avfilter/setpts: add option strip_fps to decide framerate handling

In f121d95, the outlink framerate was unconditionally unset.
This breaks/bloats outputs from CFR muxers unless the user explicitly
sets 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
general form `PTS+constant` which preserves the input framerate.

Default value is false, which restores old behaviour.

Fixes #11428

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 strip_fps (@emph{video only})
+Boolean option which determines if the original framerate metadata is unset.
+If set to true, be advised that a sane frame rate should be explicitly
+specified if output is sent to a constant frame rate muxer.
+Default is @code{false}.
+
 @end table
 
 The expression is evaluated through the eval API and can contain the following

libavfilter/setpts.c

@@ -98,6 +98,7 @@ typedef struct SetPTSContext {
     const AVClass *class;
     char *expr_str;
     AVExpr *expr;
+    int strip_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 *s = outlink->src->priv;
 
-    l->frame_rate = (AVRational){ 1, 0 };
+    if (s->strip_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 },
+    { "strip_fps",  "Unset framerate metadata", OFFSET(strip_fps), AV_OPT_TYPE_BOOL, { .i64 = 0 }, 0, 1, .flags = V|F },
     { NULL }
 };
 AVFILTER_DEFINE_CLASS(setpts);

tests/fate/hevc.mak

@@ -210,7 +210,7 @@ $(HEVC_TESTS_444_12BIT): SCALE_OPTS := -pix_fmt yuv444p12le -vf scale
 fate-hevc-conformance-%: CMD = framecrc -i $(TARGET_SAMPLES)/hevc-conformance/$(subst fate-hevc-conformance-,,$(@)).bit $(SCALE_OPTS)
 $(HEVC_TESTS_422_10BIN): CMD = framecrc -i $(TARGET_SAMPLES)/hevc-conformance/$(subst fate-hevc-conformance-,,$(@)).bin $(SCALE_OPTS)
 $(HEVC_TESTS_MULTIVIEW): CMD = framecrc -i $(TARGET_SAMPLES)/hevc-conformance/$(subst fate-hevc-conformance-,,$(@)).bit \
-	-pix_fmt yuv420p -map "0:view:0" -map "0:view:1" -vf setpts=N
+	-pix_fmt yuv420p -map "0:view:0" -map "0:view:1" -vf setpts=N:strip_fps=1
 
 FATE_HEVC-$(call FRAMECRC, HEVC, HEVC, HEVC_PARSER) += $(HEVC_TESTS_8BIT) $(HEVC_TESTS_444_8BIT)
 FATE_HEVC-$(call FRAMECRC, HEVC, HEVC, HEVC_PARSER SCALE_FILTER) +=         \

tests/fate/mov.mak

@@ -225,7 +225,7 @@ fate-mov-pcm-remux: CMP = oneline
 fate-mov-pcm-remux: REF = e76115bc392d702da38f523216bba165
 
 FATE_MOV_FFMPEG-$(call TRANSCODE, RAWVIDEO, MOV, TESTSRC_FILTER SETPTS_FILTER) += fate-mov-vfr
-fate-mov-vfr: CMD = md5 -filter_complex testsrc=size=2x2:duration=1,setpts=N*N -c rawvideo -fflags +bitexact -f mov
+fate-mov-vfr: CMD = md5 -filter_complex testsrc=size=2x2:duration=1,setpts=N*N:strip_fps=1 -c rawvideo -fflags +bitexact -f mov
 fate-mov-vfr: CMP = oneline
 fate-mov-vfr: REF = 1558b4a9398d8635783c93f84eb5a60d
 

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)):strip_fps=1