Maybe this feature exists, but I couldn't find it.

Kdenlive has a lot of features and one of them is the effect sharpening.
It is however very difficult to guess the best values for this effect as it doesn't seem to be possible to view the monitor image 1:1. There's the option 1:1 of course, but this doesn't work if the source video has a resolution the same or higher than the monitor, which these days is very common. Grabbing the frame and judging the best settings by editing the image in Rawtherapee or Gimp (and hoping the parameters match) is a bit cumbersome.

Some photo editing programs like rawtherapee offer multiple solutions for this: when clicking 1:1 it really zooms in to pixel level (cropping the sides of the image out) and there are also 'detail windows' that allow a small area of the source image to be duplicated and showed at 1:1.

I don't know if this would be possible, maybe a detail window could be made as an extra window like histogram, waveform, ...? A live update doesn't seem necessary to me, as (1) most machines aren't powerful enough anyway, (2) judging the sharpening for two or three frames should be enough and (3) it's rather difficult to judge sharpening in moving images.

The unsharp mask also has a 'size' option. Most programs do offer double or single digit precision for this number, while for kdenlive this is an integer. Do I have to divide it by 10 or so to obtain the real used size?

You can actually zoom in/out in both the clip and project monitors using Shift+Ctrl+mouse wheel. This is a probably less known feature that once Vincent told me some weeks ago. I already forgot about it myself until you were posting your feature request here. Maybe this zoom in/out already does the job for you?

TheDiveO wrote:You can actually zoom in/out in both the clip and project monitors using Shift+Ctrl+mouse wheel. This is a probably less known feature that once Vincent told me some weeks ago. I already forgot about it myself until you were posting your feature request here. Maybe this zoom in/out already does the job for you?

That looks like a good solution. I'll try it!
Update: works great!