[FFmpeg-trac] #11279(undetermined:new): Aspect ratio defined by the PNG resolution chunk “pHYs” is misinterpreted as its reciprocal

[FFmpeg]

#11279: Aspect ratio defined by the PNG resolution chunk “pHYs” is misinterpreted
as its reciprocal
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             Reporter:  goodbye       |                     Type:  defect
               Status:  new           |                 Priority:  normal
            Component:  undetermined  |                  Version:  7.1
             Keywords:  png           |               Blocked By:
             Blocking:                |  Reproduced by developer:  0
Analyzed by developer:  0             |
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 The PNG {{{pHYs}}} chunk describes the resolution/pixel density of an
 image, which can be in abstract units (in which case it only defines the
 pixel aspect ratio) or in pixels per metre.

 FFmpeg interprets the X:Y resolution ratio directly as a X:Y PAR, but in
 actuality, it’s the reciprocal.  This is because the resolution is
 specified as a “pixels per unit” value and not “units per pixel”, as is
 typically the case with PARs.  The X:Y PAR is equal to the Y:X ratio of
 resolutions specified in the chunk.

 Both of these images, from the libpng test suite, are supposed to have a
 DAR of 1:1, but FFmpeg misinterprets them to have DARs of 16:1 and 1:16:
 * http://www.libpng.org/pub/png/PngSuite/cdhn2c08.png
 * http://www.libpng.org/pub/png/PngSuite/cdfn2c08.png
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Ticket URL: <https://trac.ffmpeg.org/ticket/11279>
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