[FFmpeg-devel] [PATCH] Detect CFR/VFR in libx264.c for proper i_fps_*

[Måns Rullgård]

Rudolf Polzer <divVerent at alientrap.org> writes:

> This change adds a detection of constant/variable frame rate by the same
> heuristics as libavformat/utils.c already uses (in performing or refusing
> frame duration calculation). The heuristics is basically to check whether
> the time base is larger than 1ms, and assume CFR then, VFR otherwise.
>
> Without it, x264 estimates a huge macroblock rate, and thus forces H.264 level
> 5.1 even if the input could perfectly fine pass as e.g. level 2.1.
>
> Signed-off-by: Rudolf Polzer <divVerent at alientrap.org>
> ---
>  libavcodec/libx264.c |   23 +++++++++++++++++++++--
>  1 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/libx264.c b/libavcodec/libx264.c
> index 0dad5cd..02f7aca 100644
> --- a/libavcodec/libx264.c
> +++ b/libavcodec/libx264.c
> @@ -217,8 +217,27 @@ static av_cold int X264_init(AVCodecContext *avctx)
>      x4->params.i_height             = avctx->height;
>      x4->params.vui.i_sar_width      = avctx->sample_aspect_ratio.num;
>      x4->params.vui.i_sar_height     = avctx->sample_aspect_ratio.den;
> -    x4->params.i_fps_num = x4->params.i_timebase_den = avctx->time_base.den;
> -    x4->params.i_fps_den = x4->params.i_timebase_num = avctx->time_base.num;
> +    x4->params.i_timebase_den = avctx->time_base.den;
> +    x4->params.i_timebase_num = avctx->time_base.num;
> +
> +    if (avctx->time_base.num * 1000LL > avctx->time_base.den) { // "if time base > 1ms"
> +        // this is a heuristic to distinugish variable from constant frame rate
> +        // encoding
> +        // NOTE: this is not a new hack; compute_frame_duration() in
> +        // libavformat/utils.c uses the same heuristics
> +        x4->params.i_fps_num = avctx->time_base.den;
> +        x4->params.i_fps_den = avctx->time_base.num;
> +        // not doing VFR
> +        x4->params.b_vfr_input = 0;
> +    } else {
> +        // set this to a sensible but guessed frame rate (x264 source also
> +        // often uses this rate if none is available, but not in any occasion,
> +        // so we cannot set these to 0/0)
> +        x4->params.i_fps_num = 25;
> +        x4->params.i_fps_den = 1;
> +        // IMPORTANT: tell x264 rate control that we are doing VFR
> +        x4->params.b_vfr_input = 1;
> +    }

This kind of thing doesn't really belong at the individual codec level.
An override if the heuristic makes a wrong guess might be nice too.

-- 
M?ns Rullg?rd
mans at mansr.com