[FFmpeg-devel] [PATCH] lavf/mov.c: Guess video codec delay based on PTS while parsing MOV header.

[Michael Niedermayer]

On Mon, Nov 20, 2017 at 08:27:05PM -0800, Sasi Inguva wrote:
> Signed-off-by: Sasi Inguva <isasi at google.com>
> ---
>  libavformat/mov.c                | 50 ++++++++++++++++++++++++++++++++++++++++
>  tests/fate/mov.mak               |  7 ++++++
>  tests/ref/fate/mov-guess-delay-1 |  3 +++
>  tests/ref/fate/mov-guess-delay-2 |  3 +++
>  tests/ref/fate/mov-guess-delay-3 |  3 +++
>  5 files changed, 66 insertions(+)
>  create mode 100644 tests/ref/fate/mov-guess-delay-1
>  create mode 100644 tests/ref/fate/mov-guess-delay-2
>  create mode 100644 tests/ref/fate/mov-guess-delay-3
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index fd170baa57..afb0d4ca5c 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -3213,6 +3213,54 @@ static int64_t add_ctts_entry(MOVStts** ctts_data, unsigned int* ctts_count, uns
>      return *ctts_count;
>  }
>  
> +static void mov_guess_video_delay(MOVContext *c, AVStream* st) {
> +    MOVStreamContext *msc = st->priv_data;
> +    int ind;
> +    int ctts_ind = 0;
> +    int ctts_sample = 0;
> +    int64_t curr_pts = AV_NOPTS_VALUE;
> +    int64_t min_prev_pts = AV_NOPTS_VALUE;
> +    int64_t prev_max_pts = AV_NOPTS_VALUE;
> +    int num_steps = 0;
> +
> +    if (st->codecpar->video_delay <= 0 && msc->ctts_data &&
> +        st->codecpar->codec_id == AV_CODEC_ID_H264) {
> +        st->codecpar->video_delay = 0;
> +        for(ind = 0; ind < st->nb_index_entries && ctts_ind < msc->ctts_count; ++ind) {
> +            curr_pts = st->index_entries[ind].timestamp + msc->ctts_data[ctts_ind].duration;
> +
> +            // Everytime we encounter a new max_pts we reset num_steps and compute again.
> +            if (curr_pts > prev_max_pts) {
> +                st->codecpar->video_delay = FFMIN(FFMAX(st->codecpar->video_delay, num_steps), 16);
> +                num_steps = 0;
> +                prev_max_pts = curr_pts;
> +                min_prev_pts = curr_pts;
> +            } else {
> +                // Compute delay as the length of the path from max PTS to min PTS.
> +                // Frames: I0 I1 B0 B1 B2
> +                // PTS:     0  4  1  2  3 -> num_steps = delay = 1 (4->1)
> +                //
> +                // Frames: I0 I1 B1 B0 B2
> +                // PTS:     0  4  2  1  3 -> num_steps = delay = 2 (4->2, 2->1)
> +                if (min_prev_pts != AV_NOPTS_VALUE) {
> +                    if (curr_pts < min_prev_pts) {
> +                        ++num_steps;
> +                        min_prev_pts = curr_pts;
> +                    }
> +                }
> +            }

Can you explain why this algorithm is correct ?
(iam asking as i suspect it is not correct, but i may be wrong)

What this should do is find the minimum buffer size to sort the stream
of frame timestamps.


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Its not that you shouldnt use gotos but rather that you should write
readable code and code with gotos often but not always is less readable
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