lavfi/vf_libplacebo: allow fps conversion

This exposes libplacebo's frame mixing functionality to vf_libplacebo,
by allowing users to specify a desired target fps to output at. Incoming
frames will be smoothly resampled (in a manner determined by the
`frame_mixer` option, to be added in the next commit).

To generate a consistently timed output stream, we directly use the
desired framerate as the timebase, and simply output frames in
sequential order (tracked by the number of frames output so far).

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index ef48a4cf31..70bb0996d3 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -16044,6 +16044,13 @@ and @code{(oh-ph)/2}.
 Set the output placement width/height expressions, default values are @code{ow}
 and @code{oh}.
 
+@item fps
+Set the output frame rate. This can be rational, e.g. @code{60000/1001}. If
+set to the special string @code{none} (the default), input timestamps will
+instead be passed through to the output unmodified. Otherwise, the input video
+frames will be interpolated as necessary to rescale the video to the specified
+target framerate, in a manner as determined by the @option{frame_mixer} option.
+
 @item format
 Set the output format override. If unset (the default), frames will be output
 in the same format as the respective input frames. Otherwise, format conversion