I am just wondering if there are any viewpoints or warnings about using purity boost in a similar fashion to using saturation boost options in AgX. I read the user guide and interpret that purity boost will have limited effect in highlights.
I often boost the blue to ‘improve’ blue skies in landscape, but I am toying with the idea of using master boost instead of the saturation slider in other images.
I can’t find the post currently, but iirc Kofa suggested using the primaries to tune the N6/highlight behaviour and using other tools for most colour adjustments. The math for the purity boost can apparently also break down for higher values.
On the other hand: if it works for you I guess there is no wrong way of boosting saturation.
I find Color Balance to give a more natural saturation than the purity boost in AgX. However, adding contrast in AgX naturally comes with increased saturation, in a very pleasing way.
(It pains me a little to say this, as this has been a bug-bear of mine with Lightroom, where contrast and saturation were similarly linked, before they introduced the luma tone curve. And now I actuallylike that linkage 
)
Here’s how I think about this today: there are two types of contrast adjustments:
There’s a technical need to increase contrast and saturation to compensate for the loss of dynamic range from scene to screen. Going from 30k nits in a scene to a 300 nit screen or a 100 nit print reduces our perception of contrast and saturation. To retrieve the original perception despite the lowered brightness, contrast and saturation need to be boosted. It makes sense to link contrast and saturation for this purpose, and that’s where I see AgX.
Second, however, is artistic contrast and saturation in the rendered images. Here the two controls should be independent, and indeed should be split into highlights, shadows, saturation, purity, brilliance. Color Balance is the much better tool at this stage.
Go to AGX Color Transform Simulation
Click the button Blender defaults.
Enable the display of the ‘outset’ space.
Then, try drastically increasing the purity boost, and keep an eye on the ‘outset’ space, and on the colour diagram.
In other words: don’t overdo it (and it’s not that hard to overdo it).
Thanks for the replies. I confess that I like to do my saturation and chroma tweaks usually in color balance RGB and I should have mentioned this. However, I like how I can boost blues alone in the primaries of AgX and getting more satisfying skies. So today when editing an image that included blue skies and a rainbow from Victoria Falls I experimented with boosting all three color sliders and then noticed the master purity and boost and wondered about the difference or similarity to the saturation slider in AgX.
I went to the AgX link Kofa sent above and saw problems with lifting boost in that model, but when I opened my image in DT and boosted master purity boost to 200% I didn’t get any adverse effect except a very colorful image. It probably looked nicer than the heavy handed comparison I did with the saturation slider in AgX.
It seems to me that the primary boost sliders for RGB are a nice way to boost individual color channels that may need a lift. Similar to color zones or color equalizer.
Well, that JS simulation may have somewhat different maths, it’s purely LLM coded, I didn’t want to spend too much time on it.
But here is a quick demo in darktable. Note that by boosting blue, we don’t only affect blue:
Thank you Kofa for taking time to reply and demonstrate what you mean. It seems only a problem if you do sledge hammer edits with the sliders rather than subtle changes when boosting colors. AgX is a wonderful tool. It is a complex tool that has achieved simplicity in its use.
It‘s important to keep in mind that AgX is a tonemapper and not intended to be an all purpose editing module. All those primaries sliders are there to mitigate or fine tune effects in tonemapping process.
