CMYK document with levels adjustment in rgb color mode

[NotMyFault]

Hi,

1. Create any cmyk document 
2. add some content, e.g. rectangular shape with any color gradient (over CMY, not K only)
3. add gradient map adjustment, using cmyk colors
4. add levels adjustment: no visual change
5. set adjustment layer color space to RGB (or LAB): drastic shift in colors. All colors become severely reduced.
6. same happens when using channel mixer instead of levels
7. nothing happens when using curves adjustment Update: you need to add one node in curves before it has any effect, but then you get identical results to levels / channel mixer.

By design or bug?

it smells more like a bug.
 

PS: probably related to gradient map adjustments. added test document.

cmyk gradient map issue.afdesign

[Lisbon]

17 hours ago, NotMyFault said:

probably related to gradient map adjustments. added test document.

I agree.
The gradient map is doing something (sorry for the choice of words) and it seems related to the luminosity component. The color seems to be fine.

The values remain the same if you add a Levels adjustment set to Colour blend mode and changing CMYK to RGB/Lab
If you do the same in Luminosity blend mode, you will see a change.

I only tested this with your file. I havent tested it with other gradients.

[Old Bruce]

17 hours ago, NotMyFault said:

• set adjustment layer color space to RGB (or LAB): drastic shift in colors....

Why wouldn't there be? Smaller gamut vs larger gamut.

[NotMyFault]

4 hours ago, Old Bruce said:

Why wouldn't there be? Smaller gamut vs larger gamut.

In this case it is just the opposite way. I would understand your argument in a RGB document using a levels adjustment in CMYK mode. But here we have a CMYK document and RGB mode adjustment. Does not make sense-

[NotMyFault]

I may have found the cause.

The black to white gradients where based on CMY colors (no K).

The adjustment layers in RGB color mode convert to K only colors (CMY set to 0).

unfortunately there is a shift in brightness introduced.

by adding a curves adjustment it is possible to compensate this almost

 

[Old Bruce]

1 hour ago, NotMyFault said:

But here we have a CMYK document and RGB mode adjustment. Does not make sense-

Why not use the CMYK mode for the adjustment?

The block from the RGB disappears if I do that. Difficult to see but it is there.

RGB

CMYK

 

[NotMyFault]

8 hours ago, Old Bruce said:

Why not use the CMYK mode for the adjustment?

Because this option is there and i want to fully understand how it is working.

And if an adjustment with all sliders to „zero“ creates a visible effect, i have a problem to understand how it works.

This thread is not about how to achieve a certain effect, it is more specifically about to understand why the different methods (working in sRGB) are not working properly in CMYK. It is about „why“. And please be specific, generic statements about how the color models differ do not help in this case. I know about additive / subtractive color models, different color gamuts, etc. This should be no factor for grey gradients.

 

[NotMyFault]

14 hours ago, Lisbon said:

I agree.
The gradient map is doing something (sorry for the choice of words) and it seems related to the luminosity component. The color seems to be fine.

The values remain the same if you add a Levels adjustment set to Colour blend mode and changing CMYK to RGB/Lab
If you do the same in Luminosity blend mode, you will see a change.

I only tested this with your file. I havent tested it with other gradients.

Yeah this leads into the right direction. 

The luminosity shift is causing the issue. And it is caused by converting CMY based gray tones into K based grey tones in a way not giving the same luminosity as before, this is the whole bug.

[thomaso]

On 7/12/2022 at 10:44 PM, NotMyFault said:

• add levels adjustment: no visual change
• set adjustment layer color space to RGB (or LAB): drastic shift in colors. All colors become severely reduced.
• same happens when using channel mixer instead of levels
• nothing happens when using curves adjustment 

I seem not to understand what you are asking for. Your document does not contain your mentioned Level and Curves Adjustments, so it's unclear to me where you did place them . If I add them as top layers I do see a visual change for all layers below and regardless which colour mode I use for the change but with different results in the various modes.

Also I wonder why UCR versus GCR was not mentioned in this thread since it affects conversion between the 3 channel vs. 4 channel colour modes. There is no such a choice at any place in Affinity UI, which makes it more difficult to know what gets used and when.

It also reminds me to this recent response to the question about an Invert Adjustment in CMYK.

Another trial: I would expect that a 50% colour would not change if inverted but it appears that RGB 50/50/50 results in a quite dark / black occurrence while CMYK 50/50/50/50 keeps its luminance but gets slightly tinted. I assume the latter is related to cmy vs cmyk (GCR/UCR) but how come that the 50% mid RGB gray results black?

[NotMyFault]

50 minutes ago, thomaso said:

It also reminds me to this recent response to the question about an Invert Adjustment in CMYK.

That thread is what actually got me started to dig deeper - and try to understand how all this works in Affinity.

Inversion kind of works if you limit it to either CMY or K, but does not work if you mix both cases. 
I use levels/curves/channel mixer as replacement for invert adjustment because you can control per color channel, whereas invert uses all CMY and K channels.

