5 Replies Latest reply on Feb 11, 2010 9:02 AM by Gernot Hoffmann

# How to convert the CMYK color percentage to RGB color percentage?

How to convert the CMYK color percentage to RGB color percentage values. Is there any formulae for to convert CMYK to RGB percentage values. Kindly advise me.

Thanks for looking into this.

• ###### 1. Re: How to convert the CMYK color percentage to RGB color percentage?

Maria,

There are no RGB percentages.

RGB colours are in proportions x/255 of the full colour brightness, with values from 0 to 255. Furthermore, this scale is inverse as compared to the CMYK scale from 0 - 100%, in both cases for each colour component/ink.

Blacks form a special case, where CMYK black may be a simple CMYK 0/0/0/100, which corresponds to something like RGB 33/33/37, whereas RGB 0/0/0 corresponds to something a bit below CMYK 100/100/100/100. A rich black in CMYK may be different values of C, M, and Y, together with 100 K, the values depending on other colours in the artwork, and the total ink coverage (the sum C+M+Y+K) up to somewhere above/below 300, dpending on paper etc.

The best way to find a match is to set the values in CMYK or RGB and change the option in the Color palette/panel to the other kind to see the corresponding values. Be aware that the values will depend on the Document Color Mode, and note the sometimes significant differences, both caused by different colour gamuts.

• ###### 2. Re: How to convert the CMYK color percentage to RGB color percentage?
Is there any formulae for to convert CMYK to RGB percentage values

The problem is: there are myriad formulae for converting CMYK to RGB (and vice-versa). You're talking about converting four values to three. Or three values to four.

That's what all the color management stuff is about. The closest CMYK approximation for a given RGB color is dependent upon multiple variables. So no, no one can give you a one-size-fits-all formula.

JET

• ###### 3. Re: How to convert the CMYK color percentage to RGB color percentage?

Could you please explain and provide me the myriad formulae for converting CMYK to RGB (and vice-versa).

Thanks

• ###### 4. Re: How to convert the CMYK color percentage to RGB color percentage?

No.

"Myriad" means "many". Conversions between CMYK and RGB are not simple conversions like "this CMYK value equals this RGB value."

Theoretically, in a perfect world, RGB and CMY would be straightforward conversions to complementary colors 180 degrees across the color wheel. But CMY pigments are imperfect and are not capable of reproducing many colors that are visible in RGB. K (black) is added to compensate for the "muddiness" inherent in straight RGB to CMY conversion. Addding the fourth color means you must now convert from three colors to four and there are many many different ways to do that. The "best" way depends upon variables which include the color and absorbancy of the paper, type of printing device, type of ink, and more.

Moreover, an "RGB color" is not really a specific color. It's just a set of three 8-bit values. Different RGB devices (monitors) respond differently to those values. So a given set of RGB values being sent to your TV set quite likely results in a different color than the same RGB values being sent to your computer monitor.

That's what all this color calibration technology is about, with rendering intents and embedded profiles, collectively called "Color Management." There is no unambiguous "this CMYK color equals this RGB color" cross reference table that anyone can give you.

You ask "how to convert the CMYK color percentage to RGB." But you don't explain what exactly you are trying to do in any context. One can answer your question by simply telling you to doubleClick a swatch in Illustrator and change its color mode from CMYK to RGB. Illustrator will try to pick RGB values based upon your current color management settings. But the document will still be a CMYK document.

You need to explain what you are trying to do. Are you looking for some kind of algorithm you can use in scripting? (You post many similar questions in the scripting forum.)

There is no single, simple conversion table or formula for what you are asking.

JET

• ###### 5. Re: How to convert the CMYK color percentage to RGB color percentage?

Maria,

as already mentioned: these color space conversions are
nowadays handled by ICC profiles which are not characterized

by 'formulae' but by huge multidimensional tables.

The only one generic transformation by formulas was introduced

by PostScript for DeviceRGB to DeviceCMYK and vice versa
(see PostScript Language Reference. Third Edition).

All variables are normalized for the range 0.0 to 1.0.
R is complementary to C, G to M and B to Y.
'Device' means: unspecified color space.

DeviceRGB to DeviceCMYK:

c = 1.0 - R
m = 1.0 - G
y = 1.0 - B
k = min (c,m,y)

C = min (1.0, max (0.0 , c - UCR(k) ))
M = min (1.0, max (0.0 , m - UCR(k) ))
Y = min (1.0, max (0.0 , y - UCR(k) ))
K = min (1.0, max (0.0 , BG(k) ))

UCR(k) means 'UnderColor Removal'
BG(k) means 'Black Generation'
(1) UCR(k) = 0;  BG(k) = 0;
Result is CMY with K=0
(2) UCR(k) = k;  BG(k) = k;
Result is CMYK with replacement of equal values c=m=y by k
Anything between (1) and (2) may be used.

DeviceCMYK to DeviceRGB

R = 1.0 - min (1.0, C+K )
G = 1.0 - min (1.0, M+K )
B = 1.0 - min (1.0, Y+K )

These transformations are of almost no use.

If the spaces are unknown then they may be better than nothing.

Also for color wheels in tutorials.

Best regards --Gernot Hoffmann