[Image-SIG] Convert to CMYK

[Gary Speer]

Hans & Kevin -
As a user that does a lot of design for print, it does not make sense to me
to build a complex converter to CMYK.  Just about every printer driver has
an optimized CMYK GCR/UCR converter that closely matches the printer's
characteristics when you send it an RGB print job.  A PIL based generic
converter to a light (under 20%) GCR will always look worse than the printer
driver's formula optimized to the specific printer's characteristics.  If
the converter matching the printer is not adequate, then no amount of PIL
tweaking will come close to adjusting on the client side using something
like Photoshop.  There is no real on-screen way to produce a browser app
that will display the results of the separation curve since it will always
go back to the same RGB values on the browser screen.  Unless I'm missing
something, I just don't see the value in converting server-side.

Kevin - I would hate to see you waste time on an optimized converter without
a purpose.  All I can think of as a benefit is a method of increasing
contrast with on-line tools without throwing in a color shift.  If so,
switch to LAB and expand the luminosity histogram - a much easier and
reliable method than doing so in RGB or CMYK.  Now, color enhancing using
colorspace shifting filter curves - that could be cool!

Hans - If you can expand on the purpose, perhaps we can suggest a better
technique.  Otherwise, the RGB version of the JPEG will always look better
on your CMYK printer because of the printer-specific optimized driver is
doing the grey color replacement optimized to avoid over-inking.and produce
even color replacement based largely on the paper selection and its total
ink limits.
Gary
.
----- Original Message -----
From: "Kevin@Cazabon.com" <kevin@cazabon.com>
To: "Hans Svedåker" <hans.svedaker@bytbil.com>; "image-sig"
<image-sig@python.org>
Sent: Friday, July 05, 2002 7:58 PM
Subject: Re: [Image-SIG] Convert to CMYK


> The built-in RGB/CMYK conversion in PIL is not an optomized one... there's
> no GCR/UCR, it's basically a RGB/CMY flop.
>
> Formulas for UCR/GCR are fairly easily available, maybe we should look at
> implementing a tunable conversion method in the core of PIL?  I've
mentioned
> this before, I just haven't had the time to do anything myself yet...
>
> Kevin.
>
>
> ----- Original Message -----
> From: "Hans Svedåker" <hans.svedaker@bytbil.com>
> To: "image-sig" <image-sig@python.org>
> Sent: Thursday, July 04, 2002 1:43 AM
> Subject: [Image-SIG] Convert to CMYK
>
>
> > Hi
> >
> > I'm trying to convert JPG image to CMYK format. And it works fine,
> > except for one thing. When there is black (or gray) in the image it is
> > converted to (255,255,255,0) instead of (0,0,0,255). There are never any
> >   black in the image, insted i blends max cyan, magenta and yellow to
> > get black. How shuld i do to get that?
> >
> > /Hans Svedåker
> >
> >
> >
> > _______________________________________________
> > Image-SIG maillist  -  Image-SIG@python.org
> > http://mail.python.org/mailman/listinfo/image-sig
> >
>
>
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>