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In a discussion of creating a saturation mask, @dmstraker and @smadell referred to an equation involving channels:

Saturation=(max(SR,SG,SB)-min(SR,SG,SB))/max(SR,SG,SB)

The only way I can see to access these Source Channels is in  Apply Image. However Apply Image deals with two images, a Source and a Destination. This calculation deals with just one. The information on Field Equations in Filter > Distort > Equations  makes no mention of channels. 

How would I create a greyscale saturation layer from an RGB layer, which I can then convert to a mask?

John


Windows 10, Affinity Photo 1.8,5 Designer 1.8.5 and Publisher 1.8.5 (mainly Photo), now ex-Adobe CC

CPU: AMD A6-3670. RAM: 16 GB DDR3 @ 666MHz, Graphics: 2047MB NVIDIA GeForce GT 630

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55 minutes ago, John Rostron said:

In a discussion of creating a saturation mask, @dmstraker and @smadell referred to an equation involving channels ...

Do you have a link to that discussion?


Affinity Photo 1.8.4, Affinity Designer 1.8.4, Affinity Publisher 1.8.4;  2020 iMac 27"; 3.8GHz i7, Radeon Pro 5700, 40GB RAM; macOS 10.15.6
Affinity Photo 
1.8.4.186 & Affinity Designer 1.8.4.4 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 13.3.1

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Hey, John and R C-R...

First of all, the link to that series of posts is:  https://forum.affinity.serif.com/index.php?/topic/76900-saturation-masks-a-simple-method/

Second, Apply Image really only needs a single image (in fact, a single layer) since there is a button near the top of the dialog box that says "Use Current Layer as Source." Clicking that lets you apply the layer to itself. Here is a screen shot where I swapped the Red and Green channels in a photo (original on the left, swapped channels on the right).

1265296221_ApplyImageChannelSwap.thumb.jpg.c4958992bf40e7c51c2766f2940bedf0.jpg

The more important point might be that your last question is easier to answer than Dave Straker's math makes it out to be. I have posted a series of macros which will (i) create Saturation masks and Vibrance masks (inverted Saturation masks); (ii) create greyscale visualization layers for those masks; and (iii) allow you to apply the masks to a layer stack. You can access those macros here: https://forum.affinity.serif.com/index.php?/topic/87004-macro-updates-for-v17/

 

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59 minutes ago, R C-R said:

Do you have a link to that discussion?

Sorry it was not there originally. I cannot copy/paste the url when using Chrome on my tablet. Anyway, here is the link.

John


Windows 10, Affinity Photo 1.8,5 Designer 1.8.5 and Publisher 1.8.5 (mainly Photo), now ex-Adobe CC

CPU: AMD A6-3670. RAM: 16 GB DDR3 @ 666MHz, Graphics: 2047MB NVIDIA GeForce GT 630

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