blend modes in CMYK

[NotMyFault]

Hi,

how are blend modes difference and subtract defined for CMYK documents?

1. create new cmyk document
2. add a colourful image
3. add HSL adjustment, set situation to 0
4. merge visible
5. deactivate or delete HSL adjustments.
6. now play with blend mode of top (desaturated) layer. I get full black (meaning exactly 0 values) for blend mode difference and subtract. this is unexpected for me.

Anyone able to explain?

[walt.farrell]

I see the same thing, but I have no explanation.

[Old Bruce]

10 hours ago, NotMyFault said:

how are blend modes difference and subtract defined for CMYK documents?

1. create new cmyk document
2. add a colourful image
3. add HSL adjustment, set situation to 0...

I am curious as to whether or not you are seeing a truly desaturated "colourful image" after setting the saturation to 0 (zero). Here on Mac I get a colour cast at that point. Not so with an RGB image.

So I think the problem is with CMYK. Keep in mind that their are four (greyscale) channels and they are negatives in comparison to the three channels and them being positives.

[NotMyFault]

1 hour ago, Old Bruce said:

I am curious as to whether or not you are seeing a truly desaturated "colourful image" after setting the saturation to 0 (zero). Here on Mac I get a colour cast at that point. Not so with an RGB image.

So I think the problem is with CMYK. Keep in mind that their are four (greyscale) channels and they are negatives in comparison to the three channels and them being positives.

Yes, in CMYK there is still about 10% saturation left after desaturating.

after googling around a bit on stackoverflow etc everybody says blend mode and adjustments are not suitable form CMYK because of the subtractive color model. 

It seems nobody made a tutorial what works in CMYK and works doesn’t work.

The basic advise (i fully agree with) is to do all editing in RGB (or GREY or LAB), and only flatten / export finally to CMYK. You can use normal / paththrough blend modes, and single-channel adjustments, but the rest is „creative art“ and not predictable.

[NotMyFault]

I just found the explanation:

• blend mode subtract gives full black on the K channel (this is something else to investigate)
• this 100%K will overshadow all other color channels. If you use the channels panel to inspect the CMY channels individually or combined but excluding K, you see a correct result per channel. 
• but if you activate the K channel, the result will be full black.

To summarize: blend mode subtract correctly works on individual CMYK channels by subtracted them individually, but you must inspect the result individually.

[lacerto]

Obsolete.

[lacerto]

Obsolete.

[NotMyFault]

Thanks @lacerto. The main topics of my question were:

1. Do the 4 arithmetic blend modes work natively in CMYK, or is there a temporary transformation to e.g. RGB or HCL color format (like in BM HUE, COLOR)?
   Answer: they work natively in CMYK. Meaning every color channel is calculated individually, like in RGB
2. Is it possible to use BM subtract/difference in CMYK in a similar way as in RGB to visualize differences between layer content ?
   Answer: Only restricted. You need to use channels panel to focus on one single color channel, otherwise the K channel will render everything black. Another hurdle is that channels panel misses a feature to automatically invert color channels (selectable as toggle), otherwise the brightness is always inverted when switching from all channels to individual channels. 
3. I will continue trying to find a way representing small differences between layers in CMYK in a more practical way. 

[lacerto]

Obsolete.