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

Hi,

The way GEGL currently exposes each numerical parameters is as follow:

1) A nominal range, say [a, b], which represents the range where the operation is expected to work
2) An UI range, [a', b'] included in [a, b], representing the area of interest of the parameter.
3) An exponent, (like a gamma correction), to make the slider not behave linearly on the [a', b'] range.

Currently, GIMP's sliders "show" only [a', b'], but allows values in [a, b]. The exponent is the way GEGL 
handles the "mutli-scale" problem and it allows to have more precision on small value. Those values are all 
in GEGL, look for eg gegl_chant_double_ui in /operations/common/*.c

I will let others comment on what is the optimal UI range for those two operations. But changing [a', b'] 
would be really easy, and it should change the "unit" as well. I am not sure about making that range "user 
configurable", it makes sense imho.

In the GEGL Gaussian blur, parameters are the std_dev along x and y, describing mathematically the Gaussian 
curve used as kernel. Though, I have to read the code to know where the infinite curve is clipped to have a 
"only-one-pixel" actual radius. Which is not obvious...

Other than that, I don't really understand the "Masks" comment. You can see the actual mask by checking "Show 
the layer mask", but perhaps you want to see both the mask and the resulting image at the same time?

Hah, and the "computational explosion" with bad parameters is a known bug in a lot of GEGL operations, it may 
even make GIMP crash. It probably needs to be fixed in each operation individually, but this is not 
obvious... Perhaps we will need to make the "process" step of GeglOperations cancellable at some point? 
Unclear...

Also keep in mind that this is all "work in progress", Mitch once said that the ui infrastructure for GEGL 
Operations in GIMP is not done. And most of GEGL Operations still need some love.

Regards,

Téo


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.


Units:

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!

Elle
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