help with CMYK -> RGB

help with CMYK -> RGB

Post by Shai Deke » Tue, 14 Apr 1998 04:00:00



To all color experts

I am using the (known) simple (though non-linear) formulas for display
of CMYK images:

==============================================
CMYK -> RGB

Red  =(1-Black)*(1-Cyan)

Green=(1-Black)*(1-Magenta)

Blue =(1-Black)*(1-Yellow)

C, M, Y, K, R, G, and B have a range of [0;1].
==============================================

I obtained the above formulas from various faq's and tiff library
codes.

The display looks o.k. but is far from the result obtained using
PhotoShop which looks MUCH better!!! I was told that PhotoShop does not
use the above formulas but uses instead hard-coded conversion tables (+
interpolations between table values). It looks like I'm missing a lot of
know-how (or quite a few hours of reversed engineering) !?

    thanks

        Shai

 
 
 

help with CMYK -> RGB

Post by Robert Collett » Tue, 14 Apr 1998 04:00:00



> To all color experts

> I am using the (known) simple (though non-linear) formulas for display
> of CMYK images:

> ==============================================
> CMYK -> RGB

> Red  =(1-Black)*(1-Cyan)

> Green=(1-Black)*(1-Magenta)

> Blue =(1-Black)*(1-Yellow)

> C, M, Y, K, R, G, and B have a range of [0;1].
> ==============================================

> I obtained the above formulas from various faq's and tiff library
> codes.

> The display looks o.k. but is far from the result obtained using
> PhotoShop which looks MUCH better!!! I was told that PhotoShop does not
> use the above formulas but uses instead hard-coded conversion tables (+
> interpolations between table values). It looks like I'm missing a lot of
> know-how (or quite a few hours of reversed engineering) !?

Unfortunately, I don't know that algorithms that Photoshop uses to
convert between RGB and CMYK.  However, I know that they use Lab as an
intermediate color space.  Thus, converting from CMYK to RGB, they would
convert CMYK to Lab using the Printing Inks Setup.  The Lab to RGB
conversion would be determined by the monitor setup parameters (White
Point, Phosphor Chromaticities, Gamma).

Hope this helps a little.

Bob Collette
Eastman Kodak Co.

 
 
 

help with CMYK -> RGB

Post by Hiram Berr » Tue, 14 Apr 1998 04:00:00


Shai,

I have also encountered this problem, and the best solution I have found is
semi-empirical. That shouldn't be surprising, since these color models are
somewhat subjective anyway-- your Cyan ink may not be exactly the same shade
as my Cyan, and even if we agree on a standard we might use different
screens so that it gives a different shade of green when screened with a
particular Yellow.? The same CMYK can look significantly different with
different print jobs.? So I established a local standard for RGB-- a scanner
with invariant settings.? Then for a particular set of print conditions, I
print a set of samples: for the linear case it is 5 swatches consisting of
the unprinted paper and one each of pure C,M,Y, and K; for the quadratic
model 15 swatches consisting of the linear ones plus a 50% print of each
primary and a 50%:50% swatch of each possible combination of 2 different
colors; the cubic and quarternary models require 35 and 70 sample swatches
respectively, and the choice of color composition is analogous.

Then you scan them on the local standard scanner.? You now have (in the
quadratic case) 15 sets of defined points ((RGB),(CMYK)), which give 15
equations of the form

R = q0 + q1*C + q2*M + q3*Y + q4*K + q11*C^2 + q12*C*M + [rest of q terms]

containing 15 undetermined q coefficients.? Solve this system of eqns for
the q's.? The G and B cases are analogous; you now have a general mapping
from CMYK to RGB for the particular print conditions.? Notice that you can
make this model as precise as you want by increasing the order of the
solution.

HTH,

Hiram.
?
?


> To all color experts

> I am using the (known) simple (though non-linear) formulas for display
> of CMYK images:

> ==============================================
> CMYK -> RGB

> Red? =(1-Black)*(1-Cyan)

> Green=(1-Black)*(1-Magenta)

> Blue =(1-Black)*(1-Yellow)

> C, M, Y, K, R, G, and B have a range of [0;1].
> ==============================================

> I obtained the above formulas from various faq's and tiff library
> codes.

> The display looks o.k. but is far from the result obtained using
> PhotoShop which looks MUCH better!!! I was told that PhotoShop does not
> use the above formulas but uses instead hard-coded conversion tables (+
> interpolations between table values). It looks like I'm missing a lot of
> know-how (or quite a few hours of reversed engineering) !?

> ??? thanks

> ??????? Shai

?
 
 
 

help with CMYK -> RGB

Post by G. Adam Stanisl » Sat, 18 Apr 1998 04:00:00




Quote:>CMYK -> RGB
>Red  =(1-Black)*(1-Cyan)
>Green=(1-Black)*(1-Magenta)
>Blue =(1-Black)*(1-Yellow)

That does not seem right. Off the top of my head (it is 2:30 am...), I
believe the formulae to be:

red = 1 - black - cyan
green = 1 - black - magenta
blue = 1 - black - yellow

 
 
 

help with CMYK -> RGB

Post by Stephen Plat » Sat, 18 Apr 1998 04:00:00



> Shai,

> I have also encountered this problem, and the best solution I have found is
> semi-empirical. That shouldn't be surprising, since these color models are
> somewhat subjective anyway-- your Cyan ink may not be exactly the same shade
> as my Cyan, and even if we agree on a standard we might use different
> screens so that it gives a different shade of green when screened with a
> particular Yellow.  The same CMYK can look significantly different with
> different print jobs.  So I established a local standard for RGB-- a scanner
> with invariant settings.  Then for a particular set of print conditions, I
> print a set of samples: for the linear case it is 5 swatches consisting of
> the unprinted paper and one each of pure C,M,Y, and K; for the quadratic
> model 15 swatches consisting of the linear ones plus a 50% print of each
> primary and a 50%:50% swatch of each possible combination of 2 different
> colors; the cubic and quarternary models require 35 and 70 sample swatches
> respectively, and the choice of color composition is analogous.

> Then you scan them on the local standard scanner.  You now have (in the
> quadratic case) 15 sets of defined points ((RGB),(CMYK)), which give 15
> equations of the form

Uh, aren't you making some assumptions about your scanner at this point?
(Or, what do you mean by 'standard scanner'?)

Steve

- Show quoted text -

> R = q0 + q1*C + q2*M + q3*Y + q4*K + q11*C^2 + q12*C*M + [rest of q terms]

> containing 15 undetermined q coefficients.  Solve this system of eqns for
> the q's.  The G and B cases are analogous; you now have a general mapping
> from CMYK to RGB for the particular print conditions.  Notice that you can
> make this model as precise as you want by increasing the order of the
> solution.

> HTH,

> Hiram.


> > To all color experts

> > I am using the (known) simple (though non-linear) formulas for display
> > of CMYK images:

> > ==============================================
> > CMYK -> RGB

> > Red  =(1-Black)*(1-Cyan)

> > Green=(1-Black)*(1-Magenta)

> > Blue =(1-Black)*(1-Yellow)

> > C, M, Y, K, R, G, and B have a range of [0;1].
> > ==============================================

> > I obtained the above formulas from various faq's and tiff library
> > codes.

> > The display looks o.k. but is far from the result obtained using
> > PhotoShop which looks MUCH better!!! I was told that PhotoShop does not
> > use the above formulas but uses instead hard-coded conversion tables (+
> > interpolations between table values). It looks like I'm missing a lot of
> > know-how (or quite a few hours of reversed engineering) !?

> >     thanks

> >         Shai

 
 
 

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