The perspective adjustment tool in digiKam is indispensable for editing architectural shots. It can be somewhat fiddly to use, but with enough practice, you can achieve rather good results.
The photo above is a case in point. The original image was a bad case of converging lines. In some situations, this effect can be used as part of a composition, but that wasn’t what I had in mind.
digiKam to the rescue: a few minutes of tweaking the photo with the perspective adjustment tool and all converging lines were gone. Besides perspective correction (and subsequent cropping), I applied one of my custom curve presets and sharpened the final result a bit.