[Solved] What does the CMYK option do? How is it used?

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[Solved] What does the CMYK option do? How is it used?

Postby John_Ha » Thu Oct 25, 2018 4:23 pm

A quick search of the Writer forum with cmyk shows a number of posts where people enquire about CMYK as opposed to RGB colours. The conventional wisdom seems to be that AOO only supports RGB and not CMYK.

I think the essential difference - but please correct me if I am wrong - is that RGB (Red, Green, Blue) is additive colour mixing where each colour you add to the mix adds colour. So, if you add a red light to a blue light you get a yellow colour. If you add red, blue and green you get white. So, it is used for screens, where light is created.

CMYK (Cyan, Magenta, Yellow, Black) is subtractive colour mixing where each colour you add to the mix subtracts colour. If you add cyan and magenta you get blue. If you add cyan, magenta and yellow, you get black. Subtractive colour mixing is used for printer inks or paints and commercial printing and publishing companies sometimes ask for CKYK to be used.

So, what does the CMYK option in Tools > Options > OpenOffice > Colours do? And how do you use CMYK if your printer or publisher asks you for CMYK?

Help says
Colour table

To modify, select the colour model: Red-Green-Blue (RGB) or Cyan-Magenta-Yellow-Black (CMYK).

OpenOffice uses only the RGB colour model for printing in colour. The CMYK controls are provided only to ease the input of colour values using CMYK notation.


Does this mean that you can specify a colour using CMYK values, but that when AOO prints, it will output the RGB values which are equivalent to the CMYK values you entered? If so, then any colour stored in a .odt file will presumably be stored as RGB and not as CMYK.

If that is right it suggests the conventional wisdom that AOO only supports RGB is correct.

Also, if you specify a colour as CMYK does that mean that the RGB colour created and stored by AOO can be converted back to CMYK without any colour changes? So, if a printer asks for CMYK, could you use CMYK values to define the colours you use, and tell the printer that all your colours are defined as CMYK colours, but they are expressed as RGB colours? It would work for text and graphics created by AOO, but images, photos, graphics or text copied into AOO may not use "CMYK" defined colours.
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Clipboard01.gif
CMYK option in AOO
Last edited by John_Ha on Tue Oct 30, 2018 1:07 pm, edited 1 time in total.
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Re: What does the CMYK option in AOO do? How is it used?

Postby MrProgrammer » Fri Oct 26, 2018 5:09 pm

John_Ha wrote:I think the essential difference … is that RGB (Red, Green, Blue) is additive colour mixing [and] CMYK (Cyan, Magenta, Yellow, Black) is subtractive colour mixing.
Yes. See https://en.wikipedia.org/wiki/CMYK_color_model.

John_Ha wrote:If that is right it suggests the conventional wisdom that AOO only supports RGB is correct.
I'm not a developer. I'm a user, like you are, and not familiar with how the application is written. This wisdom is precisely what the Help page you quoted says, and I believe we have to trust the page unless our testing can prove that it's wrong.

John_Ha wrote:if a printer asks for CMYK …
If your printing vendor wants CMYK, OpenOffice is the wrong program to use since it only works with the RGB color model. Read about Scribus. It definitely supports CMYK, and Scribus appears to import OpenOffice documents, though I suspect some types of formatting are not translated perfectly.

If this solved your problem please go to your first post use the Edit button and add [Solved] to the start of the title. You can select the green checkmark icon at the same time.
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Re: What does the CMYK option in AOO do? How is it used?

Postby musikai » Sat Oct 27, 2018 1:32 pm

I', sure no CMYK expert and I suppose there are a lot of fine details about the RGB to CMYK color differences when it goes into printing.

But I wonder why the printers still require their customers to deliver CMYK pdfs and don't offer a conversion as they certainly have all the powerful tools available.

Cheap programs to use with CMYK support are: Xara Page&Layout Designer, Affinity Designer or as MrProgrammer told Scribus. In Scribus you can open your OO_RGB.pdf (use "open" not "import"), activate the color management for the document and save as CMYK PDF.

I created a little colored PDF in LibreOffice to test: RGB_oo.pdf
I couldn't attach the Scribus PDF here as it is too big (the fonts were converted to outlines)

Another possibility is to use Ghostscript for converting RGB to CMYK.
Code: Select all   Expand viewCollapse view
gswin64c.exe -o outputfile.pdf -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -dProcessColorModel=/DeviceCMYK -sColorConversionStrategy=CMYK -sColorConversionStrategyForImages=CMYK inputfile.pdf


it created RGB_to_CMYK_GhostScript.pdf

In my free project in the signature (LOSA) there is a little tool that can convert all PDFs in a folder from RGB to CMYK using an installed Ghostscript.
Attachments
RGB_to_CMYK_GhostScript.pdf
(108.27 KiB) Downloaded 39 times
RGB_oo.pdf
(71.1 KiB) Downloaded 33 times
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Re: What does the CMYK option in AOO do? How is it used?

