The free program
»Perspective Image Correction« by Wolfgang Schroeder
corrects »falling lines« (»stürzende Linien«).
I got it from here, it’s available from sourceforge as well. A German version seems to be available here.
Some hints for dummies:
• The program will not correct barrel or pincushion distortions, when straight lines became curved (»Tonnenverzerrung«, »Kissenverzerrung«*).
• Work on a copy of the picture, as the program will overwrite the old picture.
• Start with »File« to »Load Source Image«. Do not dispair: Typically the picture won’t fit at first:
Do this with the left mouse button#).
If you make a mistake or want to try another rectangle, cancel with the right mouse button.
Doge’s Palace, alongside the Riva degli Schiavoni. In the »corrected image« at right this rectangle is straight, the rest follows. Danke, Wolfgang!
*) For programs to correct lens distorsions look elsewhere. I took PTLens. In olden times it was free, now it’s much better and costs $ 25.
#) If yo don’t know what’s left or right, you might be a Guugu Yimithirr, see Wikipedia. Or just think that your heart is at the left (mostly) and your liver at the right, but please don’t try to find it yourself.
An exact definition of left vs. right is so difficult that even Wikipedia has given up: » … definitions of left and right based on the geometry of the natural environment are unwieldy … « (»sperrig«).
To define left vs. right you must find a (rare) physical effect that does not follow the usual parity, perhaps here. You’ll get to the »weak fore«, something seldom seen and understood, as one of the four forces around. Grant Hutchinson: »But the principle’s the thing: the weak force is the only one that shows
parity violation, so you need to use a weak-force interaction which has
been given a unique axis in a magnetic field. That lets you label the
magnetic field direction. Then you use the interaction of moving charges
and the labelled magnetic field to give you an unequivocal left and
right. The neat thing about that interaction is that with the field
orientated up and down (which you can describe) and the current
orientated near and far (which you can describe), you get a force that
goes left and right (which you can't describe).
Link to here: http://blogabissl.blogspot.com/2015/04/perspective-image-correction-for-dummies.html