AP: Saturation mask (experimental)

[dmstraker]

I've been puzzling over how to create a saturation mask, where the darkness/lightness mask represents the saturation of the colours. You can select by hue and luminosity, but not by saturation level, which is a shame as it is possible in PS. Here's what I did, with images.

Note: All adjustment layers are above the image, with the whole set collected in a group.

Initial image has overlaid strips of RGBKMYC, fading from white and to black. They are at greatest saturation in the middle, so we should see this being selected.

First step is to add an HSL layer, dragging Saturation to zero. Then set Blend Mode to Subtract.

Here we can see the central areas of the strips are coming through, which is what we want (I tried Difference blend but it wasn't the same).

There's more detail in there, but we can't see it at the moment, so add an Invert layer.

To make it black and white for masking, add a Black/White layer.

This is all rather faded, so add a Levels layer and drag the Black up to meet the histogram and improve contrast.

We can now create a mask from this. Invert it again if you like to select the more saturated colours.

Here's a quick example, with the mask loaded to a Curves layer which has been pulled up to affect lower saturation areas (note the lighter and darker areas of the strips are lifted) but higher sauration is not (as can be see in reds such as the draining board stand).

As this effectively converts saturation to luminosity, areas of it can be selected with Blend Ranges, for example just the most or least saturated.

The principle could also be applied to any other adjustment, subtracting it from the original and turning this into a mask.

Bottom line - does this make sense? Have I fallen over any tripwires?