Bug #12694

Perspective tool changes aspect ratio

Added by Claude Jolicoeur about 1 month ago.

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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,

1-NoCrop.jpg (87.9 KB) 1-NoCrop.jpg Claude Jolicoeur, 07/17/2019 11:00 PM
2-NoCrop.jpg (98.7 KB) 2-NoCrop.jpg Claude Jolicoeur, 07/17/2019 11:00 PM
3-NoCrop.jpg (95 KB) 3-NoCrop.jpg Claude Jolicoeur, 07/17/2019 11:00 PM
4-NoCrop.jpg (97.4 KB) 4-NoCrop.jpg Claude Jolicoeur, 07/17/2019 11:00 PM
3-M.jpg (101 KB) 3-M.jpg Claude Jolicoeur, 07/17/2019 11:13 PM
2-M.jpg (133 KB) 2-M.jpg Claude Jolicoeur, 07/17/2019 11:13 PM
4-M.jpg (119 KB) 4-M.jpg Claude Jolicoeur, 07/17/2019 11:13 PM

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