Tom Yan:
> the top_field_first bit is only used to indicate the field order
> when the picture is a frame picture (which consists of two fields)
> but not when it is a field picture (which consists of one single
> top or bottom field).
>
> also removing the unnecessary progressive_sequence check (the bit
> is mandated to be 0 if progressive_frame is 0 on any picture in the
> sequence).
>
Just because something is mandated does not mean that it is so;
spec-incompliant files exist.
> Signed-off-by: Tom Yan <tom.t...@gmail.com>
> ---
> libavcodec/mpegvideo_parser.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/libavcodec/mpegvideo_parser.c b/libavcodec/mpegvideo_parser.c
> index c5dc867d24..004ff602f6 100644
> --- a/libavcodec/mpegvideo_parser.c
> +++ b/libavcodec/mpegvideo_parser.c
> @@ -181,6 +181,7 @@ static void
> mpegvideo_extract_headers(AVCodecParserContext *s,
> break;
> case 0x8: /* picture coding extension */
> if (bytes_left >= 5) {
> + s->picture_structure = buf[2] & 0x3;
> top_field_first = buf[3] & (1 << 7);
> repeat_first_field = buf[3] & (1 << 1);
> progressive_frame = buf[4] & (1 << 7);
> @@ -198,13 +199,15 @@ static void
> mpegvideo_extract_headers(AVCodecParserContext *s,
> }
> }
>
> - if (!pc->progressive_sequence && !progressive_frame)
> {
> - if (top_field_first)
> - s->field_order = AV_FIELD_TT;
> - else
> - s->field_order = AV_FIELD_BB;
> - } else
> + if (progressive_frame)
> s->field_order = AV_FIELD_PROGRESSIVE;
> + else if (top_field_first ||
> + /* top_field_first is mandated to be 0 when
> + the picture is not a frame picture) */
> + s->picture_structure ==
> AV_PICTURE_STRUCTURE_TOP_FIELD)
> + s->field_order = AV_FIELD_TT;
> + else
> + s->field_order = AV_FIELD_BB;
> }
> break;
> }
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