CMYK to RGB and RGB to CMYK Conversion

[mrd0619]

Here's the problem: In Illustrator I created a CMYK swatch with the values 100.0.81.18, when I switched the swatch to RGB the mix comes out as 0.143.92. BUT, if I start with a new RGB swatch and change it to CMYK, the mix then changes to 86.20.82.5.

Shouldn't my RGB values convert back to the original CMYK values of 100.0.81.18? Where is this other random CMYK mix getting pulled from? Why is this happening? Could it just be a bug with the algorithm that determines the conversion?

[John Mensinger]

rab9485  wrote

Shouldn't my RGB values convert back to the original CMYK values

No. Your theory is based on a false equivalency.

Where is this other random CMYK mix getting pulled from?

It's not random. It's the CMYK approximation of the RGB mix from which you converted. The RGB mix was also an approximation of your original CMYK. RGB and CMYK do not have a direct relationship in a way that would result in the same values converting back and forth. In fact, I submit that if you were to continue converting back and forth, the results would continue to depart farther and farther from your original values.

[mrd0619]

I feel like this makes sense however, when I take the CMYK values 86.20.82.5 and convert to RGB, I get the original RGB values 0.143.92 again, which is even more confusing.

[John Mensinger]

Well, my explanation is generalized and can't rule out the possibility that there will be certain gamut-shared mixes that convert back and forth with direct equivalency, under the right conversion-profile circumstances. There may be ways to find them, and of course this is all finite and quantifiable. Typically though, a standard workflow doesn't include bi-directional conversions that become subject to all this theory, and given the number of possible combinations, one might as well consider it happenstance.

[Monika Gause]

If one of the values is out of gamut in the target space, then it gets converted so another value that is inside the color space. And that's wenn your equation gets thrown off.

[Jacob Bugge]

rab,

This reminds me of the old Cold War story about machine translation between English and Russian to improve mutual understanding and  prevent an undesired event, the first test being translation from English to Russian and back again of Out of sight, out of mind which became Invisible insanity.

In addition to what John said, the approximations also depend on the chosen colour management settings.

[Ton Frederiks · Accepted solution]

There are many ways to convert RGB to CMYK.

Because you go from 3 component (RGB) to a 4 component color (CMYK) you can vary the amount of black (grey component) and the CMY colors to keep the same color appearance.

Here is an example how various CMYK values can result in the same RGB color values:

[Lina28215554ixaf]

Hi, 

Digging this old post as i'm pulling my hair off from this RGB/CMYK thing.
Thank you for illustrating the different CMYK values a RGB can be converted to, depending on the profile. 

I used to only use hex colors to create my logo - finding the matching RGB. but when my printer asks me for the CMYK, i'm lost.. 
When looking for the equivalent of my RGB on web converters, most of them revert back similar values for CMYK. 

example: 

#04aa85 -- RGB 4 170 136  -->  CMYK 98, 0, 20, 33

https://www.colorhexa.com/04aa88  

https://rgb.to/hex/04aa88 

I get that those numbers mean nothing without any mention of profile....  YET, when I look for this color on Illustrator, RGB and hex values match (for my FOGRA 39 profile) but the CMYK values are way off. 

I'm fine with using the right hex and rgb values on my computer but what should I provide to my printer ??? 

- the "adobe CMYK for this profile" : so 76 0 57 0  - telling him that i used a coated fogra39 profile

or the one "commonly" seen online, that I also get on Inkscape for a s-rgb1966 profile : 98 0 20 33 ? 

Thank you !! 

[Stephen_A_Marsh]

You have a CMYK recipe of values that was created by hand, it did not come from an ICC profile conversion.

When converting to RGB, a CMYK profile is used as a description of what those numbers/values mean, this creates an intermediate Lab colour value. The intermediate Lab colour value is then converted to RGB.

When going from RGB to CMYK, the same thing happens with the profile conversion: RGB > Lab > CMYK… The “Lab” part is not Lab colour mode as in Photoshop, it is the PCS or Profile Connection Space.

The CMYK profile uses a different recipe value, which as closely as possible creates the same Lab colour value. The constant here is the Lab colour value, which is being retained (as much as possible) in both the RGB and the CMYK colour conversions.

If you converted from RGB > CMYK > RGB > CMYK you should see the same CMYK numbers/values, as the CMYK values have come from the ICC profile conversion each time.

[ceyhun_akgun]

ICC PROFILLER interrupts when returning from RGB to CMYK color world. Rebuilds the colors in the ICC profile as CMYK.

This happens every time in mutual conversions. Colors affect one another. The ICC profile will reconstruct the existing color as close as possible to the target color space.

[mrd0619]

I've learned that this is not as straightforward as I originally thought. Thank you all for your help!