Postby John_Ha » Sat Oct 27, 2018 3:10 pm

musikai wrote:But I wonder why the printers still require their customers to deliver CMYK pdfs and don't offer a conversion as they certainly have all the powerful tools available.

I agree - any printer should be able to accept RGB and print it.

The Difference Between CMYK and RGB in Digital Printing has the interesting point that many RGB colours cannot be converted to equivalent CMYK colours. Hence, if you print from an RGB item, the printed item will have different colours compared with how the RGB item looks on screen. It says:

It is important to view the image in CMYK before printing it out.

It is common for color changes to be evident when an image is converted from RGB to CYMK.

The discrepancy is evident because the majority of the shades in RGB cannot be easily reproduced by using CMYK inks.

This is the reason why the image should be converted to CMYK before printing.

Major color changes to the images can be noticed, especially in the color areas that are extremely intense.

So, if you are happy that the colours may be slightly(??) different on printing, then send RGB to the printer, and accept the variation.

If you want to ensure that the printed item replicates exactly what you see on the screen, then use an application to convert the RGB to CMYK. I assume that the CMYK to RGB conversion works OK and will view correctly on a screen. It is just the RGB to CMYK conversion which does not work well, where the most affected colours are the RGB bright colours which appear as much more dull CMYK colours when printed.

If you are printing only AOO created text and only AOO created graphics, then you could set the AOO colour palette to use only CMKY colour, which is presumably where the AOO CMYK option comes in. You then know that "what you see on screen" and "what you see on the print" will be the same. However, if you bring in any colour photo or any coloured text or any coloured graphic, it almost certainly will be using colours not in the palette you defined and therefore will be printed slightly differently.
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Re: What does the CMYK option in AOO do? How is it used?

Postby John_Ha » Sat Oct 27, 2018 3:18 pm

Also see Important Information About RGB and CMYK which has this comparison chart where you can see that there are far fewer colours (the gamut) available in CMYK (the inner blue line) compared with those available in RGB (the outer yellow line). Both have far fewer colours than the human eye can see.

Clipboard01.gif

It also says:

You most likely won't notice this kind of color shift in a color photograph.

It is more likely to happen if you pick a very rich, vibrant color for a background or some other element of your layout. It probably won't look bad, it just won't look exactly the same. But it may not be noticeable at all either.

suggesting my caution about photos above may be wrong.
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Re: What does the CMYK option in AOO do? How is it used?

Postby musikai » Sat Oct 27, 2018 3:53 pm

John_Ha wrote:I assume that the CMYK to RGB conversion works OK and will view correctly on a screen.


I don't think they will look exactly the same. I remember Xara's Page&Layout Designer having a special CMYK preview mode for that.
And then there is the problem of the monitor itself. There are some expensive monitor color calibration tools to purchase.
A friend who is a professional graphic designer always worked with a printed brochure showing only palettes of all different colors and their CMYK values, as this was the only reliable way to exactly get correct results on defined colors.
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Re: What does the CMYK option in AOO do? How is it used?

Postby John_Ha » Sat Oct 27, 2018 4:18 pm

musikai wrote:I don't think they [CMYK to RGB conversion] will look exactly the same

But, if you look at the image, every CMYK colour has an equivalent RGB colour.

But the RGB colours outside the CMYK gamut do not have equivalent CMYK colours.

So, although AOO uses an RGB colour model, AOO carefully chooses its RGB colours to be inside the blue line so that they have an exact equivalent CMYK value for when the document is printed.
Attachments
Clipboard02.gif
Last edited by John_Ha on Tue Oct 30, 2018 11:39 am, edited 1 time in total.
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Re: What does the CMYK option in AOO do? How is it used?

Postby musikai » Sat Oct 27, 2018 6:28 pm

Mh, ok, this makes sense! Great graphic!
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Re: [Solved] What does the CMYK option do? How is it used?

Postby John_Ha » Wed Oct 31, 2018 2:04 pm

The free GIMP - Gnu Image Manipulation Program uses RGB but offers rudimentary CMYK support with the Separate and Separate+ plug-ins - see GIMP/CMYK support.

The free Scribus offers both RGB and CMYK support - see How to create your own colours.

Googling RGB CMYK gets many hits, many of which are contradictory.
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