Digital BW, The Print

> From: garethjolly <garethjolly@...>
> 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.