Converting RGB to CMYK canon Rules, and possible script

[-Dain-]

Hi guys,

I'd like to get your input on something I found weird about Conversions from RBG to CMYK.

The Issue : discrepancies in results when converting colors from RGB to CMYK via PS/IL/ID/Online

The datas :

a- When using color pickers in PS/ Illustrator the result values are the same. Let's call it Value 1.

b- When using ID, the result is way off from the results obtained from PS/Il. Let's call it Value 2
c- When using online converters such as https://www.rapidtables.com/convert/color/rgb-to-cmyk.html we get another result. Let's call it Value 3.

My questions :
1- Is the conversion something clear and follows some math rules ? something that is linked to a color profile and that can explain the discrepancies of the Values 1-2-3 ? Or is it just a bug or something ?
2- Following the website above, it seems to me that conversion follows a mathematical equation. Shouldn't it be canon ?

The why about this topic : a script that converts swatches RGB to CMYK 

Following some color registration issues with our printing houses (moiré), I want to implement a change in dealing with colors on texts. Rather than using 4C, I'd like to go for 3C or even 2C when it's possible. During offset printing, removing 1 color saves quite some time, money, and possible problems. It's a win-win change I believe and since the change is only applied to regular texts, possible variations in colors won't even be seen by clients.
This means creating a script that would :

- gather the swatches
- propose an accurate conversion of the color

- create the new color with a custom name
Do any of you know about some existing script that would do that ? I not, I'll be happy to do it (surely with some help from you guys, at some point)

Thanks for your input

[Test Screen Name]

Let's take 3 first. These results are junk. What, all the different web sites? Yes, junk. They are based on the myth, the lie, that there is an algorithm, a single correct conversion.  Made by people who understand web sites but don't understand Color management. 

[Test Screen Name]

So, why isn't there an algorithm? It would certainly be convenient!

The reason is that CMYK isn't a colour, it's a recipe. Many designers are not trained in this. They are told to convert to CMYK because that's what prints. But here's the problem: different brands of CMYK inks are actually different. Worse, the same inks look utterly different on shiny magazine stock versus cheap grey newsprint. So the conversion to CMYK has to allow for those differences to make an appearance similar to each other and the original. That's colour management. That's why we have to set CMYK (and RGB) profiles. There is no case of "default" or "absent" profile.

[Test Screen Name]

So, how does RGB to CMYK conversion actually work (in a proper colour managed app). To oversimplify a bit too much

- there is a standard colour space, Lab

- RGB and CMYK profiles are tables and stuff to convert between Lab and RGB, or Lab and CMYK.

- To convert RGB to CMYK, first the RGB profile converts RGB to Lab. Then the CMYK profile converts Lab to CMYK. Job done!

- So this conversion involves reading profile files, and doing conversions including 4D table lookup and interpolation.

- A CMM (Color Management Module) is a piece of software to do this sort of job

- You'd be crazy to write your own CMM. (It could be fun but take maybe years). Or rather your boss would be crazy to pay you to write your own CMM.

[Peter Spier]

As to why your conversions might be different among Adobe apps, you must be sure that both the RGB working sapce and the CMYK conversion space are the same across all the apps.

[rob day]

Hi @-Dain- , Also, the source and destination for an RGB to CMYK Color Picker conversion would depend on whether there are documents open when you pick the color. As Peter suggests the RGB and CMYK Color Settings Working Spaces would be used for a Color Picker RGB to CMYK conversion, but only if there are no documents open.

Here I have AdobeRGB set as my RGB Working Space, Coated GRACoL 2006 as my CMYK Working Space, and matching Conversion Options (which also affect a conversion), Relative Colorimetric with Black Point Compensation turned on. Note that Illustrator requires an open document to get at its Color Picker, so I have a CMYK Illustrator doc open with the Coated GRACoL 2006 CMYK destination assigned. The Color Picker conversions in the 3 apps match:

If there is a document open, the document’s assigned profile(s) are used—the document assignment could be different than the Color Settings Working Space, so you have to check Edit>Assign profiles....

InDesign allows you to mix RGB and CMYK colors in the same document, so an ID doc has both an RGB and CMYK profile assignment. Here I’ve changed the RGB and CMYK document assignments to sRGB and US Sheetfed Coated respectively, and you can see the color managed conversion of 100|100|150 sRGB to US Sheetfed Coated CMYK has changed from 70|63|13|1 to  66|56|9|0:

A CMYK profile handles the GCR and black generation on a conversion to CMYK, so you can reduce the amount of black by creating a profile for the press with lighter black generation. I don’t think there would be a way to script GCR for an existing CMYK color and keep its color’s appearance for the destination press.