[FFmpeg-trac] #10912(undetermined:new): d_fov and h_fov not working well when converting equirectangular to rectilinear (v360)

[FFmpeg]

#10912: d_fov and h_fov not working well when converting equirectangular to
rectilinear (v360)
-------------------------------------+-------------------------------------
             Reporter:  0kajuna0     |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:  v360,        |               Blocked By:
  equirect, rectilinear              |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the bug: When trying to convert a 360 frame from
 equirectangular to rectilinear with the v360 filter, the d_fov (diagonal
 field of view) works well, but h_fov (horizontal) and v_fov (vertical)
 don't seem to produce the expected results. The image gets distorted. This
 also happens if no fov is specified.
 How to reproduce:
 (tested with recent git master versions)
 Equirectangular test file:
 https://en.wikipedia.org/wiki/Equirectangular_projection#/media/File:Plate_Carr%C3%A9e_with_Tissot's_Indicatrices_of_Distortion.svg

 No fov results in a distorted output
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear a.png
 }}}
 https://imgur.com/dxlo01O

 d_fov results in correct proportions:
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:d_fov=125 b.png
 }}}
 https://imgur.com/Ti96A7o

 Using h_fov and v_fov to output a different aspect ratio results in a very
 distorted image
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:h_fov=56:v_fov=112 c.png
 }}}
 https://imgur.com/B0tJctX

 We can try to correct this by specifying width and height, but the result
 is still distorted
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:h_fov=56:v_fov=112:w=1440:h=2880
 d.png
 }}}
 https://imgur.com/tl6CbJq

 As a partial workaround, we can use d_fov and apply a roll and transpose
 to output a correct vertical ratio
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:d_fov=125:roll=90,transpose=1
 e.png
 }}}
 https://imgur.com/XydgshW

 This also works for converting the image back to equirectangular
 {{{
 % ffmpeg -i e.png -vf transpose=2,v360=rectilinear:e:id_fov=125:roll=-90
 h.png
 }}}
 https://imgur.com/uNHThfJ

 Another workaround is to use d_fov and trigonometry to define the output
 resolution
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:d_fov=125:w=1440:h=2880 f.png
 }}}
 https://imgur.com/evVUX5i

 This allows us to output any ratio
 {{{
 % ffmpeg -i in.png -vf v360=e:rectilinear:d_fov=125:w=1440:h=1440 g.png
 }}}
 https://imgur.com/OC8pwQE

 But unfortunately bringing that back to equirectangular breaks proportions
 {{{
 % ffmpeg -i g.png -vf v360=rectilinear:e:id_fov=125 i.png
 }}}
 https://imgur.com/TP8wXzF

 Is this a bug or am I doing something wrong?
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/10912>
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