Re: [PR] avcodec/h264_parser: align field_order default with the decoder (PR #24128)

On Thu, 13 Aug 2026, 16:37 ngaullier via ffmpeg-devel, <
[email protected]> wrote:

> PR #24128 opened by ngaullier
> URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24128
> Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24128.patch
>
> Use case: interlaced AVC Intra is typically missing the required
> information
> for proper tff detection.
> Currently, the decoder defaults to tff, but the parser defaults to
> progressive.
>
> ./ffmpeg -f lavfi -i testsrc=1920x1080 -codec libx264 -pix_fmt yuv422p10le
> \
>     -x264-params avcintra-class=100:tff=1 -frames 1 tff.h264
> ./ffprobe tff.h264 -of flat -show_entries
> stream=field_order:frame=interlaced_frame,top_field_first
>
> Before:
> frames.frame.0.interlaced_frame=1
> frames.frame.0.top_field_first=1
> streams.stream.0.field_order="progressive"
>
> After:
> frames.frame.0.interlaced_frame=1
> frames.frame.0.top_field_first=1
> streams.stream.0.field_order="tt"
>
> Issue since field_order setting in 3f1a7ceb2c604deff.
>
>
> >From 9e94cff9dcd4d5f4abfd483a6d39d263a73007b6 Mon Sep 17 00:00:00 2001
> From: Nicolas Gaullier <[email protected]>
> Date: Thu, 13 Aug 2026 17:00:42 +0200
> Subject: [PATCH] avcodec/h264_parser: align field_order default with the
>  decoder
>
> Use case: interlaced AVC Intra is typically missing the required
> information
> for proper tff detection.
> Currently, the decoder defaults to tff, but the parser defaults to
> progressive.
>
> ./ffmpeg -f lavfi -i testsrc=1920x1080 -codec libx264 -pix_fmt yuv422p10le
> \
>     -x264-params avcintra-class=100:tff=1 -frames 1 tff.h264
> ./ffprobe tff.h264 -of flat -show_entries
> stream=field_order:frame=interlaced_frame,top_field_first
>
> Before:
> frames.frame.0.interlaced_frame=1
> frames.frame.0.top_field_first=1
> streams.stream.0.field_order="progressive"
>
> After:
> frames.frame.0.interlaced_frame=1
> frames.frame.0.top_field_first=1
> streams.stream.0.field_order="tt"
>
> Issue since field_order setting in 3f1a7ceb2c604deff.
>
> Signed-off-by: Nicolas Gaullier <[email protected]>
> ---
>  libavcodec/h264_parser.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/h264_parser.c b/libavcodec/h264_parser.c
> index 9d64fc603f..af43cad609 100644
> --- a/libavcodec/h264_parser.c
> +++ b/libavcodec/h264_parser.c
> @@ -423,6 +423,7 @@ static inline int parse_nal_units(AVCodecParserContext
> *s,
>
>              if (sps->frame_mbs_only_flag) {
>                  p->picture_structure = PICT_FRAME;
> +                s->field_order = AV_FIELD_PROGRESSIVE;
>              } else {
>                  if (get_bits1(&nal.gb)) { // field_pic_flag
>                      p->picture_structure = PICT_TOP_FIELD + get_bits1(&
> nal.gb); // bottom_field_flag
> @@ -541,8 +542,11 @@ static inline int
> parse_nal_units(AVCodecParserContext *s,
>                          s->field_order = AV_FIELD_TT;
>                      else if (field_poc[0] > field_poc[1])
>                          s->field_order = AV_FIELD_BB;
> -                    else
> -                        s->field_order = AV_FIELD_PROGRESSIVE;
> +                    else if (sps->mb_aff) {
> +                        /* Default to top field first
> +                         * This is the same as what the decoder does */
> +                        s->field_order = AV_FIELD_TT;
> +                    }
>                  }
>              } else {
>                  if (p->picture_structure == PICT_TOP_FIELD)
> --
> 2.52.0
>
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LGTM

>
>
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