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Re: CMYK-RGB conversion (was: Epson or HP for alt-phot



Mitch Valburg wrote:
>   If you print using a CMYK RIP (EIEIO?) then you have full control
> over the four ink channels, bypassing the printer driver.  This is also the
> only way to directly or precisely control the amount of black ink laid down,
> since even CMYK files sent directly from Photoshop are converted by the printer
> driver back into RGB before printing.


For those for whom the implications of the above are not immediately
obvious, I'd just underscore Mitch's comment by adding that converting a
CMYK file back to RGB is not a benign act; in this conversion
information is almost always lost from the file. This is why you should
send an RGB file rather than a CMYK file to most inkjet printers unless
you have a CMYK RIP installed, because as Mitch said, otherwise the file
will be converted back to RGB and then to the printer driver's CMYK
space. Since information is lost in the CMYK-RGB conversion, the
resulting CMYK file will be different from the original CMYK file, even
aside from the added complication of the possible differences between
the Photoshop CMYK space and the printer driver CMYK space. Staying
within Photoshop for sake of simplification, if you start with an RGB
file, then convert that to CMYK and later back to RGB, the resulting RGB
file will be different from your original RGB file, because information
from the original file will have been lost in the back-conversion. And
by extension, if that corrupted RGB file is then re-converted to CMYK,
it won't be the same as the earlier CMYK file. This is why there is a
feature in Photoshop that allows you to see what an RGB file would look
like in the CMYK space you have specified, without actually making the
conversion. 
Katharine Thayer