[Gimp-developer] Gimp from git Gegl Gaussian Blur and Unsharp Mask useability issues

Hi all,

As Gimp 2.10 seems to be closer to being a reality, I wanted to bring up some minor but annoying useability issues with Gegl Gaussian Blur and Unsharp Mask.

Gegl USM slider range - can't use the slider for small values:

There are many algorithms to add "local contrast enhancement" to an image. But my favorite algorithm remains high radius low amount USM (http://www.luminous-landscape.com/tutorials/contrast-enhancement.shtml).

For a sample camera-generated jpeg, in Gimp 2.8, Radius 50.0, Amount 0.15, Threshold 0 does a nice job. Experimenting, the Gimp 2.9 USM settings that produce a similar result are Std. Dev. 4.00, Scale 0.20. These are very small values on a very large slider range.

For regular sharpening I use values like 0.5/0.3 or 0.75/0.25. The Gimp 2.9 Gegl USM sliders aren't useable for modifying these small values to see whether "a little more, a little less" might be better. And typing in "slightly larger, slightly smaller" values is awkward.

No doubt there are use cases for very large (10 or 20 or higher) USM slider values, but they would seem to fall into the category of special effects rather than normal image editing. Normal image editing would be easier if the sliders were easier to use when dialing in small values.

It would help a lot if the max value for the slider ranges could be set to say 10 (or a user-specifiable value), with an option to multiply by 10 or 100 to get larger max values.

Gegl Gaussian Blur slider range:

Many times a large radius blur is needed. Many times a small radius blur is needed. The Gegl blur "unit" is larger than a single pixel and it's not easy to choose a small value using the slider. So to test the effect of various small radius blurs, the only alternative seem to be to type the values in.

A smaller "max value", perhaps 255, might help, perhaps with a check box to multiply the values by 10 for a maximum of 2550, if anyone actually uses values that large (my computer would crawl into a corner and croak if I tried to use a 2550 blur radius).

Masks and Gaussian Blur:

Many times seeing the effect of blurring on the image is the most important thing, which the new Gegl Gaussian blur makes possible (yeah!). But when modifying a mask, it's important to see the amount of mask blur directly.

The old, non-Gegl Gaussian Blur dialog shows a small box at 100%. Albeit with a lot of scrolling around, that little box helps avoid situations where too small a blur on a mask creates a line of abrupt tonal transition in the image, that might not become obvious until later in the editing process or when the image is viewed at 100%.

Keeping the old dialog would help, but it would be fantastic to see the entire blurred mask rather than just a tiny portion of it at a time.


What is the Gegl Gaussian blur "unit" and what's its mathematical relationship to a 1-pixel Gaussian blur?

What is the USM Std. Dev. and Scale mathematical relationship to the old Radius and Amount? And what happened to USM Threshold? (I don't use Threshold, but a lot of sharpening tutorials mention it.)

Gegl Gaussian Blur and Unsharp Mask sliders are touchy:

Sometimes when I'm trying to type in a value, instead I accidentally cause the slider to move all the way to the right side of the slider box, and Gimp/Gegl starts doing a very time-consuming unwanted operation. The cancel button takes a long time to respond, probably because the cancel signal has to fight for a share of the CPU, which pegs at 100%.

The ability to set the max values on the sliders would solve this problem (I'd still be clumsy with the mouse, but the price of clumsiness would be a lot smaller!).

Complaints aside, Gimp from git is great. Otherwise there'd be no reason to complain about the details!


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