[ColorWidget] Add CMYK support

[troopa81]

This is the first PR regarding CMYK QEP

Adds CMYK support in color widget so we can select CMYK color.

A new setting activeCmykComponent has been introduced, while the old one activeComponent refers now only to the Rgb/Hsv part

Funded by Métropôle de Bordeaux

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 1ad2b4b)

[nyalldawson]

@troopa81 There's a fair amount of wasted space in the dialog caused by putting the combo box in the top left corner. Could we try moving it here instead?

How does this look in the vertical orientation mode used in the styling dock color buttons too? Can you share a screenshot showing that?

[nyalldawson]

Question: is the use of rounded integer 0-255 values an issue when using CMYK colors? It's been on my wishlist for a while to revisit these widgets and make them return qreal values for the color components, so that we aren't losing information when a color is changed via the widget and are better equipped to handle higher precision color models in future (since these are already supported by both Qt and our color reading/writing code, it's really only the color widget itself holding us back).

[troopa81]

Question: is the use of rounded integer 0-255 values an issue when using CMYK colors?

As much as RGB color I think. Do you have a specific issue in mind ? or just the lack of precision when you use the color wheel widget for instance

[troopa81]

@troopa81 There's a fair amount of wasted space in the dialog caused by putting the combo box in the top left corner. Could we try moving it here instead?

Way better indeed. Done and updated video accordingly

[spacehobo]

Question: is the use of rounded integer 0-255 values an issue when using CMYK colors?

As much as RGB color I think. Do you have a specific issue in mind ?

One reason to use CMYK is so that you can hit very specific spot colours used by your printers. This allows prints to get very high DPI at the tones you specify, because they don't need to use screens to dither various colours together.

If I can't specify the exact percentages from my printer's catalogue of spot colours, and have them propagate all the way through to a CMYK-colourspace PDF put out by the Layout Manager, then the result will be a very expensive print of a map with degraded quality.

[troopa81]

@nyalldawson I remoed the warning message

@spacehobo Good point, I'll try to deal with this in another PR if I have the time/budget.

[nyalldawson]

Just one little change left

[qgis-bot]

@troopa81

This pull request has been tagged for the changelog.

• The description will be harvested so please provide a "nearly-ready" text for the final changelog
• If possible, add a nice illustration of the feature. Only the first one in the description will be harvested (GIF accepted as well)
• If you can, it's better to give credits to your sponsor, see below for different formats.

You can edit the description.

Format available for credits

• Funded by NAME
• Funded by URL
• Funded by NAME URL
• Sponsored by NAME
• Sponsored by URL
• Sponsored by NAME URL

Thank you!

[tannenfreund87]

Question: is the use of rounded integer 0-255 values an issue when using CMYK colors?

Usually CMYK values are given as percentages of the maximum amount of ink for the specific print. At least here in Germany. So if I get values, I need to convert them from percent to integer values. Otherwise, the colours become too light. See for example the table starting on page 11 of this pdf:

Anhang_zu_den_Richtlinen_für_die_Forsteinrichtung_im_Körperschaftswald.PDF

https://www.gesetze-bayern.de/Content/Document/BayVwV257553-78

[troopa81]

Usually CMYK values are given as percentages of the maximum amount of ink for the specific print.

That makes sense.

GIMP displays it in percentage (CMYK in its last version)

https://librearts.org/2022/08/gimp-2-99-12-cmyk/gimp-2-99-12-sample-points.webp

Krita offers to switch between % and 0-255 range

Not sure this is relevant to display CMYK in 0-255, so I'll propose a modification to display it always in percent.

[andreasneumann]

I just double checked with the "Swiss World Atlas" team (the printed school atlas) - and they confirmed that they enter values in percentages (usually 5% step increments).

[troopa81]

@andreasneumann thanks for the confirmation. Let's go for percent so!

[nyalldawson]

@troopa81 a couple of thoughts:

1. We should offer a toggle (like gimp, inkscape, etc do) to switch between raw values and percent
2. This should be respected by both CMYK color components + RGB + alpha
3. We should ensure we aren't using setting the QColor components via integer values, but rather using the float methods to avoid loss of precision

[troopa81]

We should offer a toggle (like gimp, inkscape, etc do) to switch between raw values and percent

I was afraid you said so :) Why not, but I'm not sure exactly what is the avantage of selecting CMYK in raw value, or RGB in percent ?

We should ensure we aren't using setting the QColor components via integer values, but rather using the float methods to avoid loss of precision

Yes, It's in my todo list

[nyalldawson]

@troopa81

I was afraid you said so :) Why not, but I'm not sure exactly what is the avantage of selecting CMYK in raw value, or RGB in percent ?

I'm unsure if there's any valid use case for setting CMYK as raw values, and I'm not opposed to just showing in percent always. (We've always shown HSV/alpha values as percent and not the raw values and there's been no requests in decades to expose these as raw values!)

RGB as percent is important if we remove the forced integer rounding... and I'd argue that for many users setting a value to 50% percent is a much more natural choice then "127.5". But I don't want to add GUI work for you for my wants, so I'm happy to add this toggle myself after we get the non-int rounding in place 👍

[spacehobo]

I'm unsure if there's any valid use case for setting CMYK as raw values, and I'm not opposed to just showing in percent always. (We've always shown HSV/alpha values as percent and not the raw values and there's been no requests in decades to expose these as raw values!)

My primary goal for CMYK colours is to be compatible with printers' "spot colours" when generating a PDF for submission. All printers I'm aware of use percentages (although they don't show the % symbol) and I'm slightly nervous that conversion errors between byte ratio values and decimal could "break" a spot colour back into a screen mix.

[troopa81]

@spacehobo This has been taken care in #58376 so now CMYK color are edited in percent, not raw values.