vf_vapoursynth: pass through container FPS value

This is basically a hack; but apparently a needed one, since many
vapoursynth filters insist on having a FPS set.

We need to apply the FPS override before creating the filters. Also
change some terminal output related to the FPS value.

DOCS/man/vf.rst

@@ -771,6 +771,15 @@ Available filters are:
         Display size of the video. Can be different from video size if the
         video does not use square pixels (e.g. DVD).
 
+    ``container_fps``
+        FPS value as reported by file headers. This value can be wrong or
+        completely broken (e.g. 0 or NaN). Even if the value is correct,
+        if another filter changes the real FPS (by dropping or inserting
+        frames), the value of this variable might not be useful. Note that
+        the ``--fps`` command line option overrides this value.
+
+        Useful for some filters which insist on having a FPS.
+
 ``vapoursynth-lazy``
     The same as ``vapoursynth``, but doesn't load Python scripts. Instead, a
     custom backend using Lua and the raw VapourSynth API is used. The syntax

player/video.c

@@ -180,6 +180,7 @@ static void recreate_video_filters(struct MPContext *mpctx)
     d_video->vfilter->hwdec = d_video->hwdec_info;
     d_video->vfilter->wakeup_callback = wakeup_playloop;
     d_video->vfilter->wakeup_callback_ctx = mpctx;
+    d_video->vfilter->container_fps = d_video->fps;
 
     vf_append_filter_list(d_video->vfilter, opts->vf_settings);
 
@@ -294,6 +295,13 @@ int reinit_video_chain(struct MPContext *mpctx)
     d_video->fps = sh->video->fps;
     d_video->vo = mpctx->video_out;
 
+    if (opts->force_fps) {
+        d_video->fps = opts->force_fps;
+        MP_INFO(mpctx, "FPS forced to %5.3f.\n", d_video->fps);
+        MP_INFO(mpctx, "Use --no-correct-pts to force FPS based timing.\n");
+    }
+    update_fps(mpctx);
+
     vo_control(mpctx->video_out, VOCTRL_GET_HWDEC_INFO, &d_video->hwdec_info);
 
     recreate_video_filters(mpctx);
@@ -317,16 +325,6 @@ int reinit_video_chain(struct MPContext *mpctx)
     reset_video_state(mpctx);
     reset_subtitle_state(mpctx);
 
-    if (opts->force_fps) {
-        d_video->fps = opts->force_fps;
-        MP_INFO(mpctx, "FPS forced to be %5.3f.\n", d_video->fps);
-    }
-    if (!sh->video->fps && !opts->force_fps && !opts->correct_pts) {
-        MP_ERR(mpctx, "FPS not specified in the "
-               "header or invalid, use the -fps option.\n");
-    }
-    update_fps(mpctx);
-
     return 1;
 
 err_out:

video/filter/vf.h

@@ -123,6 +123,8 @@ struct vf_chain {
     struct mp_image_params output_params;
     uint8_t allowed_output_formats[IMGFMT_END - IMGFMT_START];
 
+    double container_fps;
+
     struct mp_log *log;
     struct MPOpts *opts;
     struct mpv_global *global;

video/filter/vf_vapoursynth.c

@@ -590,6 +590,7 @@ static int reinit_vs(struct vf_instance *vf)
 
     p->vsapi->propSetInt(vars, "video_in_dw", p->fmt_in.d_w, 0);
     p->vsapi->propSetInt(vars, "video_in_dh", p->fmt_in.d_h, 0);
+    p->vsapi->propSetFloat(vars, "container_fps", vf->chain->container_fps, 0);
 
     if (p->drv->load(vf, vars) < 0)
         goto error;