RGB value associated with a CMYK value.

[Ickis]

Hello,

Background

I have got a couple of designs which have been created in Inkscape (a vector program) and are to be printed. The issue I'm having is that the printer requires CMYK colours. Inkscape does not really have any support for CMYK, but it does allow me to choose colours from the CMYK palette, but it will still save them as the equivalent RGB colours. I'm aware the term CMYK is not too helpful without specifying a colour profile, but as I say Inkscape doesn't really support CMYK in any meaningful way, so the concept of colour profiles is lost on Inkscape. Nevertheless, I know exactly what CMYK colours within the Coated Fogra39 profile, that I want to use, but the design and colours which must be the RGB equivalents must be set within Inkscape, and then have the SVG imported into Photoshop for rasterisation and conversion to the required CMYK profile.

Problem

My main problem is that I am trying to get the sRGB colour from Photoshop that equates to the required CMYK Coated Fogra39 colour, so that when Photoshop imports the sRGB SVG file, and I convert it to a CMYK Coated Fogra39 profile, I end up with the CMYK colour I want… I hope that makes sense.

Here’s what I’m doing:

1. Create a new Photoshop document with CMYK colours, with the Coated Fogra39 profile.
2. Choose from the colour picker the CMYK colour I want and fill the page with the colour – i.e. CMYK 40, 60, 0, 40.
3. Use the “Convert To Profile” to convert to the sRGB profile.
4. Use the Eyedropper tool to get the converted colour from the page – sRGB 119, 85, 125.
5. So, by rights that sRGB code should map to the chosen CMYK Coated Fogra39 profile.
6. But if I then convert this back to the CMYK Coated Fogra39 profile and check the eyedropper tool, I end up with a wildly different CMYK value of 61, 71, 29, 12.

Question:

So, my question is, how can I get an sRGB value that equates to a particular CMYK value within the Coated Fogra39 profile, so that importing an SVG containing that sRGB value would land me with the required CMYK value after converting to the Fogra39 profile?

Many thanks!

 

[thebestcpu]

Hi @Ickis

Another forum member may have a better way to achieve your desired result yet I will provide my inputs and approach.
First my inputs
- Your approach on using the eyedropper tool is a good starting point yet has some issues
- The sRGB value you get depends on some of the conversion settings such as if you have black point compensation turned on or not and also which rendering intent you have selected. It also can be potential impacted if you have the Use Dither option selected as well.
- Even more important is that there are many CMYK settings which will generate the same viewing color. The reason being is that when creating a CMYK color, the gray level can be partially achieved by using just K or a combination of CMY inks. You can check this by looking at the Lab numbers in with the eye dropper or in the color picker in Photoshop and see that using the came conversion options (e.g. black point etc) that the results Lab numbers are virtually the same.

It seems that if one could fix the K value one wanted to end up with in a CMYK conversion you might get a lot closer to what you want yet I do not know a way of doing that within Photoshop

However, though tedious it might be one way that would be spot on would be to bring you SVG image into Photoshop as a sRGB document, convert to CMYK and then manually one by one change the resulting CMYK colors with the final desired CMYK colors. Of course this is only feasible if the number of colors you need to convert is relatively small.

I do know that having specific CMYK numbers can be important especially in the printer has been calibrated with those specific CMYK numbers in mind.

Sorry I would not provide a cleaner answer (maybe another forum member will provide one) yet at least I hope you can understand the difficulties in the getting specific CMYK numbers in the conversion process.

John Wheeler

 

[Ickis]

Hi John,

Thank you for the reply. I've had a good play around with the conversion settings as you mentioned, but to no avail I'm afraid... Thank you nevertheless, shall await further contributions.

 

[thebestcpu]

Hi @Ickis
In seeing what Photoshop can do for you so exact CMYK values will be used from RGB images, I have a few questions.

- Is the Inkscape content a limited number of solid colors or is it more complex?
- I assume for each unique color you have specific CMYK numbers that must be used. Is that correct?
- do you have an example vector image (or representative portion of a vector image) that you could link to on a post to try out some techniques?
- When converted to a raster image does the image need dithering for softer edges or it will be at a high enough resolution so that does not matter?
- Is it critical that a specific CMYK number be used or can an equivalent CMYK number (same hue, sat, luminosity) be used? Just looking if there is any leeway or not.

Answers to the above will help to bound the problem which may help find a solution.

