Printing in RGBK from Photoshop 5

[Dan Culbertson <danculb@concentric.net>]

If you like doing your last few color corrections in CMYK so you can see
what you are going to get in terms of ink channels --or if you have an
obscure need to work in CMYK (like for doing quadtones the right way!) here
is a way to use StylusRIP without using StylusRIP (on the 3000, probably
other Epson printers as well).

After you have done your basic color corrections in RGB convert to CMYK
with the appropriate StylusRIP CMYK profile selected in CMYK setup (better
yet, make your own CMYK profile or set the CMYK ink colors assignments if
you know how to do that).  Be sure you convert a copy, not your original
RGB document!  Now you have a CMYK document that you can tweak (except that
StylusRIP doesn't really let you tweak the K channel but go ahead and do it
anyway).  After you are all set, convert first to Multichannel, then back
to RGB.  Photoshop 5 will convert your K channel to a spot channel.  The
RGB colors will look wrong but if you just go ahead and print through the
RGB driver (much faster than StylusRIP) with "No Color Correction" selected
the colors will print the way you had them in CMYK and the same as if you
had printed through StylusRIP from CMYK.  The K channel will be integrated
into the RGB channels by some miracle formula of the driver (or
Photoshop??) but you now have a lot more control over it than just working
in RGB.  100% K patches come out a bit weak but 100% CMY blacks come out as
full blacks.  Lets you put an adjustment curve on the K spot channel to get
a last minute tweak of the blacks and shadows.  But don't try to correct
the second generation RGB colors -- if you still need more color work go
back to the original RGB file (before you converted to CMYK) or you can
seriously degrade the file.

To see how this works, create a CMYK test patch file (one with 100%C,
100%Y, 100%M, and 100% blends of any two to get Blue, Red, and Green).
Convert to Multichannel then to RGB.  The C will convert to a 255G+255B
cyan, the Y to 255R+255G, etc. and the RG and B patches will convert to
pure 255 RG&B.  Then print through the RGB driver with the "No Color
Correction" option and you will see that pure color patches print (no stray
colors dithered in).  If you create a CMYK document with the Epson
StylusRIP profile selected in CMYK setup (or you have the printing inks
colors set correctly) the monitor will soft proof how you will print in
RGBK.  If you start with an RGB file, then convert to CMYK with the correct
profile, you can bypass all the RGB profile stuff and get a bit better
control in CMYK before you convert back to RGB for the final print.  Not
near as good as working with a true CMYK RIP but a *lot* faster and you
have a better black control than just working with a CMYK preview while in
RGB mode.  Between the CMYK preview and this final conversion to CMYK and
back, you can get a pretty good bit of control out of that old Epson RGB
driver!

Works especially well if you want to work in CMYK for direct control of
quadtone channels  or if you are using some really odd ink colors in your
Epson (I do odd things with inks you know) and if you know how to set the
ink colors in Photoshop.  A full CMYK RIP will give you much better control
of the total ink and black channel but this procedure does about everything
you can get from StylusRIP without the long processing time.  For
conventional RGB files  and conventional ink/paper combinations, staying in
RGB with a professionally made RGB profile may very well work better -
though you will have limited visualization until Photoshop XX comes up with
an RGB version of CMYK preview!

Dan Culbertson

PS -- When you convert to the RGBK file the K is a *spot* channel -- which
means you can not only lay a curve on it to adjust its contrast etc. but
you can also click on the channel and change its color from black to (say)
a nice deep blue!  Nice way to moderate those nasty dead black shadows you
sometimes get!  But you want to be delicate about it rather than garish
(although " garish" does have its fans!).




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