[FFmpeg-devel] [PATCH] H.264: decode picture timing SEI message and get field order

[Haruhiko Yamagata]

Hi,

I wrote a patch which implements decoding of picture timing SEI.

[Current problems]
libavcodec derives field order from POCs.

cur->top_field_first = cur->field_poc[0] < cur->field_poc[1];

It assumes BFF if two POSs are equal. This is not always correct.

And it derives AVFrame::interlaced_frame from used decoding process.

cur->interlaced_frame = FIELD_OR_MBAFF_PICTURE;

Sometimes this is not correct. Interlaced/progressive mode of post-decoding 
processes such as deinterlacing and color space conversion is not always 
same as the mode used during decoding. H.264 for example allows to encode 
an interlaced frame using a progressive algorithm. Sample: premiere-paff.ts 
(http://x264.nl/h.264.samples/premiere-paff.ts).

[Solutions]
Picture timing SEI message contains pic_struct which defines interlace/progressive
mode of post decoding processes and field order. Most streams have the SEI for 
every frame or field. My patch implements decoding of the SEI and use of pic_struct. 
Now the field order is always correct. Interlaced picture is judged interlaced. 
Field 1 repeat flags are also set.

[Limitation]
As usual, there are exceptions: badly encoded streams. Some progressive 
streams from television have pic_struct 3, which indicates interlacing. 
Fortunately such streams are rare. This is not my fault, other decoder 
show the same behavior.

Please review.

----------------------------- for svn log -----------------------
H.264: Implement decoding of picture timing SEI message
Now correct values are set to interlaced_frame, top_field_first
and repeat_pict in AVFrame structure.
patch by ffdshow tryouts
--------------------------------------------------------------

Best regards,
Haruhiko Yamagata
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