site but it's a lot of "this is" and not a lot of
> "because of this" - ie a lot of factual stuff but little "why".   
> Roy has
> done a tremendous job filtering through all this stuff for our  
> benefit.)
>
> Linearising vs Profiling
>
> In theory a more linear printer means profiling is easier.  This is  
> because
> it is impractical for the profile to have every possible  
> observation in it
> (although it is much more practical for greyscale than colour) and  
> so it
> needs to interpolate between observations.  Linearity makes  
> interpolation
> easy.  But get enough observations in the sample and linearity  
> becomes, I
> think, less important.  I suspect, although I am not knowledgeable  
> enough on
> this stuff to be definitive, that for B&W so long as the printer has a
> semblance of linearity then 51 observations is more than enough for  
> sensible
> output and linearising the printer becomes less relevant.  (I think  
> the
> whole Epson Colorbase trip we all went on a couple of weeks back  
> was a wild
> goose chase.)  My bet is that 51 observations would be enough to  
> manage even
> the crude Black Only luminance scale for example.  Only testing  
> will tell.
> Remember 51 observations (even 21) is a huge number of observations  
> for a
> single axis - imagine doing that number for each combination of 3  
> axes R, G
> and B.
>
> Now to practicalities.
>
> One can pay the QTR shareware fee and use QTR Create ICC even if  
> you don't
> have an expensive spectrophotometer.  You can simply plug the hue data
> (Lab's a* and b*) with zeros.  You will need a densitometer that  
> either
> outputs L* or XYZ_Y and then you can convert that to L* - ie you  
> need to be
> able to measure luminance.  But in the grand scheme of things these  
> are
> relatively cheap.  You will not be able to soft proof the hue of  
> your output
> but I think this is a secondary concern.  What I like about the QTR  
> Create
> ICC profiles the most is that my print workflow is now really very
> automated.  I know that the profile will scale the image luminance  
> properly
> for my print media and simply need to focus on getting the image  
> looking
> right on screen.  The first soft proof really is the print "at size".
>
> Even if one has to spot read the 21 or 51 patches this isn't so bad.
> Getting the data into the right format is easy once you understand  
> what
> formats are allowed.  A simple text editor can do this.
>
> Of course if you do have a spectrophotometer - and it doesn't have  
> to be an
> EyeOne - then you can include the a* and b* information and get a  
> soft proof
> of hue as well as luminance.
>
> I hope this helps.
>
> Steve
>