How do I use the “Standard Deviation” and “Scale” parameters in Gimp 2.10's GEGL-based unsharp mask?

Question

Previously, Gimp 2.8 and earlier used a dialog with the familiar "radius, amount, threshold" parameters as described at Unsharp mask - what do the parameters do?, like this:

But in Gimp 2.10, we instead have this:

with "Standard Deviation" and "Scale".

The old dialog has Radius capped at 500 (useful for local contrast on high-megapixel images) and Amount capped at 10.00 (useful only for identifying where halos and artifacts are going to happen and then backing way, way down.) The new dialog has both new parameters capped at 300 if you type the number, but the sliders are non-linear and if you click with the mouse as close to the right as you can go without hitting the arrows, you get something around Standard Deviation of 40 and Scale of 10.

Threshold is to avoid sharpening noise; that's just gone, as far as I can tell.

How do the new parameters relate to the "classic" terminology?

I generally use the filter in three different ways:

• Very small (0.3 or so) radius, relatively high amount (0.5 to 1.0) for "clarity"
• Small (3 to 5) radius for edge sharpening with low amount (0.05-0.12), depending on image content and viewing size
• Very large radius (100 or 200) with low amount (0.05 to 0.10) for local contrast enhancement

How can I do these things with the new tool? I searched the Gimp mailing lists, but the results I got back are very mathy and not directly practical.

Answer 1

The documentation of the corresponding GEGL operation can be found here. There, the meaning of the parameters is briefly explained. Standard deviation would translate to radius and scale to amount, but there may be differences in implementation of the founding operations such as the gaussian blur which may lead to different results or different parameter scaling. The functionality represented by the threshold slider seems not to be implemented in GEGL yet. This parameter causes small values in the difference “mask” of original image and blurred image, that is later added to the original image, to be neglected. If you need this functionality, you could use the G'MIC plug-in version of that filter, which is (as its description states) inspired by the original unsharp mask of gimp but with more parameters in the UI to control what happens.

Answer 2

Well I think the fact that the GEGL version has a check box marked "temp hack" tells you all you need to know about that dialog.

I would suggest not getting too deep into the settings for these 2.9 tools, as they strike me as likely to change, rather than likely to remain the same. I suspect they'll be evolving over time even after 2.10 ( or the mythical 3.0 ).

I think you should treat the new tools as exactly that - new tools with their own way of behaving. You'll need to learn them to find how they fit into your way of working. At this point I'd expect the code to have quirks that are not intentional, so, again, maybe experiment with 2.9, but expect it to change a lot.

In broad terms all USM routines are quite alike, so I think you'd find the std-deviation to relate to radius and scale to relate to amount, and they're probably the same thing in new names.

Answer 3

I cannot provide you the exact numbers which would correspond to your previous settings but Standard deviation behaves very similarly to Radius and Scale behaves like Amount.

You could try these Standard deviation and Scale pairs:

1. 0.4 and 5.0
2. 3.0 and 0.5
3. 50 and 0.15

See how you like them as a starting point and tweak to your liking.