@gungan - thanks very much! for that link, which is interesting and in turn leads to other very interesting links with lots of good information.
Regarding the PhotoShop “gamma” checkbox, I don’t have access to current PhotoShop, and have to go way out of my way to set up a very old version of PhotoShop. But see this article:
Other people have checked various versions of PhotoShop for me, and I can confirm for an older version of PhotoShop, that only some operations use linear RGB even when the “gamma” checkbox is checked. Except for 32-bit floating point precision - then AFAIK PhotoShop linearizes all operations.
Run the tests in the article on GIMP-2.10 using “default” for everything, and then again using “legacy”. Please use 32-bit floating point precision. At 8-bit precision there is not enough shadow information to prevent posterization when working on linear RGB, which I think is the source of what you have described as “not smooth” when using default brush strokes and blend modes.
The whole point of high bit depth GIMP 2.10 is that unlike 8-bit GIMP-2.8, GIMP-2.10 operates on linearized RGB whenever appropriate to avoid “gamma” artifacts. Most GIMP operations do use linear RGB except of course for “legacy” mode. The few operations that don’t operate on linear RGB, use perceptual RGB for various good and carefully considered reasons. For example, Posterize is supposed to group tones “perceptually”, not linearly. Some operations make it really easy to operate on linear or on perceptual RGB at the user’s choice. And most operations do allow to choose one way or the other, but “how” isn’t always obvious.
See this article and especially look at the pictures:
Parts of the article are out of date. For example, now GIMP does allow operating on linear RGB for Levels and Curves. And changing between linear or perceptual precision does no longer affect the layer blend modes, for GIMP-2.10 that’s done using “right-click/change Composite/Blend/etc” as we’ve already talked about, or by using “Legacy” blend modes.
For people who’ve always painted using 8-bit images and so necessarily perceptual RGB instead of linear RGB, getting used to working with linear RGB takes some time. The Blender/VFX/3D people started making that change many years ago and after sufficient ranting and user education and changes in software, finally it’s easy to use linear RGB in the relevants softwares.
It’s odd how long it’s taken artists/photographers and painting/editing software to start using/allowing painting/editing using linear RGB. But after years of everyone “doing it wrong” because of 8-bit software limitations, switching to using linear RGB really is an adjustment, which is the point of GIMP-2.10’s “legacy” blend modes. And sometimes artistic choice trumps “technically correct”, which is why usually in GIMP-2.10 there’s a way to “do the technically wrong thing for the right artistic reason”.