Digital BW, The Print

> From: Roy Harrington <roy@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Thu, 20 Oct 2005 23:07:46 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] ICC v. Transfer Function in Epson driver
> 
> Steve,
> 
> I think you kind of have this backwards.

No I think we are just talking across purposes.


>All your Converts are changing the
> file values.  

Correct

>By Converting you no longer have an even stepwedge.

Understood but then you are changing observations.  My point was that the
visual separation of an image doesn't change.  If I work up an image in Gray
Lab or GG2.2 or GG 1.8 and print it with conversion to the print profile
each image will look the same.  So visual separation evident in the image is
NOT dependent on workspace (unlike a Same as Source workflow).

Of course the colour represented by 90% grey in GG 2.2 is not the same
colour as 90% grey in GG 1.8 - that's what is meant by the fact that these
two colour spaces have different gamma.  90% grey in GG 2.2 is the same
colour as 94% grey in GG 1.8.  (It is L* 6 in both cases because it is the
same colour in both cases.)  90% grey in GG 1.8 is a lighter colour than 90%
grey in GG 2.2 - and it will have a different L*.  If you are looking at two
colours in an image (forget their corresponding workspace-dependent file
values for a moment) their separation in print is not altered by workspace
in a colour managed workflow.  So when thinking about what happens to your
image on conversion to a print profile don't be confused about these gamma
differences in workspaces.  In each case the file value sent to the printer
is adjusted by the appropriate amount to render the proper colour.

This is what I meant by an illusion of sample.  If we choose to sample 94%
grey then yes this colour and its "separation" from 100% K moves with
workspace - or more accurately with gamma.  This IS the definition of gamma.
But by so doing we are sampling different colours.  94% only has meaning
when attached to a profile.  When we print an image we do not see a bunch of
numbers on the page but a bunch of colours.  The separation of the print
space is determined by the print space NOT the workspace.

(To emphasise the point, get a step wedge and tag it first with GG 2.2 and
print it with the printer profile, then convert it to 1.8 and print it with
the printer profile.  The prints will look the same regardless of your
workspace.  In each case you are printing different file values and
different observation points on each greyscale but satisfy yourself that you
are printing the same colours.)

It's pedantic point but an important one if you are coming from a Same as
Source workflow where file values are printed without conversion with
respect to the print space.  In that case workspace does matter.  A lot of
people get confused by this stuff.

>What you 
> see
> with the K and Lab values are the actual values.  K is calculated by
> (255-gray)/2.55
> and L is the actual calculated Luminosity based on the grayscale profile.
> 
> BTW, if you are looking at RGB values throughout this exercise they will
> absolutely
> confuse the situation to no end.  You would be seeing another profile
> conversion to
> the working RGB space.  The numbers are fairly removed from what's in the
> data.
> 
> The one feature of PS that shows what's really there is the Histogram.  A
> stepwedge
> should have nice even combs there.
> 
> You need to Assign profiles to try the different grayspaces.

I would constrain this to:  you need to Assign a profile to see what colour
the same file number represents in the new space.

Assign keeps the numbers and changes the colour.

Convert changes the numbers and keeps the colour.

When we send a target to a printer for profiling we do so Without colour
management.  This is because we want to see how the printer reacts to
getting a particular number (which is of course all they react to other than
a good thump when they are clogged!).  We don't want this number changed at
any time by the colour management module.  Collect enough reactions to
numbers pairs and we can create a picture of how it will likely react to all
possible numbers.  With this profile, the colour management module does the
conversion of numbers sent (ie changes the numbers) so that the new numbers
represent the same colours on paper - subject of course to its policies with
respect to managing out-of-gamut colours.

Steve