Author Topic: Control over CMYK black export;  (Read 688 times)

Yanis

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Control over CMYK black export;
« on: October 20, 2020, 04:29:00 pm »
Hello to all and thank you in advance for your time!

Is there a way to ensure that the cmyk values I choose for a colour (and, most importantly, black) remain the same after export? My document contains rgb images. If I use a cmyk black backgound (30,30,30,100) and then export for print the background CMYK value has changed and K is not even 100! It is also obvious in the pdf that it is less black than the black of the (initally rgb) images.

And another thing I don't understand. I imported an rgb (0,0,0) image to see how that would work as a background and it was assigned as registration colour. It looks good when exported, but I understand that registration colour should not be used for content.

Thnx again!

mnawij

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Re: Control over CMYK black export;
« Reply #1 on: October 22, 2020, 05:08:33 am »
Hi Yanis, first a few questions, is it pdf format that you are exporting to? Color shift might be from color management if you have it turned on... Which operating system are you using and which version of Scribus are you using? Could you upload a sample scribus file and corresponding pdf file to look at? Thanks!

Cheers!

mnawij

Yanis

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Re: Control over CMYK black export;
« Reply #2 on: October 22, 2020, 02:55:50 pm »
Hi mnawij and thank you for your time. Yes, I am exporting to PDF for print. No, I don't have color management activated. I am using Ubuntu 20.04.1 and Scribus 1.5.5. I have attached a three layer sample (the top third of the page is an rgb(000) black image I made in Gimp).
Thnx again!

jghali

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Re: Control over CMYK black export;
« Reply #3 on: October 22, 2020, 03:47:00 pm »
>> If I use a cmyk black backgound (30,30,30,100) and then export for print the background CMYK value has changed and K is not even 100!

I checked the PDF by opening it with a PDF object browser. The CMYK values of both rectangles are correctly exported:
0.60000 0.40000 0.40000 1.00000 k
0.30000 0.30000 0.30000 1.00000 k

>> It is also obvious in the pdf that it is less black than the black of the (initally rgb) images

CMYK devices cannot reach blacks with luminosity as low as RGB black. If I checks the RGB(0, 0, 0) conversion with Scribus default profiles, CMYK (30,  30, 30, 100) values are too low to reach the blackness of resulting CMYK values (75, 67, 63, 95).

Here are the luminosity expected for these different values in CIELab space:
- RGB(0, 0, 0) : 0
- CMYK(75, 67, 93, 95) : 11.67
- CMYK(60, 40, 40, 100) : 11.23
- CMYK(30,  30, 30, 100) : 12.88

In CIELab space, any color difference above 1 is expected to be noticeable. The luminosity differences between RGB(0, 0, 0) and the different CMYK values is hence quite more than noticeable.

Yanis

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Re: Control over CMYK black export;
« Reply #4 on: October 22, 2020, 05:27:28 pm »
Thank you for your help jghali. Can you recommend a PDF browser for linux that allows to view pixel color values? If I open it with photoshop or libreoffice draw the values are not correct :(

a.l.e

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Re: Control over CMYK black export;
« Reply #5 on: October 22, 2020, 06:27:46 pm »
you can open the pdf with scribus and see what is inside of it...

jghali

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Re: Control over CMYK black export;
« Reply #6 on: October 22, 2020, 06:36:52 pm »
>> Can you recommend a PDF browser for linux that allows to view pixel color values?

Unfortunately I don't know any. As a.l.e mentionned it, you could still "open" the PDF with Scribus. However you have to always remember that Scribus is not a PDF editor and that sometime we have to make some adjustments to accommodate with Scribus graphics capabilities.

>> If I open it with photoshop or libreoffice draw the values are not correct

Photoshop will usually apply some color management when opening PDFs, especially if PDF contains CMYK colors and PDF is rendered to RGB. Don't know about libreoffice, but I'm not aware libreoffice has decent CMYK color support.

Yanis

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Re: Control over CMYK black export;
« Reply #7 on: October 22, 2020, 07:02:10 pm »
Thank you a.l.e as well for your contribution.

I opened my file (not the test file that I have attached on my first post) with scribus to find out that Scribus assigned two different values to images which were initially rgb blacks ((6,6,9) and (13,12,11)). (for the record, two same-value blacks appear the one as gray and the other as brown in my eyes). In the file I attached the rgb black values is (5,6,6).

Am I to understand that Scribus rgb to cmyk algorithm will assign rgb(000) a cmyk value according to the rest of the image or even document content?

Extra question: Why Scribus "sees" rgb colours? Isn't the "for print" pdf export a cmyk document?

jghali

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Re: Control over CMYK black export;
« Reply #8 on: October 22, 2020, 07:30:04 pm »
>> Extra question: Why Scribus "sees" rgb colours? Isn't the "for print" pdf export a cmyk document?

Not always, depending on settings Scribus can export RGB colors as is, noticeably if any "use profile" option is checked in color tab of PDF export dialog. This enable printers to perform color management on RIP with ICC profiles usually more accurate than the ones available to user.
« Last Edit: October 22, 2020, 07:45:05 pm by jghali »

jghali

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Re: Control over CMYK black export;
« Reply #9 on: October 22, 2020, 07:40:10 pm »
Also Scribus color picker does not support CMYK. You will only get RGB colors from it. So you cannot use Scribus color picker to analyze imported CMYK content. You can only go to Edit > Colors and Fills and check how CMYK colors have been imported. For images you would need to disembed them for Scribus file and inspect them in a CMYK capable image editor.

Yanis

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Re: Control over CMYK black export;
« Reply #10 on: October 27, 2020, 12:27:39 am »
Ok, I understand there is a lot I don't know about...

To return to the base of my concern, is there a way to ensure that I will have the same black in different pages/images?

(For the record, when opening pdf with Scribus, it did "read" correctly the cmyk values that had been defined in it when creating the document. The problem occured with the png/jpg images.)

a.l.e

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Re: Control over CMYK black export;
« Reply #11 on: October 27, 2020, 10:22:00 am »
well, i guess that the solution is probably to use the same black?

i'm a bit lost in all the tries you did, but if you know which black you have in your image, just use the same one for the background shape.

and let the color management handle the conversion.

but you have to know the value, not guess it with the system or scribus color picker.

utnik

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Re: Control over CMYK black export;
« Reply #12 on: October 27, 2020, 01:35:28 pm »
…if you know which black you have in your image, just use the same one for the background shape.

and let the color management handle the conversion.

but then you should set the same 'rendering intent' for images and solid colors. with the standard settings ('perceptual' for images and a 'colorimetric' variant for solid colors) you may still get visual differences!

utnik