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Photos in real-estate listings typically present a distorted reality in which rooms look longer than they are—see for example https://photographyforrealestate.net/2016/07/19/is-it-possible-to-get-by-shooting-interiors-with-just-a-24mm-effective-lens/ I assume this is mainly accomplished by using an overly wide field of view (but maybe there's more to this than lens angle? I'm not a photographer and lack the theoretical knowledge).

What digital post-processing methods are suitable for correcting for this? I have fiddled with the "perspective" transformation of Gimp, but that simply allows you to apply an affine transformation to the 2-D pixel array—the results are not convincing. I think the real solution would need to go deeper, using some sort of artificial intelligence to model the scene in 3D, in order to virtually change the field of view. Can anyone point me in the right direction?

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How big objects appear in a picture and from which side they are seen only depends on the location of the objects and the camera. So if you want to see how a picture would look is it was taken with a higher focal length lens, you can simply crop the picture.

If you want to know how a picture would look if it was taken with a longer lens and from a larger distance, then things get very complicated. For every object you would need to determine how big it should be, and from which side it is seen. It is not possible to do this with a single picture. You are missing information on what objects look like from a different side, and information about what is behind objects if they need to be smaller.

This is actually a very important issue for graphics card designers and game/VR designers. There is a lot of information available on forums and in books in these fields.

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  • Thanks. Are you able to give pointers to specific information in any of the forums/books? – jez May 13 at 16:00
  • The wiki page on "Rendering (computer graphics)" has some interesting information. There is also the gamedev SE (stack exchange). I'm not really an expert, just stumbled on to some information at some point, and I found it interesting enough to remember. – Orbit May 13 at 18:33
  • if you want to see how a picture would look is it was taken with a higher focal length lens, you can simply crop the picture – not if the lens that took the original wide angle image was not a rectilinear lens. Cropping anything off-center from a non- pinhole projection image does not fix the distorted projection. – scottbb May 14 at 4:19
  • So the correct disclaimer would be that it only applies to rectilinear lenses, or images cropped from the middle? – Orbit May 14 at 8:57
  • That’s correct. How much of a crop from the middle is a matter of taste, I guess. The more the crop in the middle, the closer it appears to be rectilinear. Of course, with too much crop, there’s a substantial loss of resolution. – scottbb May 14 at 17:48
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Photos in real-estate listings typically present a distorted reality by using an overly wide field of view. What digital post-processing methods are suitable for correcting for this?

Depends on what you mean by "correcting this". A wide angle lens captures a spherical sector of the world and maps it onto a flat plane. This is called a projection mapping. This is very similar to projections of the globe onto a lfat map, such as the common Mercator projection. So, assuming you want to see even just half of the globe, how to you "reproject" the globe onto a flat plane, and maintain real-world distances, angles, and areas (to scale, of course) in the "fixed" map? You can't. It fundamentally is not possible.

Such is the same with wide angle fields of view in cameras. You can change the projection mapping, but realize that all projection mappings are compromises between keeping straight lines straight, or maintaining relative angles, etc. Simply: the wider the angle of view of the lens, the more projection distortions you'll have to accept at the edges and corners.

I have fiddled with the "perspective" transformation of Gimp, but that simply allows you to apply an affine transformation to the 2-D pixel array—the results are not convincing.

You are correct that Gimp's perspective transformation is basically just an affine transformation. Your use-case is the wrong application of that function. Perspective control transformation does the same thing that keystone correction does on projectors. Perspective transformation is the post-processing fix to a problem that could also have been solved in-camera using lens shift (with a shift-only or tilt-shift perspective-control lens). See also, How do I prevent / fix out of square images?

I think the real solution would need to go deeper, using some sort of artificial intelligence to model the scene in 3D, in order to virtually change the field of view.

Not necessary. You just need to reproject the image with a different projection mapping function. Panorama Tools, which has a plugin for Gimp, can handle the reprojection.

See also:

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  • Many thanks. From the panotools wiki I also found a link to the "Wide Angle" Gimp filter which looks promising: members.ozemail.com.au/~hodsond/wideangle.html (am unable to try either of them yet because they're not off-the-shelf ready-to-install and I got bogged down trying to get gimptool to work). But I'm starting to think I may have misunderstood the issue. In real estate photos the phenomenon I'm talking about is rooms looking longer than they are: perhaps there's more to that than lens angle? I've edited the question. – jez May 15 at 14:56
  • @jez There’s really nothing more to it than basic composition and viewing distance. A wide angle lens squeezes more field of view into an image, but we don’t really change our viewing distance to the image. This is both a physical and psychological phenomenon. The exaggerated size of nearby objects (such as the bed footboard) compared to further objects (like headboard) don’t look like our mental image of the room. When we are there in person, our peripheral vision, as well as stereoscopic separation, gives us a better sense of scale, and our brains compensate. – scottbb May 15 at 19:30
  • For a GUI & workflow for panotools there is hugin (wiki.panotools.org/Hugin) which can be downloaded and run as a single item. It includes options for "Architectural Projection" i.e. correcting the shot from the bottom of a building where it narrows in perspective to one without narrowing. This might do the job that you are looking for. – Steve Barnes May 16 at 5:27
  • @SteveBarnes the architectural projection is the affine transformation that OP was dissatisfied with. That's not enough to compensate/re-project a wide angle field of view. – scottbb May 16 at 17:14
  • @scottbb My main point was that hugin is a much simpler installation, usually, for people to get working than the GIMP plugin for Panorama Tools which is actually the same toolchain. – Steve Barnes May 17 at 4:24

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