Digital BW, The Print

Thank you very much for a very comprehensive response.  Some of it's a
little beyond me at the moment - I really must buy Real World Colour
Management.  I'll add it to the growing list!

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Steve Kale
<stevekale@b...> wrote:
>
> 
> 
> 
> > From: garethjolly <garethjolly@b...>
> > Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> > Date: Thu, 29 Dec 2005 20:55:43 -0000
> > To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> > Subject: Re: [Digital BW] Advanced B&W printing and ICC profiles
> > 
> > Can you talk me through that?
> > 
> > Logically, a colour profile must include white,
> 
> Yes
> 
> >shades of grey 
> 
> Not really.  Yes there is a greyscale axis in the 3D LUTs but this
is not
> necessarily generated with stimulus-response observations of greyscale
> patches.  Greyscale points are just the points in the 3D LUT where
R=G=B.
> Have a look at the 4096 patch test target on Bill Atkinson's site.
> 
> >and
> > black? 
> 
> Yes
> 
> >Or am I missing something?  I suppose I must be, because
> > otherwise you wouldn't bother with black and white profiles.
> 
> The B&W profiles we are talking about do not attempt to manage hue, only
> luminance.  So a greyscale image can be printed sepia with these
greyscale
> ICC profiles.  Hue management is left to the user and normally
selected via
> QTR/IJC tone "curves" or the Epson Adv B&W picker.  A colour profile
does
> manage hue and so will try to bend the printer so that the image
comes out
> "neutral".
> 
>  
> >Perhaps
> > the issue is the translation from grayscale using a colour profile
> > could introduce a slight colour shift?  In which case, couldn't a
> > conversion program from colour to B&W ICC remove this?
> 
> Even if you could strip out the greyscale axis you have no control
over how
> this greyscale is made up.  When you print greyscale with a colour
profile
> it looks up the greyscale axis in the profile (this axis is not separate
> from any of the other colours in the LUT) and prints accordingly. 
But as
> each point on the greyscale is just one point in an overall 3D
colour space
> what we typically find is that we get metamerism.  We would rather
control
> more tightly the inks used in greyscale generation.
> > 
> > How does a profile actually work?  I suppose I assume that a colour
> > profile is essentially a set of curves in RGB whereas a B&W profile is
> > a grayscale curve.  Is that right?
> 
> Depends.  Most colour profiles use lookup tables rather than curves.
 So you
> have a 3D array rather than 3 curves.  I recommend taking a look at
Bruce
> Fraser's Real World Colour Management.
> 
> The other thing to remember is that often the greyscale printing
driver is
> completely different from the colour driver.  When we select Adv B&W or
> Black Only we are no longer using the colour driver.  As a result, the
> stimulus-response behaviour used to make the colour profile is not
at all
> valid.
>