Re: [FFmpeg-devel] [PATCH] mpegvideo_parser: check picture_structure for field order

[James Almer <jamrial@gmail.com>]

On 2/5/2022 9:51 AM, Tom Yan wrote:
> 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).
> 
> Signed-off-by: Tom Yan <tom.ty89@gmail.com>
> ---
>   libavcodec/mpegvideo_parser.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/mpegvideo_parser.c b/libavcodec/mpegvideo_parser.c
> index c5dc867d24..2ddcdb0f37 100644
> --- a/libavcodec/mpegvideo_parser.c
> +++ b/libavcodec/mpegvideo_parser.c
> @@ -108,7 +108,7 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
>       uint32_t start_code;
>       int frame_rate_index, ext_type, bytes_left;
>       int frame_rate_ext_n, frame_rate_ext_d;
> -    int top_field_first, repeat_first_field, progressive_frame;
> +    int picture_structure, top_field_first, repeat_first_field, progressive_frame;
>       int horiz_size_ext, vert_size_ext, bit_rate_ext;
>       int did_set_size=0;
>       int set_dim_ret = 0;
> @@ -181,6 +181,7 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
>                       break;
>                   case 0x8: /* picture coding extension */
>                       if (bytes_left >= 5) {
> +                        picture_structure = buf[2] & 0x3;

You could maybe also set s->picture_structure with this.

>                           top_field_first = buf[3] & (1 << 7);
>                           repeat_first_field = buf[3] & (1 << 1);
>                           progressive_frame = buf[4] & (1 << 7);
> @@ -199,7 +200,9 @@ static void mpegvideo_extract_headers(AVCodecParserContext *s,
>                           }
>   
>                           if (!pc->progressive_sequence && !progressive_frame) {
> -                            if (top_field_first)
> +                            /* top_field_first is mandated to be 0 when
> +                               picture_structure is not 3 (i.e. not a frame picture) */
> +                            if (top_field_first || picture_structure == 1)
>                                   s->field_order = AV_FIELD_TT;
>                               else
>                                   s->field_order = AV_FIELD_BB;

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