Re: UCR vs GCR...or Neither - RAW CMYK?


samarsh@...
 

Dan writes:

I suspect that the answer probably depends on how many
printers use
expensive scanners and scan directly into CMYK. If so, they're
almost
certainly going with the manufacturer's default, which is
ordinarily a
light black, UCR style, but Scitex products will produce a darker
black
than Screen products which in turn produce a darker black
than anything
from Heidelberg/Linotype/Hell. If they're using Photoshop to
make the seps,
odds are that they're going with a heavier GCR, because most
printers don't
know much about the topic--the knowledge, if anywhere, is in
prepress, and
it's fairly rare there.
Dan, this is a good opening for bringing up a subject which has
been bugging me for a while.

I use one of those expensive Scitex scanners you mention - and I
would have to agree with your take on their default tables black
generation. I have not been able to make a custom black
generation curve in Photoshop that even comes close.

In part this is due to how hard the black generation curve is to
edit - and that Photoshop _only_ gives the option of UCR or GCR
separation methods.

To mimick the defualt scanner sep, I would need to make two
Photoshop conversions, one using CMY only and another to
make a custom black plate to plug into the CMY file!

The Scitex scanner in default CMYK tables settings scans into
CMYK with GCR and UCR turned OFF. You get 'raw' CMYK.

By evaluating the separation setup curves in the scanner for the
off setting - one can get a basic understanding of the process.

It seems that CMY are used throughout the image to describe
neutral tones. I guess this is similar to using GCR with no black
generation. This is the closest description to the CMY behaviour
that I can come up with, and is not a good example.

Black curve seems to start at around 40% and seems irrelevant
to the 'linear' neutral CMY curve (where cyan is run higher).

Shadow aimpoints are very high in all four colours, often
delivering TAC of 360%.

This is out of my 'desktop' experience and must be a throwback
to more traditional scanning or separation. This 'raw' CMYK is
very different to Photoshops UCR or GCR separations.

Basically - there seems to be no way to make the built in settings
of Photoshop behave in a similar way - since UCR or GCR is
always in use for a separation.

Now back to UCR or GCR.

The Scitex software does use the % term for GCR. There is no
mention of none, light, medium, heavy or maximum black
generation - as in Photoshop.

As a side note - my 'antique' copy of Macromedia xRes had a
similar approach.

The conversion from RGB to CMYK was 'raw'. After the file was in
CMYK, then the UCR/GCR command was run...you could even
apply this command to a CMYK file created in other software.

A dialog box was presented with the choice of UCR or GCR and
their related controls, including start point for the grey
component, black limit, TAC and percent of UCR or GCR (with
UCA as an option as well).

All this was applied as a post separation move - instead of
during separation...which seems weird to my Photoshop
experience, but it may have some uses. I have not kicked these
options around much - since xRes gathers actual and virtual
dust.

As for your comment on the knowledge being is pre press, and
that it is usually lacking - I have to agree.

Before reading your works, I used GCR med black generation as
a general rule - but did experiment with light black generation.

Now I use GCR light black or UCR most of the time, unless
more black can help (when doing Photoshop seps). This is very
much an individual image and output based decision - gone are
the days when one setting suited all (now that I know better). I
have not reached this level on the scanner yet.

But most users that I have seen use GCR Med and the other
defaults in Photoshops built in settings as a standard - if they
are not using a profile such as SWOP v2 or US Flatsheet v2 etc.

Rather than just use the Photoshop terminology, here's
what they have to say:

"Current recommendations suggest that a safe range of GCR
to use is between
30% and 60%. A 50% GCR setting removes 50% of the gray
component normally
produced by the chromatic color and compensates by adding
an equivalent
amount of black."

This naive description is roughly equivalent to me telling you
that I would
like to meet you at 2 p.m. today on the corner of First and Main,
without
telling you in what city. Any sane method of GCR will use a
relatively low
percentage of black in light greys and a relatively high one in
darker
greys. The SWOP definition is meaningless, worthless. And if
SWOP doesn't
know what it's talking about, how can we expect the individual
web printers
to?
Well, if using xRes then I guess I would enter 50% as the
percentage of UCR or GCR - with a start point of around 25%. It
seems that xRes does work this way - there are no light,
medium, heavy or max options.

The same for my Scitex separation method - if GCR is chosen
then you have no choice but to enter a percentage value for UCR
or GCR, and a black start point %, black limits, TAC etc.

Dot gain is never explicityly mentioned in the scanner software.

But since xRes is history, then the question is a lot harder for
Photoshop - which does not use these terms. Instead we have
UCR, or GCR - none (cmy) light, med, heavy and max black
generation.

But from what you are saying, knowing the amount of UCR or
GCR is pointless without knowing the black generation method,
as in the start point for black?

I take it that the black start point in more traditional separation
software is what Photoshops named black generation settings
do, or perhaps the custom black generation curve option?

I thought that the use of the term 50% GCR was an old drum
scanner or traditional separation terminology. Since I entered
pre press through typesetting - I just presumed that this was a
gap in my knowledge, that would have been covered if I originally
trained in repro instead of type.

Many printers still list this in their separation spec sheets -
instead of the more 'regular' or 'standard' Photoshop
terminology. But just like SWOP, they do not mention a start point
- only the amount of UCR or GCR.

As for the original post - I work for an Australian commercial
printer and we will take pretty much any separation you can throw
at us (for good coated stock). We use CTP and only have one
four colour flatsheet litho press - so things are pretty tight in
process control. Before working here, I would have considered
the seps we produce mud - but everyone seems happy...

I am glad that I am not the only one having problems
understanding all this...I thought it was just me. <g>

Regards,

Stephen Marsh.

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