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transforming images rgb to cmyk

 
 
indide_designs
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      05-22-2005, 01:35 PM
I want to know how can i transform rgb images to cmyk without loosing
original colors in te printing process
thanks


 
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Tacit
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      05-22-2005, 03:05 PM
In article <42908a9b$0$12857$(E-Mail Removed)>,
"indide_designs" <(E-Mail Removed)> 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
 
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Mike Russell
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      05-22-2005, 10:04 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


 
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Hecate
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      05-22-2005, 10:30 PM
On Sun, 22 May 2005 14:35:20 +0100, "indide_designs"
<(E-Mail Removed)> 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
(E-Mail Removed)
Fashion: Buying things you don't need, with money
you don't have, to impress people you don't like...
 
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hearsay@att.net
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      05-23-2005, 01:07 PM


Hecate wrote:
>
> On Sun, 22 May 2005 14:35:20 +0100, "indide_designs"
> <(E-Mail Removed)> 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.


I recall reading somewhere that converting to/from LAB is "lossless". If
that is true, will RGB to LAB to CMYK work?
 
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Bill Hilton
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      05-23-2005, 01:41 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".

 
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Hecate
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      05-23-2005, 09:50 PM
On Mon, 23 May 2005 13:07:08 GMT, (E-Mail Removed) wrote:

>
>
>Hecate wrote:
>>
>> On Sun, 22 May 2005 14:35:20 +0100, "indide_designs"
>> <(E-Mail Removed)> 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.

>
>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
(E-Mail Removed)
Fashion: Buying things you don't need, with money
you don't have, to impress people you don't like...
 
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tacit
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      05-24-2005, 12:32 AM
In article <(E-Mail Removed)>, (E-Mail Removed) 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
 
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hearsay@att.net
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      05-24-2005, 11:53 AM


tacit wrote:
>
> In article <(E-Mail Removed)>, (E-Mail Removed) 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.


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?
 
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Mike Russell
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      05-24-2005, 10:39 PM
(E-Mail Removed) wrote:
[re conversion of RGB to CMYK]
> 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?


There are a host of technical issues with out of gamut colors. The ICC
standard defines a "tag" that defines out of gamut colors.

But the gamut tag is not always included in a profile, and when it is, it is
not used consistently by different manufacturers. So programs like
Photoshop generally ignore the gamut tag, perferring instead to rely on
color values calculated using the profile. One rather simple method of
doing this is to see if converting the color back and forth gives the same
result. For example, converting the out of gamut RGB (255,0,0) into CMYK
and back again gives a different RGB value from the original (RGB(199,3,3),
say). When this happens, that particular color is considered out of gamut.

There are other, better, ways to calculate out of gamut colors, and I
suspect the Photoshop engine uses one of those. BTW, out of gamut colors
are calculated by the engine, not by Photoshop itself. You can experiment
by telling Photoshop to use the Microsoft (or Apple) ICM engine in your
color prefs.

But this only says that the profile generates unique values for a particular
color, and not whether the printer itself yields a different color when that
color and colors close to it in value are printed. Given the limits of the
current technology, particularly the poor support of the gamut tag, the only
way to tell for sure is to print the colors, and scan them with a photometer
or spectrophotometer. You can't even look at the darn things to tell if
they are different, because the differences are very subtle. No wonder
there is so much confusion, so much being spent on calibration equipment,
and relatively little result to show for it.

But most of us realize that our inkjet printers do a pretty good job. If
printers were watches, at this time in history they would accurate to a
couple of minutes a day. As was the case with watches, there are those who
require (or think that they require) greater accuracy, and are willing to
pay for it. As has happened already with watches, at some point in the
future printers will probably be extremely accurate and inexpensive. Until
that happens, most of us can easily live comfortably with less than
perfection.

Here are some more thoughts that I hope will give pause to some of you who
have bought into the conventional wisdom that larger gamuts are always
better than smaller ones.

1) What was the last out of gamut object you photographed? Blue sky, for
example, is not really that saturated, particularly near the horizon where
most of our cameras are pointed. Red objects - even bright bird plumage, is
a far cry from RGB(255,0,0). For most of us, the answer to the question is
"none".

2) RGB is not the last word in color spaces. Consider pure yellow objects,
which are not all that uncommon, are not at all well represented in the RGB
color space. For most monitors RGB(255,255,0) is brighter, but less
saturated, than CMYK(0,0,100,0) will be in print. Magenta and cyan objects
have similar problems.

Recent monitor developments have made an end run around this problem,
resulting in the apparent miracle of a monitor capable of displaying the
Adobe RGB color space directly. This is achieved by filtering the RGB
phosphors to create purer colors, sharpening and stretching the three
corners of the RGB gamut (to see these shapes, check out Curvemeister's
Labmeter, a free gamut plotter image). But even purity can have its limits,
and the extermely sharp spectral characteristics of these monitors are
bringing a new problem, viewer metamerism, to the forefront. With this
latest advance, color consistency is literally in the eye of the beholder.
The color on these monitors simply look funny to some people.

3) Consider that a larger gamut such as Adobe RGB sacrifices color
gradation. An Adobe RGB image uses a smaller number of color values to
represent the same range of colors than an sRGB image. This issue may be
addressed by working in 16 bits, but there is a more serious problem to
working in a large gamut space: the high probability that someone else may
look at your image on an sRGB monitor and conclude that your work is too
drab. Both of these issues - particularly the second one - are reasons for
working in sRGB, particularly since real world objects such as pure blue sky
do not come close to exceeding the gamut of even sRGB.

4) Print has a much smaller gamut than a CRT. The big money in photography
is still in printed images, whether they be published images, or wall-sized
art.

My suggestion, as always, is to trust what you can see and verify, and not
spend too much time or money on getting your "watch" to run within one
second of correct. Todays color and printer technology is excellent, and
most of us - the vase majority - can live with its imperfections, provided
we understand them, and base our understanding on common sense.
--

Mike Russell
www.curvemeister.com


 
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