A few weeks ago I added a new feature to the Free-Rotation imageplugin: 'Automatic (Horizon) Correction'.
This is nothing new, Gimp and Photoshop (and for sure other applications) have such a feature, too.
You set two points in the preview widget and hit "Adjust" to rotate the image accordingly to these markers.
The difference is that it is suitable to adjust horizontal and vertical lines and determines automatically which of the modes it should use, based on
This is a companion discussion topic for the original entry at https://www.digikam.org/news/2009-05-06_new_auto-correction-mode_for_the_free_rotation_image_plugin/