[FFmpeg-trac] #4168(undetermined:new): defect : mpeg2 interlaced yuv420 chroma incorrectly decoded

[FFmpeg]

#4168: defect : mpeg2 interlaced yuv420 chroma incorrectly decoded
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             Reporter:  clam         |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
              Version:  git-master   |  undetermined
             Keywords:               |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0
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Comment (by clam):

 Yes, I disagree :)

 A lot of codecs can compress videos with interlaced chroma or not. I don't
 speak of codecs like VC-3 (DNxHD) or ProRes : they're broadcast formats
 using 4:2:2 chroma so the chroma coding is the same for interlaced or
 progressive frames. For instance if you compress a 1080/50i or a 1080/25p
 sequence in ProRes, the video stream will be exactly the same but
 somewhere in the files there will be a flag telling than one is interlaced
 and the other progressive (often added by the editing software).

 For MPEG2, DV and H264 it's a different matter since they use 4:2:0 chroma
 coding and they must indicate in the stream (or the file's header) that
 the content is interlaced or not. It also change the way macroblocks are
 coded.

 MPEG2 is mostly used for interlaced videos (it's what you get on a DVD or
 a DVB broadcast), I only saw interlaced DV in my life, and H264 might be
 used for interlaced content using MBAFF.

 The decoders must know they work on interlaced pictures since it changes
 the way macroblocks are encoded in the stream. A quick peek at mpeg12dec.c
 and dvdec.c confirms this.

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Ticket URL: <https://trac.ffmpeg.org/ticket/4168#comment:19>
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