Colour management with inkjet printers

Mike wrote:
Hello,

I'm working my way through Dan Margulis's Professional Photoshop book, which
is heavily geared towards colour correction using curves. I have acheived
some excellent results using his techniques, but I am a little confused
about one area in particular and was hoping somebody here who's used the
book could help me.

I am able to acheive very pleasing colours working in CMYK and setting
correct black and white points, correct skin tones, correct neutral colours
etc, as per Dan's advice. However, my confusion arises when I want to print
a corrected image on my inkjet printer. As I understand it, an inkjet
printer needs an RGB image which it then converts to its own CMYK profile
for print. However, when I convert my image back to RGB for printing, there
is sometimes a subtle change in some of the colours on screen (nothing
dramatic, but enough for me to notice that some colours are not the same as
they were in CMYK).

Is this a limitation of using an inkjet (ie. the fact that inkjets need RGB
images)? Or have I failed to understand something? Also, would I be better
off saving important images in CMYK and taking them to a decent lab for
printing instead?

Thanks for any help.

Standard ink jet printer drivers and most RIPs going to even pro level
inkjet printers ie Epson pro series need an image in an RGB color
space. As has been said work in Photoshop in the color space you need.
Switching to CMYK greatly reduces your color gamut by switching back to
RGB you are just place an image with a CMYK gamut into an RGB color
space, no magic here once you have lost the color you can't get it
back. So from what I understand, the printer driver will try to conform
to the color space the imge is in and extrapolate the colors from the
color truncated image (CMYK to RGB) so you get some funny looking
colors. CMYK only makes sense when going to offset, and then the CMYK
the printers take is so device specific it is often better to let the
person doing the printing convert the file. When inkjet evolved from 4
color printers to the 6+ color printers now used, CMYK lost its
usefullness.

Tom

Thanks for all replies to this thread - they've been very useful.

Raphael Bustin wrote:
...
CMYK is 100% device-specific, and there is no unique
representation of any given color in CMYK space. So
basically, any conversion *out* of CMYK space is at
best an educated guess.
...

The fact that there is no unique representation of any given color in
CMYK means that any conversion *out* of CMYK space is strictly defined,
while conversions *to* CMYK space have to employ "educated guesses".

--
Best regards,
Andrey Tarasevich

On Wed, 19 Jul 2006 18:28:58 -0700, Andrey Tarasevich
wrote:

Raphael Bustin wrote:
...
CMYK is 100% device-specific, and there is no unique
representation of any given color in CMYK space. So
basically, any conversion *out* of CMYK space is at
best an educated guess.
...

The fact that there is no unique representation of any given color in
CMYK means that any conversion *out* of CMYK space is strictly defined,
while conversions *to* CMYK space have to employ "educated guesses".


I don't follow that logic at all. Any CMYK gamut is device-
specific. In likelihood, there will be clipping or compression
during the conversion to CMYK. Those tones are forever
lost and cannot be recovered in the conversion back to
CMYK.

There are other issues as well. Try sending a CMYK file
through any standard print driver. You'll get caca, because
of the implicit CMYK to RGB conversion that precedes
the final separation.

Unless you are printing through a real bona-fide CMYK
RIP, there's no point converting an image to CMYK.


rafe b
www.terrapinphoto.com