[FFmpeg-trac] #7623(avcodec:closed): Wrong interpretation of (m)jpeg aspect ratio
FFmpeg
trac at avcodec.org
Thu Dec 20 14:50:19 EET 2018
#7623: Wrong interpretation of (m)jpeg aspect ratio
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Reporter: UlfZibis | Owner:
Type: defect | Status: closed
Priority: normal | Component: avcodec
Version: git-master | Resolution: invalid
Keywords: mjpeg aspect | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Changes (by Gyan):
* status: new => closed
* resolution: => invalid
Comment:
As per the JPEG specification at
https://www.w3.org/Graphics/JPEG/jfif3.pdf, (last tip on pg 7)
if you only want to specify a pixel aspect ratio, put 0 for the units
field in the special APP0 field. Xdensity and Ydensity can then be
programmed for the desired aspect ratio. Xdensity = 1, Ydensity = 1 will
program a 1:1 aspect ratio. Xdensity and Ydensity should always be non-
zero.
Also, see on pg. 5
units = 0: no units, X and Y specify the pixel aspect ratio
units = 1: X and Y are dots per inch
units = 2: X and Y are dots per cm
Our encoder always writes units as 0, so Irfanview is at fault here.
`put_bits(p, 8, 0); /* units type: 0 - aspect ratio */`
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Ticket URL: <https://trac.ffmpeg.org/ticket/7623#comment:10>
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