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Re: Printer CMYK vs RGB gamut!

Hallo Henk,

thank's for your post.
You are absolutly right in your explanation.
But in Oliviers post, for me it is clear, that we are talking about
the projection of the gamut of

ONE DEVICE,
ONE KIND OF INKS and
ONE KIND OF SUBSTRAT

and building a RGB printer profile or CMYK printer profile for THIS DEVICE.


Am 20.11.2007 um 15:11 schrieb Olivier Desmaison:
Thanks Clemens.
So you're saying the gamuts are similar in both CMYK and RGB for a given
printer (K3) on a given medium, correct ?
I had a quick look at ColorXact site, worth visiting for sure.
Olivier

Most of the inkjet printers expect RGB data as printer driver input and handle
them as CMY + a predefined GCR (internal colormatching = off).
With the light and dark inks (like light cyan and cyan), they are doing a predefined
ink mixing as well, so there is nothing to do for a profile here.

But you are right, we had the wrong wording.
We were not talking about colorspaces, but about the color models RGB and CMY
to describe the device color of ONE DEVICE.

Best regards,
Clemens

Am 20.11.2007 um 16:17 schrieb Henk Gianotten:

In my opinion, this is a very dangerous and confusing statement.
First it's important to define color spaces.
Which RGB? Which CMYK?
Both are device dependent and only if you determine a standard
space, one is able to make the right comparison.
Recently there was the sRGB versus aRGB discussion.
sRGB is a great standardized color space and a great solution
for private image shots and web representations.
It has a fixed 6500 color teperature. Adobe RGB is much larger.
We experience these problems if one tries to reproduce cyan and
green colors.
For example sRGB cannot reproduce the green we print if
cyan and yellow are printed together. Based on
ISO 12647-2 Papertype 1 standards 2007 amand 1:2007.
Adobe RGB is able to do that. RGB = 0, 139 and 147.
sRGB does have a smaller gamut and lacks in blue and green colors.
So there is no fixed relation between RGB and CMYK.
There are fixed relations between a specified RGB
(for example eciRGB_v2 or sRGB) and fixed CMYK's
(for example CMYK based on the Output Condition Fogra39).
There is already a lot of confusion.
As long as you talk about one specific printer printing
on one type of substrate with one set of (specified) inks
and one set of RGB values based on one specified RGB space
your explanation is valid.
Regards, Henk



At 14:16 20-11-2007 +0100, you wrote: 
Am 20.11.2007 um 13:41 schrieb Olivier Desmaison:

I read a strange statement: printer CMYK gamuts are narrower than
the same
RGB gamuts. Since it's the same device I'm puzzled.
Olivier


Hallo Oliver,

this statemant is correct, as long as you are talking about the same
device.

For example:

You can make a RGB printer profile for an Epson printer, using the
manufactures printer driver
or a CMYK printer profile using a RIP.

Printing RGB 0-255-255 to the printer using the epson driver will
result in a color patch of 100% cyan.
Printing RGB 255-0-255 to the printer using the epson driver will
result in a color patch of 100% magenta.
Printing RGB 255-255-0 to the printer using the epson driver will
result in a color patch of 100% yellow.

So, when you are measuring this patches using a spectro, you'll end
up with the same Lab values like
printing the CMYK patches with a RIP (my assumption in this example
is, that both, the printer driver and the
RIP are pre linearized to the same amount of ink per chanel).

The gamut must be the same.

The story of the small CMYK gamut comes from the fact, that most of
the printing presses deliver a smaller
gamut than RGB input devices like scanners, digicams or syntetic RGB
profiles like ADOBE RGB etc.
But this is a completely different strory.

Best regards,
Clemens Beisch




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References: 
 >Printer CMYK vs RGB gamut (From: "Olivier Desmaison" <email@hidden>)
 >Re: Sunlight (From: Beisch Clemens <email@hidden>)
 >Re: Sunlight (From: Roger Breton <email@hidden>)



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