 

[NotMyFault]

To give you a fresh and focused view on this topic. I create a new test document, which contains 3 gradients:

1. top CMY only - 100/100/100/0 to 0/0/0/0
2. middle CMYK - 100/100/100/1000 to 0/0/0/0
3. bottom K only - 0/0/0/100 to 0/0/0/0

combined (2)

CMY channels only (1)

K channel only (3)

If you add a levels adjustment (neutral settings) in CMYK: all fine

If you change mode to sRGB, 

You see that the histogram changes, more larger values.

In the channels panel, you can see that actually all colors are converted into K channel. CMY became pure white. 

But the conversion shifted the lightness drastically. this shift is unexpected, I assume a bug.

cmyk grey contribution.afphoto

[thomaso]

1 hour ago, NotMyFault said:

But the conversion shifted the lightness drastically. this shift is unexpected, I assume a bug.

I see what you mean. It is indeed confusing that this level adjustment without any change does influence the colours as soon its colour mode gets switched from CMYK to RGB (respectively as soon an 'empty' adjustment with RGB mode selected gets activated, as in your document).

I don't understand what's going on. It reminds me to the difference when the "K Only" button gets pressed for images: from a CMYK image it picks 1 channel only and simply hides the other 3 – while for RGB it does a colour calculation of its 3 channels.

Applied to images your observed change appears even more strange, as if colour gets added (for K channel only) respectively subtracted (for CMY) as soon this adjustment gets activated – as if a colour conversion from CMY to K is happening if RGB mode in an adjustment gets activated.

[wonderings]

No answers but why are you trying to mix CMYK and RGB? If the intent is for final output in print then I would say do everything in CMYK as your RGB is going to have to be converted anyways and better to work with the colour family that will be printing it. And if not printing why not use RGB completely as it does have a wider gamut? 

[Old Bruce]

With all due respect I must say that this is like asking "Why are different things different?"

[NotMyFault]

4 minutes ago, wonderings said:

No answers but why are you trying to mix CMYK and RGB? If the intent is for final output in print then I would say do everything in CMYK as your RGB is going to have to be converted anyways and better to work with the colour family that will be printing it. And if not printing why not use RGB completely as it does have a wider gamut? 

Because the function to use curves/levels/channel mixer with deviating color mode is available in Affinity, and i want to understand how it works (for what it is usable).

The most prominent use case is use it from sRGB (or CMYK - this is the question of this thread) documents, set to LAB, and use the lightness channel of LAB to adjust lightness without affecting saturation. Another is to use GREY for quick&dirty B&W conversions, without need to enter the correct factors in B&W adjustment.

As James Ritson is stating in his tutorials - just experimenting. And learning. As there is virtually no information available covering these topics (in help / video tutorial / spotlight / googling around), i can only try it all these functions and kind of reverse engineer what there are actually doing.

[NotMyFault]

10 minutes ago, Old Bruce said:

With all due respect I must say that this is like asking "Why are different things different?"

Are you reflecting to wondering post or my thread?

Assuming my original question:

sorry to disagree. I want to get an exact, complete, understandable definition what these functions do. Affinity is providing lots of functions who are „well known“, either form PS, or from publicly available sources (Wikipedia, text books, …), or who are 100% „unique“ by Affinity, and nobody is able to use them consistently without getting a proper documentation how they work, or what results you are getting. So we need to experiment and try to understand what how it works. 

 

This thread is not „how does RGB differ from CMYK“. This thread is about what does all this adjustments, filters, blend modes etc give you as result, how does this match what is public knowledge (mostly related to PS as de-facto standard, or explicitly documented in help or tutorials by Affinity).

[thomaso]

27 minutes ago, Old Bruce said:

With all due respect I must say that this is like asking "Why are different things different?"

If I understand the reason for a difference, I don't have to learn the difference by heart. – While in this thread the expected appearance additionally matters … if suddenly the apple looks like a pear ;•)

(compare the various debates about "leading override", some say its entirely useless, some want it to be the paragraph leading there in the character panel, and all critics asks why it is the way it is.)

[Leigh]

Both Technical Support/QA have been looking into this and we "think" it's likely expected behaviour. I quickly tried to compare the same process in similar apps to see if they gave different results but was unable due to the apps requiring the same colour mode for both the adjustments and document. Have you been able to compare the results to other apps at all?

The process you're using is very specific and is not something most users will run into. I completely understand your comments about wanting to get a detailed definition on how our functions are implemented and work at a low level but this information is only something our developers can comment on. As you're aware, our developers time is finite and as stated above, this is a specific workflow that most users won't come across, so it may take some time getting the information you require, if possible.

[joe_l]

31 minutes ago, Leigh said:

The process you're using is very specific and is not something most users will run into.

Add me to those experiencing the same or similar "problem". CMYK document + RGB image with Selective Colour adjustment = Screen colours ok, Print colours too light. Changing document to RGB = Screen colours too light.

[Leigh]

On 7/15/2022 at 11:23 AM, joe_l said:

Add me to those experiencing the same or similar "problem". CMYK document + RGB image with Selective Colour adjustment = Screen colours ok, Print colours too light. Changing document to RGB = Screen colours too light.

@joe_l Seems like you have a different issue - please start your own thread and provide as much information as possible and we'll take a further look.