jalamb Posted June 13, 2019 Share Posted June 13, 2019 Hello, I am trying to do some colour corrections using curves in CMYK but don't understand what is happening (or possibly Affinity is not working correctly). I have a colour image and I open the Info Panel and add two samplers, both of which I set to CMYK. I then add a Curves layer, and select CMYK. If I choose the Black channel, and move the curve around, the values in ALL the channels in the samplers change, not just the K values. Similarly, moving the Cyan channel curve makes changes to ALL the channels. What I am trying to do is to change a value of c74 m69 y64 k83 to become c80 m70 y70 k70, but I can't see how to do this. I realise that I can't put numeric values in the Curve (as in Photoshop) but even using the picker and dragging seems completely unpredictable. Quote Link to comment Share on other sites More sharing options...
Old Bruce Posted June 13, 2019 Share Posted June 13, 2019 2 hours ago, jalamb said: If I choose the Black channel, and move the curve around, the values in ALL the channels in the samplers change, not just the K values. I have a screen shot where I chose the Cyan channel but it is also available for the Black etc. I have all the channels editable in the Channels Studio and only choose the individual channels in the Curves Layer pane. It works for me but only with % which is what I guess you want? Quote Mac Pro (Late 2013) Mac OS 12.7.4 Affinity Designer 2.4.1 | Affinity Photo 2.4.1 | Affinity Publisher 2.4.1 | Beta versions as they appear. I have never mastered color management, period, so I cannot help with that. Link to comment Share on other sites More sharing options...
jalamb Posted June 13, 2019 Author Share Posted June 13, 2019 Ah, got it.... It only works properly if the file is actually in CMYK. The file I was using was actually in LAB, and then I tried with a file in RGB. The result was the same, with the screen shots below: moving just the Cyan channel, the Magenta value changes from 70 to 71. With the Magenta channel everything moves! If I convert the file to CMYK first, rather than just selecting CYMK in the Curves panel, then it does work. So that is better than not working at all. Thank you for the help. Quote Link to comment Share on other sites More sharing options...
Ron P. Posted June 14, 2019 Share Posted June 14, 2019 Pulling the curve down for Cyan, means you're subtracting Cyan, leaving more Magenta. Quote Affinity Photo 2.4..; Affinity Designer 2.4..; Affinity Publisher 2.4..; Affinity2 Beta versions. Affinity Photo,Designer 1.10.6.1605 Win10 Home Version:21H2, Build: 19044.1766: Intel(R) Core(TM) i7-5820K CPU @ 3.30GHz, 3301 Mhz, 6 Core(s), 12 Logical Processor(s);32GB Ram, Nvidia GTX 3070, 3-Internal HDD (1 Crucial MX5000 1TB, 1-Crucial MX5000 500GB, 1-WD 1 TB), 4 External HDD Link to comment Share on other sites More sharing options...
jalamb Posted June 14, 2019 Author Share Posted June 14, 2019 No. In CMYK Cyan Magenta Yellow are primaries so do not affect each other. Just the same as in RGB changing the amount of Red does not impact Blue. R C-R 1 Quote Link to comment Share on other sites More sharing options...
R C-R Posted June 14, 2019 Share Posted June 14, 2019 2 hours ago, jalamb said: No. In CMYK Cyan Magenta Yellow are primaries so do not affect each other. Just the same as in RGB changing the amount of Red does not impact Blue. But as @Ron P. said, if you pull the curve down for any of them, it means you are leaving more of the others. Quote All 3 1.10.8, & all 3 V2.4.1 Mac apps; 2020 iMac 27"; 3.8GHz i7, Radeon Pro 5700, 32GB RAM; macOS 10.15.7 Affinity Photo 1.10.8; Affinity Designer 1.108; & all 3 V2 apps for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 15.7 Link to comment Share on other sites More sharing options...
jalamb Posted June 14, 2019 Author Share Posted June 14, 2019 Of course not. There is not a set amount of ink which has to be used and if you reduce the amount of blue then more red ink will be used! But as I said previously, that does not happen with a CYMK document, only with an RGB or LAB document. So what is going on is that using a CMYK curve on a RGB document involves converting the image to CMYK, doing the curve, and changing it back. Clearly it is the conversion which is introducing the changes in the other channels. This is not something which Photoshop includes and now perhaps we see why. Quote Link to comment Share on other sites More sharing options...
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