Darktable: Add toggle button for focus peaking

[Egocentrix]

Currently the focus peaking mode is accessible with ctrl+shift+f by default. I have read some stories from people who have accidentally enabled it and don't know what has happened, and I think the lack of a visual cue that a certain mode has been enabled would help a lot.

The point of this issue is not to complain and it certainly shouldn't be high priority, it is not a problem for me personally, but to discuss whether this would improve darktable's usability.

My suggestion for a position is in the top bar, next to the overlay/group/settings icons, because those controls affect images in both the filmstrip and the lighttable. The bottom bar controls only affect the selected image (afaik) so putting a button there would be confusing I think.

[johnny-bit]

While that's a great idea, what icon could represent "focus peaking"?

[Egocentrix]

First thing that comes to mind is something like this that I stole off Flaticon:

The problem that I have with that is that it implies that the button shows what the camera _tried to focus on_, not what is actually _in focus_. So yeah, I'll brainstorm a bit, and I'm very open to suggestions.

edit with second idea: some stylised split-prism viewfinder. Will mock something up later.
edit2: like this

[aurelienpierre]

Right, I slacked on this one. I like the stigmometer (edit2).

[aurelienpierre]

On it, it's a coffee break project.

[Egocentrix]

stigmometer

Hah, I learned a new word today!