Computer Graphics with Photoshop - transforming images rgb to cmyk

Author

transforming images rgb to cmyk

indide_designs

2005-05-24, 7:18 pm

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

Tacit

2005-05-24, 7:18 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

Hecate

2005-05-24, 7:18 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...

Mike Russell

2005-05-24, 11:16 pm

hearsay@att.net 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

Tacit

2005-05-25, 7:14 pm

In article <429314AE.E5502E03@att.net>, hearsay@att.net wrote:

> By "Certain colors can not be reproduced in CMYK", I assume you meant
> out of gamut colors between color spaces.

Yes, that's correct. For example, pure RGB blue ain't never going to be
reproduced in CMYK.

> 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?

Photoshop's out of gamut detection is remarkably good; however, there
are many issues which may confound trying to identify out of gamut
colors on an inkjet print.

For starters, Photoshop's out of gamut detection uses the current CMYK
setup; if this is incorrect (for example, if you're using a SWOP
profile, intended for printing presses, rather than a profile for your
inkjet printer), Photoshop will show you the colors out of gamut for a
printing press, not your printer.

Second, when you print CMYK to an inkjet printer, the printer's drive
software converts from CMYK to RGB, then back to the printer's own CMYK.
So the data the printer sees may not match the data Photoshop sees,
since the printer driver's CMYK->RGB conversion isn't very good.

Third, consumer-grade inkjet printers don't even use pure primary CMYK
inks at all. In particular, the cyan ink that an inkjet printer uses
isn't really cyan; it's too blue. This is done to get more vibrant, more
saturated colors that consumer users like, at the expense of color
accuracy. Professional inkjet printers use inks closer to CMYK
primaries, and are easier to calibrate for press-accurate CMYK output.

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

Mike Russell

2005-05-26, 7:25 am

Tacit wrote:
>... Photoshop's out of gamut detection uses the current CMYK
> setup ...

CMYK is indeed the default, but you may set the gamut warning to use any
profile that may be used as a working space.

LabMeter, which is a free download from Curbemeister, uses this feature to
plot the gamuts of any suitable profile.
--
Mike Russell
www.curvemeister.com

hearsay@att.net

2005-05-26, 7:14 pm

Mike Russell wrote:
>
> hearsay@att.net wrote:
> [re conversion of RGB to CMYK]
>
> 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.

When soft proofing with Epson inkjet profiles, I can see an obvious
shift in some monitor colors that won't be reproduced correctly in
prints. Yet these colors are not detected as oog by PS. Does that mean
the Epson profiles do not include the gamut tag? Is there a way to find
out if a profile has a gamut tag?

> 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.

Confusing is an understatement. Take the above situation as an example.
My working space is rgb, soft proofing is in rgb, the file is sent to
the Epson as rgb, the Epson driver will print in six cmyk inks.
Meanwhile when soft proofing in PS, there is an Info Palette option to
show Proof Color (italic rgb). What is the meaning and use of Proof
Color? IOW, what is the relationship between Actual Color and Proof
Color? Adobe must have thought that Proof Color has some use.

> 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.
> --

Very well put. For hobbyists like myself, getting print colors "close"
to monitor colors is all we need. But some colors I described above can
be WAY off. That is like having a watch that is accurate to the minute
most of the time, but can be off by hours some of the time. Very
frustrating.

Will Crockett is another who choose sRGB over other wider gamut space as
working space:

http://shootsmarter.com/infocenter.html

hearsay@att.net

2005-05-26, 7:14 pm

Tacit wrote:
>
> In article <429314AE.E5502E03@att.net>, hearsay@att.net wrote:
>
>
> Yes, that's correct. For example, pure RGB blue ain't never going to be
> reproduced in CMYK.
>
>
> Photoshop's out of gamut detection is remarkably good; however, there
> are many issues which may confound trying to identify out of gamut
> colors on an inkjet print.
>
> For starters, Photoshop's out of gamut detection uses the current CMYK
> setup; if this is incorrect (for example, if you're using a SWOP
> profile, intended for printing presses, rather than a profile for your
> inkjet printer), Photoshop will show you the colors out of gamut for a
> printing press, not your printer.
>
> Second, when you print CMYK to an inkjet printer, the printer's drive
> software converts from CMYK to RGB, then back to the printer's own CMYK.
> So the data the printer sees may not match the data Photoshop sees,
> since the printer driver's CMYK->RGB conversion isn't very good.
>
> Third, consumer-grade inkjet printers don't even use pure primary CMYK
> inks at all. In particular, the cyan ink that an inkjet printer uses
> isn't really cyan; it's too blue. This is done to get more vibrant, more
> saturated colors that consumer users like, at the expense of color
> accuracy. Professional inkjet printers use inks closer to CMYK
> primaries, and are easier to calibrate for press-accurate CMYK output.

Agreed with all your points. Please see my response to Mike's post to
see what's been bugging me.

Mike Russell

2005-05-26, 7:14 pm

hearsay@att.net wrote:
[re working spaces, versus print spaces]

> Confusing is an understatement. Take the above situation as an
> example.
> My working space is rgb, soft proofing is in rgb, the file is sent to
> the Epson as rgb, the Epson driver will print in six cmyk inks.
> Meanwhile when soft proofing in PS, there is an Info Palette option to
> show Proof Color (italic rgb). What is the meaning and use of Proof
> Color? IOW, what is the relationship between Actual Color and Proof
> Color? Adobe must have thought that Proof Color has some use.

The meaning of actual and proof color, at least, is well defined. Actual
color refers to the numeric pixel color values of your image. Proof color
displays actual color values after converting from your working space to the
last selected proof color space, as specified in View>Proof Setup. It's
like a one pixel "Convert to Profile", with the target color space set to
your proof space.

The use and intent of Proof color is another issue. Photoshop is first and
foremost a tool, and providing a motivation for each feature is in the
documentation is difficult, and not necessarily Adobe's responsibility.

One very powerful application for viewing proof color values is for
verification of profile color operation. This is useful for people who
produce color profiles, or who otherwise concern themselves with the input
and output numeric values for conversions made by a particular profile. For
example, set one info palette to actual", and another to "proof". Then move
the cursor around your image to compare color values and look for trouble
spots. Instead of saying "my colors look different", you could say that your
epson color profile converts sRGB(255,0,0) to EpsonRGB(254,12,5).

Another use would be to preview what CMYK color values would be produced for
various parts of your image, without having to first convert to the proof
CMYK color space.

> Very well put. For hobbyists like myself, getting print colors "close"
> to monitor colors is all we need. But some colors I described above
> can be WAY off. That is like having a watch that is accurate to the minute
> most of the time, but can be off by hours some of the time. Very
> frustrating.

Yes, or even a watch that simply stops some of the time. I'd be interested
in exactly which colors look funny on your printer, how your printer is set
up, etc. One quick think to try is to compare whatever profile setup you
are using now with the Color Enhance setting for your printer. Terra cotta
red, for example, happens to be a tricky color for some Epson papers.
--
Mike Russell
www.curvemeister.com