John Wheeler

 

[thebestcpu]

Hi @Ickis One additional note that may or may not matter yet the gamut space of the Fogra39 color space is not completely covered by sRGB. So there are some CMYK numbers that cannot be represented by sRGB.
Below I have include a 3D graph of the color space of sRGB (wireframe) and the color space of Fogra39 (solid color).
For the most part sRGB emcompasses most of Forgra39 except in some green/cyan areas at certain luminosites. That is show as a bulge outside the if the wireframe.
Just did not want you to be surprised if that comes up.



FYI
John Wheeler

 

[Ickis]

Hello there,

Regarding your questions:

>Is the Inkscape content a limited number of solid colors or is it more complex?
No, there are around 3 CMYK solid colours and no more.

>I assume for each unique color you have specific CMYK numbers that must be used. Is that correct?
Correct.

>do you have an example vector image (or representative portion of a vector image) that you could link to on a post to try out some techniques?
Not particularly, because it's not a design as such but simply a generic background with around 3 colours on that are arranged in a fashion yet to be fully decided. So even just three circles of different colours is sufficient for now.

>When converted to a raster image does the image need dithering for softer edges or it will be at a high enough resolution so that does not matter?
Ideally I'll be rasterising at around 600 DPI, so I would have imagined that dithering isn't required.

>Is it critical that a specific CMYK number be used or can an equivalent CMYK number (same hue, sat, luminosity) be used? Just looking if there is any leeway or not.
To be honest I'm not too au fait with hue sat luminosity... But providing it is percieved to be a similar colour then no issue. However, with that said, I was planning a solid background with brush marks over it, and the brush marks a slightly different shade of the background colour.

>the gamut space of the Fogra39 color space is not completely covered by sRGB.
Oh that's surprising. I'm not very familiar with colour spaces, but was under the impression that on the whole, CMYK profiles could be represented by sRGB, even with very small inaccuracies. I wasn't aware that there was such a solid part missing from the profile... It shouldn't be an issue mind you.

I'm actually a software engineer - hence my ignorance to certain things which you perhaps would assume to be common knowledge - and I've noticed that Photoshop has a method of scripting... I'd rather not as it is a bit overkill and is more akin to fighting Photoshop than working with it, but I considered brute forcing them sRGB values to find a CMYK value with the various settings that would come as close as possible to the required value... It just feels wrong though, and I'm struggling to believe that this issue is not a little more common.

 

[thebestcpu]

Hi @Ickis
I did come up with a way to convert from Inkscape to CMYK and have force Photoshop use the exact desired CMYK colors.
It does take a bit of initial setup and overhead os for just a couple images with only a small number of colors, there is another way that I would recommend.

First is to create the color swatched that match the desired CMYK color numbers. Though I have not seen this generally documented, color swatches and the foreground and background color chips store both color numbers and the color mode used to create the color (e.g. RGB, HSL, Gray, Lab, CMYK). The trick is how to include the color mode with the swatch.

For CMYK, open up a PS document in CMYK (not necessary yet I recommend opening up in your Fogra39 color space.
Then with the color picker, only use the CMYK numbers in setting the desired color values. Do not use the Hex, RGB, HSB, or Lab sections.
When you do this the Color Swatch will recored the exact CMYK values and include it is CMYK color (no conversions to other color space numbers)

The following image emphasizes where to change the CMYK numbers to create CMYK color swatches while you are in a CMKY PS document:


Create all of you exact Color Swatches this way and best to save them in their own Color Swatch Group. This includes any background colors that you will using as well.

Since you are using Inkscape go ahead and copy down the RGB numbers to use as well from the Color Picker.

After creating your SVG file, open it in Photoshop sRGB with the dither option NOT checked. This allows all of the colors to be solid colors on a pixel by pixel basis in Photoshop.

Now convert the PS document to CMYK mode in Forgra30 color space. At this point the CMYK colors will be close yet not exact.

To make they exact, use the magic wand tool with a reasonable yet small tolerance, with no antialias, and do not check contiguous and select one of the colors.
Click the Color Swatch to load it into the foreground color chips that contains the exact CMYK colors that you want and then fill the selection with that foreground by using the fill tool or even easier use the keyboard shortcut Opt Delete (use Alt on a PC with Delete/Backspace).

This replaces the close CMYK color with the exact CMYK numbers you want.

Do the same process for other small number of colors and you are done except for saving your image in a suitable format for printing.

When you have a small number of projects with a small number of colors, this would be the approach I would recommend.

Hope this helps
John Wheeler

PS - If you have any issue or need any clarification just ask.

 

[Ickis]

Absolutely wonderful! Thank you so VERY much...

 

[thebestcpu]

Glad that works for you @Ickis and you are very welcome