[FFmpeg-devel] [PATCH 1/3] avisynth: corrected interlace detection

[Stephen Hutchinson]

On 2/8/22 6:02 AM, Stephen Hutchinson wrote:
> From: emcodem <emcodem at ffastrans.com>
> 
> AviSynth works on frame-based video by default, which can
> be either progressive or interlaced. Some filters can break
> frames into half-height fields, at which point it considers
> the clip to be field-based (avs_is_field_based can be used
> to check for this situation).
> 
> To properly detect the field order of a typical video clip,
> the frame needs to have been weaved back together already,
> so avs_is_field_based should actually report 'false' when
> checked.
> 
> Signed-off-by: Stephen Hutchinson <qyot27 at gmail.com>
> ---
>   libavformat/avisynth.c | 11 ++++-------
>   1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/libavformat/avisynth.c b/libavformat/avisynth.c
> index 350ac6d11d..1e862a6a85 100644
> --- a/libavformat/avisynth.c
> +++ b/libavformat/avisynth.c
> @@ -250,15 +250,12 @@ static int avisynth_create_stream_video(AVFormatContext *s, AVStream *st)
>       st->nb_frames         = avs->vi->num_frames;
>       avpriv_set_pts_info(st, 32, avs->vi->fps_denominator, avs->vi->fps_numerator);
>   
> -    av_log(s, AV_LOG_TRACE, "avs_is_field_based: %d\n", avs_is_field_based(avs->vi));
> -    av_log(s, AV_LOG_TRACE, "avs_is_parity_known: %d\n", avs_is_parity_known(avs->vi));
>   
> -    /* The following typically only works when assumetff (-bff) and
> -     * assumefieldbased is used in-script. Additional
> -     * logic using GetParity() could deliver more accurate results
> -     * but also decodes a frame which we want to avoid. */
>       st->codecpar->field_order = AV_FIELD_UNKNOWN;
> -    if (avs_is_field_based(avs->vi)) {
> +    /* AviSynth works with frame-based video, detecting field order can
> +     * only work when avs_is_field_based returns 'false'. */
> +    av_log(s, AV_LOG_TRACE, "avs_is_field_based: %d\n", avs_is_field_based(avs->vi));
> +    if (avs_is_field_based(avs->vi) == 0) {
>           if (avs_is_tff(avs->vi)) {
>               st->codecpar->field_order = AV_FIELD_TT;
>           }

Pushed.