Close #122 - Add CMYK representation

[aeter]

Adds a CMYK representation. References #122

Tested also with https://www.rapidtables.com/convert/color/rgb-to-cmyk.html

Tests:

$ pastel format cmyk 'rgb(255, 255, 255)'
cmyk(0, 0, 0, 0)
$ pastel format cmyk 'rgb(0, 0, 0)'
cmyk(0, 0, 0, 100)
$ pastel format cmyk 'rgb(19, 19, 1)'
cmyk(0, 0, 95, 93)
$ pastel format cmyk blue
cmyk(100, 100, 0, 0)
$ pastel format cmyk 'rgb(55, 55, 55)'
cmyk(0, 0, 0, 78)
$ pastel format cmyk 'rgb(136, 117, 78)'
cmyk(0, 14, 43, 47)
$ pastel format cmyk 'rgb(143, 114, 75)'
cmyk(0, 20, 48, 44)
$ pastel format cmyk 'rgb(143, 111, 76)'
cmyk(0, 22, 47, 44)

[sharkdp]

Thank you for your contribution.

When working on pastel in the past, I have considered adding the CMYK color space, but it was my understanding that there is no such thing as "the RGB <=> CMYK conversion formula" [1]. Another problem is that the gamuts do not overlap with sRGB.

We face similar problems with CIELAB, but at least sRGB's gamut is a subset of the "CIELAB gamut".

Could you please clarify how these issues are addressed here? Where does the conversion formula come from?

[1] https://en.wikipedia.org/wiki/CMYK_color_model#Conversion

[aeter]

@sharkdp - as a formula reference during writing the code, I used both the bash script from ticket #122 and https://www.rapidtables.com/convert/color/rgb-to-cmyk.html.

---

The top SO answer here (https://stackoverflow.com/q/2426432) says -

The conversion from RGB to CMYK is dependent on the physical device/process being used to lay down the CMYK ink. 
These are represented in software as Color Profiles.
ICC and ICM color profiles of physical devices determine the resulting colors.

If you are not concerned with true representation on a physical device then use the direct conversion formulas in other posts.

The used formula does no apply any Color Profile.

I can find the same algorithm either used or mentioned at different places online -

• at the other answers to the cited SO question
• https://www.codeproject.com/Articles/4488/XCmyk-CMYK-to-RGB-Calculator-with-source-code
• online (JS) conversion websites like https://convertacolor.com/ seem to use it

[kyndder]

Adds a CMYK representation. References #122

Tested also with https://www.rapidtables.com/convert/color/rgb-to-cmyk.html

Thank you, @aeter !

When working on pastel in the past, I have considered adding the CMYK color space, but it was my understanding that there is no such thing as "the RGB <=> CMYK conversion formula" [1]. Another problem is that the gamuts do not overlap with sRGB.

@sharkdp , I'll follow @aeter and the references provided by him...

As at a first glance this feature will not be used for physical representation, direct conversion would be an entrance for future improvements...

As I mentioned in #122 , due to a very specific issue and for an "on screen" representation provided by a specific software, a direct conversion was all I needed...

But I understand your point... A better approach for physical conversion, would be using Hue but, even this way, we would still dependent on specific profiles to achieve a satisfactory result...

Thank you for the great work, guys!

EDIT

Just for clarification, for example, here's a sample from a device profile, from one of my printers;

My target profile is ISO 12647-2 (ISO Coated v2)...
My device's profile, is the SP7900HT_720x1440...

Based on my profile, theoretically, I can reproduce the whole CMYK gamut, but not sRGB's one...

[sharkdp]

Ok - thank you for your explanations!

[codecov-commenter]

Codecov Report

Merging #123 into master will increase coverage by 0.30%.
The diff coverage is 83.67%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
+ Coverage   69.28%   69.59%   +0.30%     
==========================================
  Files          32       32              
  Lines        2676     2723      +47     
==========================================
+ Hits         1854     1895      +41     
- Misses        822      828       +6     

Flag	Coverage Δ
#macos_latest	69.64% <83.67%> (+0.30%)	⬆️
#ubuntu_latest	69.64% <83.67%> (+0.30%)	⬆️
#windows_latest	69.63% <83.67%> (+0.30%)	⬆️

Impacted Files	Coverage Δ
src/cli/cli.rs	100.00% <ø> (ø)
src/lib.rs	86.49% <83.33%> (+0.02%)	⬆️
src/cli/commands/format.rs	92.68% <100.00%> (+0.18%)	⬆️

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