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[filmscanners] Re: CMYK rant (was Digital Darkroom Computer Builders?)
Andras writes:
> OK, what I actually meant is how many people
> use CMYK colour space when manipulating images
> in PhotoShop or so.
The entire publishing world. It is a key feature of Photoshop, and a
heavily used one, and one of the most important features of Photoshop to
professionals (which is why it is absent in "cheapo" versions of PS such as
Photoshop Elements).
> Yes, printers often have CMYK components, but that
> should be hidden from the user by the printer driver.
Most of what is printed on paper in the world doesn't pass through a printer
driver on a PC or Mac. Most of it passes through large or even huge offset
or other types of printing presses, and these presses understand only CMYK.
They use plates created from films that are exposed by raster-image
processors that understand only CMYK as well. Outside the tiny world of the
PC or Mac itself, all of the printed material in the on the planet exists in
a CMYK universe.
> For manipulating and storing images, the fourth
> channel "black" is totally redundant.
No, it is not. CMYK requires black because most inks on press are not pure
enough to produce a true black from CMY alone. And the balance between
black ink and CMY must be controlled for each set of circumstances, with
parameters like GCR and UCR. Therefore images must be manipulated in CMYK
in preparation for printing.
> Also, if CMYK is based on the printer's colour
> space, why CMYK and not CMYKcm or whatever extra
> colours there are?
Uh ... because printers use only four colors in standard process color:
cyan, magenta, yellow, and black. There aren't any other colors. (There
are six-color processes, but that is exceptional.)
> There is no such thing as a standard CMYK space ...
Yes, there are standard spaces: SWOP is one such space in the U.S.
> ... each output device has different characteristics.
True, and the only way to tweak for the output device with complete accuracy
is to convert to the CMYK space that it uses. RGB doesn't cut it.
> A CMYK image file has to be converted to the
> printer's CMYK space using ICC or similar
> transforms anyway ...
No, it does not. You prepare the CMYK file in Photoshop so that it already
conforms to the space used by the press, then you create your films and
plates from that and you go to press. Nothing is transformed after you
prepare the CMYK file in PS; that file IS the final image file.
> ... just like any RGB or other image.
CMYK isn't like RGB at all.
> On a computer, the native colour space of the
> monitor is RGB, which also resembles the spectral
> response of the three colour receptors in
> the eye.
Yes. But on paper, the color space is always CMYK. No printing process
uses RGB, since RGB requires a light source, and doesn't work by reflection.
> ... it is often sufficient to work on an image
> in the monitor's colour space, which makes life
> much easier.
Not for serious printing. You have to convert to CMYK if you plan to print
on paper and you want good results.
> Any RGB format can be directly displayed on any
> monitor (with small deviations maybe), unlike CMYK.
Fine, as long as you intend only to see it on a screen. But that won't due
for printing.
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