[FFmpeg-trac] #7623(avcodec:new): Wrong interpretation of (m)jpeg aspect ratio

FFmpeg trac at avcodec.org
Thu Dec 20 12:31:06 EET 2018

#7623: Wrong interpretation of (m)jpeg aspect ratio
             Reporter:  UlfZibis      |                    Owner:
                 Type:  defect        |                   Status:  new
             Priority:  normal        |                Component:  avcodec
              Version:  git-master    |               Resolution:
             Keywords:  mjpeg aspect  |               Blocked By:
             Blocking:                |  Reproduced by developer:  0
Analyzed by developer:  0             |

Comment (by UlfZibis):

 The unit of SAR is length/dot
 The unit of x/y-resolution is dots/length
 So values 12/11 have to be switched for dpi.
 If you print the picture on paper, you will get:
 x-width = 704/12 = 58.6666... inch
 y-height = 576/11 = 52.3636... inch
 The resulting DAR then is 1.12 instead 1.333 (4:3).

 Additionally it may be better to set the dpi to a reasonable value for the
 case if one tries to print the pictures on paper. 72 dpi is a common
 value, so here 66/72 dpi may be practicable. For higher resolutions e.g.
 HD-Video, the print should not exceed a normal paper sheet.

Ticket URL: <https://trac.ffmpeg.org/ticket/7623#comment:8>
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