Perspective tool changes aspect ratio
I have done tests with the perspective tool in darktable.
These shots are of a square box, length of the side is 33 cm. All shots taken at a distance of 78 cm from box, with a Canon (APS-C sensor) and a 24mm lens.
Shot #1, looking straight at the box (no perspective effect)
Shot #2, after raising the camera by 35 cm (effectively creating an angle of 24 degrees)
Shot #3, camera still raised 35cm, and moved right 35 cm (effectively creating a compound angle, 24 degrees horizontal and 24 degrees vertically)
Shot #4, camera still moved right 35 cm, but lowered to the original level.
Now, after using the perspective correction tool and cropping (with a 1:1 ratio). I have used the manuel perspective tool which is in the crop module (Module recadrer et pivoter in French).
Shot #2, with perspective corrected shows definite flattening of the box, which isn't square any more. Its aspect ratio is now 1.14 (the width is 14% more than the height)
Shot #3 with compound angle and perspective correction does give a square box.
Shot #4, with perspective corrected is reverse of shot #2, with height larger than width.
So... Is this a bug?
I think that after applying the perspective correction, we should end-up with an image that has the same aspect ratio as the original object.
And by the way I also tried the automatic perspective correction tool (module correction de perspective in French) and it deformed the object the same way.
As a side note, I find it a bit ackward that there are 2 tools, one in the cropping module and one in its dedicated module. Personnally, I prefer the manual version that is in the cropping module, where I can drag handles at the position I want, but maybe both versions could live in the same module, with the option for the user to use the manual version or the automatic version? - just a thought...
Thanks for your consideration,