[FFmpeg-devel] [PATCH] avfilter/setpts: add option to preserve framerate

[Gyan Doshi <ffmpeg@gyani.pro>]

In f121d95, the outlink framerate was unconditionally unset.
This breaks/bloats outputs from CFR muxers unless the user explicitly
set a sane framerate. And the most common invocation for setpts seen in
workflows, our docs and across the web is `PTS-STARTPTS` or others of the
form `PTS+constant` which preserve the input framerate.

Fixes #11428
---
 doc/filters.texi          | 6 ++++++
 libavfilter/setpts.c      | 6 +++++-
 tests/filtergraphs/setpts | 2 +-
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/doc/filters.texi b/doc/filters.texi
index b926b865ae..fc352ed34a 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -31478,6 +31478,12 @@ This filter accepts the following options:
 @item expr
 The expression which is evaluated for each frame to construct its timestamp.
 
+@item keep_fps (@emph{video only})
+Boolean option which determines if the original framerate is preserved.
+If set to false, be advised that a sane frame rate should be explicitly
+specified if output is sent to a constant frame rate muxer.
+Default is @code{true}.
+
 @end table
 
 The expression is evaluated through the eval API and can contain the following
diff --git a/libavfilter/setpts.c b/libavfilter/setpts.c
index 75d96247af..c1a15a9a7c 100644
--- a/libavfilter/setpts.c
+++ b/libavfilter/setpts.c
@@ -98,6 +98,7 @@ typedef struct SetPTSContext {
     const AVClass *class;
     char *expr_str;
     AVExpr *expr;
+    int keep_fps;
     double var_values[VAR_VARS_NB];
     enum AVMediaType type;
 } SetPTSContext;
@@ -153,8 +154,10 @@ static int config_input(AVFilterLink *inlink)
 static int config_output_video(AVFilterLink *outlink)
 {
     FilterLink *l = ff_filter_link(outlink);
+    SetPTSContext *setpts = outlink->src->priv;
 
-    l->frame_rate = (AVRational){ 1, 0 };
+    if (!s->keep_fps)
+        l->frame_rate = (AVRational){ 1, 0 };
 
     return 0;
 }
@@ -320,6 +323,7 @@ static int process_command(AVFilterContext *ctx, const char *cmd, const char *ar
 #if CONFIG_SETPTS_FILTER
 static const AVOption setpts_options[] = {
     { "expr", "Expression determining the frame timestamp", OFFSET(expr_str), AV_OPT_TYPE_STRING, { .str = "PTS" }, .flags = V|F|R },
+    { "keep_fps", "Preserve input framerate", OFFSET(keep_fps), AV_OPT_TYPE_INT, { .i64 = 1 }, 0, 1, .flags = V|F },
     { NULL }
 };
 AVFILTER_DEFINE_CLASS(setpts);
diff --git a/tests/filtergraphs/setpts b/tests/filtergraphs/setpts
index 79037d1b65..c217778bb4 100644
--- a/tests/filtergraphs/setpts
+++ b/tests/filtergraphs/setpts
@@ -1,2 +1,2 @@
 settb=1/1000,
-setpts=1/(35*TB) * (N + 0.05 * sin(N*2*PI/25))
+setpts=1/(35*TB) * (N + 0.05 * sin(N*2*PI/25)):keep_fps=0
-- 
2.46.1

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