Re: [FFmpeg-devel] [PATCH 2/2] lavfi/setpts: introduce rand() function in expression

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Stefano Sabatini:
> This is useful to simulate random jitter.
> ---
>  Changelog            |  1 +
>  doc/filters.texi     | 10 +++++++++-
>  libavfilter/setpts.c | 39 +++++++++++++++++++++++++++++++++------
>  3 files changed, 43 insertions(+), 7 deletions(-)
> 
> diff --git a/Changelog b/Changelog
> index 424bfc11af..ed01c53264 100644
> --- a/Changelog
> +++ b/Changelog
> @@ -15,6 +15,7 @@ version <next>:
>  - tiltandshift filter
>  - qrencode filter and qrencodesrc source
>  - quirc filter
> +- lavfi/setpts: introduce rand() function in expression
>  
>  version 6.1:
>  - libaribcaption decoder
> diff --git a/doc/filters.texi b/doc/filters.texi
> index d1f95b9781..1d9a5d6c7d 100644
> --- a/doc/filters.texi
> +++ b/doc/filters.texi
> @@ -30946,7 +30946,7 @@ The expression which is evaluated for each frame to construct its timestamp.
>  @end table
>  
>  The expression is evaluated through the eval API and can contain the following
> -constants:
> +constants and functions:
>  
>  @table @option
>  @item FRAME_RATE, FR
> @@ -31010,6 +31010,8 @@ The timebase of the input timestamps.
>  @item T_CHANGE
>  Time of the first frame after command was applied or time of the first frame if no commands.
>  
> +@item rand(min, max)
> +a random number included between min and max
>  @end table
>  
>  @subsection Examples
> @@ -31021,6 +31023,12 @@ Start counting PTS from zero
>  setpts=PTS-STARTPTS
>  @end example
>  
> +@item
> +Apply a random jitter effect of +/-100 TB units:
> +@example
> +setpts=PTS+100rand(-100\,100)
> +@end example
> +
>  @item
>  Apply fast motion effect:
>  @example
> diff --git a/libavfilter/setpts.c b/libavfilter/setpts.c
> index 88a8d6af86..0f24a900b3 100644
> --- a/libavfilter/setpts.c
> +++ b/libavfilter/setpts.c
> @@ -32,6 +32,8 @@
>  #include "libavutil/internal.h"
>  #include "libavutil/mathematics.h"
>  #include "libavutil/opt.h"
> +#include "libavutil/lfg.h"
> +#include "libavutil/random_seed.h"
>  #include "libavutil/time.h"
>  #include "audio.h"
>  #include "avfilter.h"
> @@ -101,18 +103,39 @@ typedef struct SetPTSContext {
>      AVExpr *expr;
>      double var_values[VAR_VARS_NB];
>      enum AVMediaType type;
> +    AVLFG lfg;
>  } SetPTSContext;
>  
>  #define V(name_) \
>      setpts->var_values[VAR_##name_]
>  
> +static double drand(void *ctx, double min, double max)
> +{
> +    SetPTSContext *setpts = ((AVFilterContext *)ctx)->priv;
> +
> +    return min + (max-min) / UINT_MAX * av_lfg_get(&setpts->lfg);
> +}
> +
> +typedef double (*eval_func2)(void *, double a, double b);
> +
> +static const eval_func2 fun2[] = {
> +    drand,
> +    NULL
> +};
> +
> +static const char *const fun2_names[] = {
> +    "rand"
> +};
> +
>  static av_cold int init(AVFilterContext *ctx)
>  {
>      SetPTSContext *setpts = ctx->priv;
>      int ret;
>  
> +    av_lfg_init(&setpts->lfg, av_get_random_seed());
> +
>      if ((ret = av_expr_parse(&setpts->expr, setpts->expr_str,
> -                             var_names, NULL, NULL, NULL, NULL, 0, ctx)) < 0) {
> +                             var_names, NULL, NULL, fun2_names, fun2, 0, ctx)) < 0) {
>          av_log(ctx, AV_LOG_ERROR, "Error while parsing expression '%s'\n", setpts->expr_str);
>          return ret;
>      }
> @@ -126,6 +149,7 @@ static av_cold int init(AVFilterContext *ctx)
>      V(STARTPTS)    = NAN;
>      V(STARTT)      = NAN;
>      V(T_CHANGE)    = NAN;
> +
>      return 0;
>  }
>  
> @@ -159,8 +183,10 @@ static inline char *double2int64str(char *buf, double v)
>      return buf;
>  }
>  
> -static double eval_pts(SetPTSContext *setpts, AVFilterLink *inlink, AVFrame *frame, int64_t pts)
> +static double eval_pts(AVFilterContext *ctx, AVFilterLink *inlink, AVFrame *frame, int64_t pts)
>  {
> +    SetPTSContext *setpts = ctx->priv;
> +
>      if (isnan(V(STARTPTS))) {
>          V(STARTPTS) = TS2D(pts);
>          V(STARTT  ) = TS2T(pts, inlink->time_base);
> @@ -186,17 +212,18 @@ FF_ENABLE_DEPRECATION_WARNINGS
>          }
>      }
>  
> -    return av_expr_eval(setpts->expr, setpts->var_values, NULL);
> +    return av_expr_eval(setpts->expr, setpts->var_values, ctx);
>  }
>  #define d2istr(v) double2int64str((char[BUF_SIZE]){0}, v)
>  
>  static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
>  {
> -    SetPTSContext *setpts = inlink->dst->priv;
> +    AVFilterContext *ctx = inlink->dst;
> +    SetPTSContext *setpts = ctx->priv;
>      int64_t in_pts = frame->pts;
>      double d;
>  
> -    d = eval_pts(setpts, inlink, frame, frame->pts);
> +    d = eval_pts(ctx, inlink, frame, frame->pts);
>      frame->pts = D2TS(d);
>  
>      av_log(inlink->dst, AV_LOG_TRACE,
> @@ -250,7 +277,7 @@ static int activate(AVFilterContext *ctx)
>          return filter_frame(inlink, in);
>  
>      if (ff_inlink_acknowledge_status(inlink, &status, &pts)) {
> -        double d = eval_pts(setpts, inlink, NULL, pts);
> +        double d = eval_pts(ctx, inlink, NULL, pts);
>  
>          av_log(ctx, AV_LOG_TRACE, "N:EOF PTS:%s T:%f -> PTS:%s T:%f\n",
>                 d2istr(V(PTS)), V(T), d2istr(d), TS2T(d, inlink->time_base));

Why is this added here and not in lavu/eval so that it is available with
all expressions?

- Andreas

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