Computer Graphics with Photoshop - transforming images rgb to cmyk

Author

transforming images rgb to cmyk

indide_designs

2005-05-22, 7:14 pm

I want to know how can i transform rgb images to cmyk without loosing
original colors in te printing process
thanks

Tacit

2005-05-22, 7:14 pm

In article <42908a9b$0$12857$a729d347@news.telepac.pt>,
"indide_designs" <rica_rui@sapo.pt> wrote:

> I want to know how can i transform rgb images to cmyk without loosing
> original colors in te printing process

Impossible. The laws of physics forbid it. Certain RGB colors can not be
reproduced in the CMYK color space, period.

Sorry...

--
Art, photography, shareware, polyamory, literature, kink:
all at http://www.xeromag.com/franklin.html

Mike Russell

2005-05-22, 7:14 pm

indide_designs wrote:
> I want to know how can i transform rgb images to cmyk without loosing
> original colors in te printing process
> thanks

Tacit is right, as usual, in his comment that exact conversion from RGB to
CMYK is not possible. But as you might guess from the fact that printing
works, the situation is far from hopeless.

Certain very saturated colors - pure reds and blues for example - lose some
of their saturation when you convert from RGB to CMYK. This is a limitation
of the inks that are used in printing, and for that matter similar
limitations exist for chemical based photographic printing processes. So we
live with it.

Considerable preparation is needed before you can create CMYK images
reasonably well, targetting them for particular press conditions as well as
compensating for the relatively minor color shifts that will inevitably
occur. There is a world of misinformation out there (none of it from Tacit,
BTW :-) and I would recommend that you spend a week reading one of Dan
Margulis's books to get a handle on the issues, then work with your printer
to create CMYK images that meet the necessary total ink limit and dot gain
specifications.
---
Mike Russell
www.curvemeister.com

Hecate

2005-05-22, 11:14 pm

On Sun, 22 May 2005 14:35:20 +0100, "indide_designs"
<rica_rui@sapo.pt> wrote:

>I want to know how can i transform rgb images to cmyk without loosing
>original colors in te printing process
>thanks
>
You can't. It's a different colour space. All you can do is get
nearest approximations.

--

Hecate - The Real One
Hecate@newsguy.com
Fashion: Buying things you don't need, with money
you don't have, to impress people you don't like...

hearsay@att.net

2005-05-23, 7:16 pm

Hecate wrote:
>
> On Sun, 22 May 2005 14:35:20 +0100, "indide_designs"
> <rica_rui@sapo.pt> wrote:
>
> You can't. It's a different colour space. All you can do is get
> nearest approximations.

I recall reading somewhere that converting to/from LAB is "lossless". If
that is true, will RGB to LAB to CMYK work?

Bill Hilton

2005-05-23, 7:16 pm

>I recall reading somewhere that converting to/from LAB is "lossless".

Draw a gradient in RGB mode and look at the histogram, convert to LAB
and back to RGB and look at the histogram ... the "fuzz" is gone,
telling you it's not completely lossless, though it's certainly good
enough for most applications.

>If that is true, will RGB to LAB to CMYK work?

Actually converting from RGB to CMYK already goes thru LAB in
Photoshop, so the answer is "no".

Hecate

2005-05-23, 7:16 pm

On Mon, 23 May 2005 13:07:08 GMT, hearsay@att.net wrote:

>
>
>Hecate wrote:
>
>I recall reading somewhere that converting to/from LAB is "lossless". If
>that is true, will RGB to LAB to CMYK work?

No it's not and no.

--

Hecate - The Real One
Hecate@newsguy.com
Fashion: Buying things you don't need, with money
you don't have, to impress people you don't like...

tacit

2005-05-24, 4:14 am

In article <4291D485.A94B9C1E@att.net>, hearsay@att.net wrote:

> I recall reading somewhere that converting to/from LAB is "lossless". If
> that is true, will RGB to LAB to CMYK work?

Converting TO Lab is lossless. Converting FROM Lab is not. When you
convert a Lab image to CMYK, the same thing happens that happens when
you convert RGB to CMYK--some of the colors are imnpossible to reproduce
in CMYK, and those colors change.

Certain colors can not be reproduced in CMYK, period. The laws of
physics forbid it. There is no workaround; it does not matter how you do
the conversion; it does not matter what modes you go through first. You
simply can't get certain colors in CMYK, and no fiddling in Photoshop
(or any other program) can change that. That's one of the fundamental
rules of life you learn when dealing with print.

However, it's not that bad. The colors look flat when compared to the
RGB original; but when you see them on their own, when you make a
printout or see them on press, they don't look bad at all.

--
Art, photography, shareware, polyamory, literature, kink:
all at http://www.xeromag.com/franklin.html

hearsay@att.net

2005-05-24, 7:17 pm

tacit wrote:
>
> In article <4291D485.A94B9C1E@att.net>, hearsay@att.net wrote:
>
>
> Converting TO Lab is lossless. Converting FROM Lab is not. When you
> convert a Lab image to CMYK, the same thing happens that happens when
> you convert RGB to CMYK--some of the colors are imnpossible to reproduce
> in CMYK, and those colors change.

Thanks for the clarification.

> Certain colors can not be reproduced in CMYK, period. The laws of
> physics forbid it. There is no workaround; it does not matter how you do
> the conversion; it does not matter what modes you go through first. You
> simply can't get certain colors in CMYK, and no fiddling in Photoshop
> (or any other program) can change that. That's one of the fundamental
> rules of life you learn when dealing with print.
>
> However, it's not that bad. The colors look flat when compared to the
> RGB original; but when you see them on their own, when you make a
> printout or see them on press, they don't look bad at all.

By "Certain colors can not be reproduced in CMYK", I assume you meant
out of gamut colors between color spaces. But some colors on a monitor
that are not flagged as oog in PS still won't show up in Epson inkjet
prints, after all the fuzz of monitor calibration and custom printer
profiles. Is the PS oog detection faulty? Aside from oog detection, are
there other ways to find out what colors won't print?