lavfi: add framestep filter

This filter is inspired upon libmpcodecs/vf_framestep.c, by Daniele
Forghieri.

Only-keyframe output is not supported, since that feature can be achieved
through the more versatile select filter.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/filters.texi b/doc/filters.texi
index e6279bd4d7..da69e569c9 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -2084,6 +2084,13 @@ Desired output framerate.
 
 @end table
 
+@section framestep
+
+Select one frame every N.
+
+This filter accepts in input a string representing a positive
+integer. Default argument is @code{1}.
+
 @anchor{frei0r}
 @section frei0r