Aspect ratio defined by the PNG resolution chunk “pHYs” is misinterpreted as its reciprocal

[goodbye]

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

The issue has been present from the very beginning, since parsing the pHYs chunk was introduced in commit 8288c2b6cb072b61f664bc8ab4c1b0a5a8db0ead, and is still present as of commit 1864025458021a2d2c542f56e268ee1106f84460 (libavcodec/pngdec.c lines 648–649)

The PNG encoder has the complementary problem of writing a reciprocal ratio into the pHYs chunk, introduced with commit f58f90238fc63090da34c9fdb1d06d724d929f6d (now libavcodec/pngenc.c lines 394–395)

[Balling]

As I understand Chrome shows it correctly, right? First link is horizontal image, and second is vertical.