Definition of CMYK colors - bug or understanding of definition

[Snoopy17]

Definition of CMYK colors

I'm using Affinity Photo, Designer and Publisher (all 1.8.3.641) under Win10pro (1909). Everything works fine so far, the software is excellent. For a new project I need to work with different color representations (RGB, HSL and CMYK) which all are supported by the 3 programs.

It is clear to me, that with integer numbers due to rounding errors conversion back and forth not always come to exactly the same result. But surprisingly with the conversion from RGB to CMYK there are difficulties to understand.

Example:
I take a color (Hex: #C0260E)
When I go to the color panel, with colorspace "RGB hex" selected and enter this, it results in the correct RGB values, shown with colorspace "RGB": [192, 38 14]
When I change the colorspace to "HSL" the result is: [8, 86, 40]
This is exactly what I'm expecting.

When I then go to colorspace "CMYK" Affinity results in: [17, 98, 100, 8]
However my expectation for this result would be: [0, 80, 93, 25]

I checked this with own vba and javascript programs and by use of a conversion website like
https://www.rapidtables.com/convert/color/rgb-to-cmyk.html
always same: [0, 80, 93, 25]

Perhaps there is another definition of CMYK used in Affinity. If so, I would like to get to know. Otherwise, could it also be a bug in the 3 programs?

I appreciate any help and explanation.

[wonderings]

For RGB 192,38,14 I find CMYK 17,98,100,8 to be closer and more in the family then your expected result of CMYK 0,80,93,25 - find it too orange. 

Are you trying to achieve matching colour for print and web use which is why you have RGB and CMYK? Seems like a real headache to be working with both RGB and CMYK. 

 

 

[lacerto]

(...)

[wonderings]

10 minutes ago, Lagarto said:

All color conversions within Affinity apps are based on color profiles (typically ISO standards) and as far as CMYK is involved, actual printing conditions (inks, the kind of media you're printing on, etc.). They show significantly different values than calculators on the Internet which are often based on abstract formulas that do not take into account these kinds of factors. 

If you use other color managed apps (like Adobe apps), you get more or less identical conversions in them as long as you're using the same color profiles. 

I did my conversion with Indesign and then checked with Publisher, same conversion from RGB to CMYK.