Perspective Correction does not respect proportion

Software darktable
,
1

Hello,
I am an architect, and proportion is a very important issue in facades. When I use the wonderful Perspective Correction module combined with the Lens Correction module, I get truly orthogonal results. The problem is that these are out of proportion. A square window becomes a rectangle.

10006941975_b57b156896_o
10006941975_b57b156896_o4104Ă—2736 6.43 MB
10006941975_b57b156896_o_dt_pc
10006941975_b57b156896_o_dt_pc5041Ă—2743 3.87 MB

Is this possible to achieve using the implemented features? Or would that need extra dev work?

(other softwares which can achieve the square proportion are PTLens, or Hugin, but they require lots of user fiddling and fall out of Darktalbe workflow)

2

The results you show are what I’d expect. After correcting the perspective, you need to crop the image.

To get this right in camera, either square your camera so it is parallel to the object plane, or look at getting a tilt shift lens.

2 Likes
3

I think the simplest way is to use the scale tool in Gimp to fix the proportion. I dont know if that is possible in dt too.

4

I am not talking about cropping the image, not about he aspect ration of the overall document dimensions. I am talking of the proportion of the perspective correction. Below, I attach the result fromm PTLens: notice the square proportion of each “window”.

10006941975_b57b156896_o_ap_ptlens
10006941975_b57b156896_o_ap_ptlens4104Ă—2736 2.64 MB

5

Your windows do look square (I did not measure them, though).
If you mean your image doesn’t look like one, see the above comment. Then, I guess PTLens and/or Hugin are showing you the properly cropped final image. In DT all you need to do is crop the processed image.
If, on the other hand, you mean that you shoot squares and after perspective correction they are not squares then there may well be something for the developers here.

1 Like
6

I do mean that I [quote=“bc_the-path, post:5, topic:12668”]
shoot squares and after perspective correction they are not squares then there may well be something for the developers here.
[/quote] !

rect4186
rect41863810Ă—1945 8.75 MB

1 Like
7

The 240 moucharabiehs of Paris Institut du Monde Arabe are all perfect squares. They are transformed to rectangles, mainly at the top of image, by the perspective correction. Seems like a bug.

From inside
image

1 Like
8

Use the “lens model” → “specific” option in dt module, instead of “generic”

Capture%20d%E2%80%99%C3%A9cran%20de%202019-05-22%2017-52-53
Capture%20d%E2%80%99%C3%A9cran%20de%202019-05-22%2017-52-533840Ă—2160 6.68 MB

4 Likes
9

Well, thank you very much. It worked perfectly! Thank you for forums and good hearted people!

1 Like
10

Now I found another show stopper. Is there a limit in the vertical lens shift? I cannot seem to be able to correct this image to orthogonality.

VA110463
VA1104634032Ă—3024 2.57 MB

11

Although PTLens seems to have the same limitation as Darktable, I was able to achieve the following result with Hugin.

09
097249Ă—6050 6.02 MB

12

Indeed, it seems the limit is too conservative. Please fill a bug report on Github, someone will take a look when they have time.

13

Issue opened.

https://github.com/darktable-org/darktable/issues/2617

2 Likes
14

Wow, I thought that being an experienced user I wouldn’t learn anything new. Was I ever wrong! For years I’ve been been correcting the resulting lopsided dimensions in Gimp after a perspective correction in Darktable (or even leave the perspective correction to Digikam which also introduces dimensional issues). Thank you ever so much!!!

3 Likes