[FFmpeg-devel] [PATCH v2] avcodec/ccaption_dec: Make real-time latency configurable

[Pavel Koshevoy]

On Sat, Jun 5, 2021 at 11:40 AM Pavel Koshevoy <pkoshevoy at gmail.com> wrote:

> Un-hardcode the 200ms minimum latency between emitting subtitle events
> so that those that wish to receive a subtitle event for every screen
> change could do so.
>
> The problem with delaying realtime output by any amount is that it is
> unknown when the next byte pair that would trigger output will happen.
> It may be within 200ms, or it may be several seconds later -- that's
> not realtime at all.
> ---
>  libavcodec/ccaption_dec.c | 4 +++-
>  libavcodec/version.h      | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/libavcodec/ccaption_dec.c b/libavcodec/ccaption_dec.c
> index de05d037a8..27c61527f6 100644
> --- a/libavcodec/ccaption_dec.c
> +++ b/libavcodec/ccaption_dec.c
> @@ -238,6 +238,7 @@ struct Screen {
>  typedef struct CCaptionSubContext {
>      AVClass *class;
>      int real_time;
> +    int real_time_latency_msec;
>      int data_field;
>      struct Screen screen[2];
>      int active_screen;
> @@ -906,7 +907,7 @@ static int decode(AVCodecContext *avctx, void *data,
> int *got_sub, AVPacket *avp
>      }
>
>      if (ctx->real_time && ctx->screen_touched &&
> -        sub->pts > ctx->last_real_time + av_rescale_q(200, ms_tb,
> AV_TIME_BASE_Q)) {
> +        sub->pts >= ctx->last_real_time +
> av_rescale_q(ctx->real_time_latency_msec, ms_tb, AV_TIME_BASE_Q)) {
>          ctx->last_real_time = sub->pts;
>          ctx->screen_touched = 0;
>
> @@ -927,6 +928,7 @@ static int decode(AVCodecContext *avctx, void *data,
> int *got_sub, AVPacket *avp
>  #define SD AV_OPT_FLAG_SUBTITLE_PARAM | AV_OPT_FLAG_DECODING_PARAM
>  static const AVOption options[] = {
>      { "real_time", "emit subtitle events as they are decoded for
> real-time display", OFFSET(real_time), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0,
> 1, SD },
> +    { "real_time_latency_msec", "minimum elapsed time between emitting
> real-time subtitle events", OFFSET(real_time_latency_msec),
> AV_OPT_TYPE_INT, { .i64 = 200 }, 0, 500, SD },
>      { "data_field", "select data field", OFFSET(data_field),
> AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 1, SD, "data_field" },
>      { "auto",   "pick first one that appears", 0, AV_OPT_TYPE_CONST, {
> .i64 =-1 }, 0, 0, SD, "data_field" },
>      { "first",  NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 0 }, 0, 0, SD,
> "data_field" },
> diff --git a/libavcodec/version.h b/libavcodec/version.h
> index 48165b9ac4..5b1e9e77f3 100644
> --- a/libavcodec/version.h
> +++ b/libavcodec/version.h
> @@ -29,7 +29,7 @@
>
>  #define LIBAVCODEC_VERSION_MAJOR  59
>  #define LIBAVCODEC_VERSION_MINOR   1
> -#define LIBAVCODEC_VERSION_MICRO 100
> +#define LIBAVCODEC_VERSION_MICRO 101
>
>  #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
>                                                 LIBAVCODEC_VERSION_MINOR, \
> --
> 2.26.2
>
>

Ping.  If there are no objections may this be applied?

Thank you,
    Pavel.