vo_opengl: add smoothmotion frame blending

SmoothMotion is a way to time and blend frames made popular by MadVR. It's
intended behaviour is to remove stuttering caused by mismatches between the
display refresh rate and the video fps, while preserving the video's original
artistic qualities (no soap opera effect). It's supposed to make 24fps video
playback on 60hz monitors as close as possible to a 24hz monitor.

Instead of drawing a frame once once it's pts has passed the vsync time, we
redraw at the display refresh rate, and if we detect the vsync is between two
frames we interpolated them (depending on their position relative to the vsync).
We actually interpolate as few frames as possible to avoid a blur effect as
much as possible. For example, if we were to play back a 1fps video on a 60hz
monitor, we would blend at most on 1 vsync for each frame (while the other 59
vsyncs would be rendered as is).

Frame interpolation is always done before scaling and in linear light when
possible (an ICC profile is used, or :srgb is used).

1 files changed, 9 insertions, 0 deletions

diff --git a/DOCS/man/vo.rst b/DOCS/man/vo.rst
index 6190526bd7..e0845d0e54 100644
--- a/DOCS/man/vo.rst
+++ b/DOCS/man/vo.rst
@@ -572,6 +572,15 @@ Available video output drivers are:
         Color used to draw parts of the mpv window not covered by video.
         See ``--osd-color`` option how colors are defined.
 
+    ``smoothmotion``
+        Use frame interpolation to reduce stuttering caused by mismatches in
+        video fps and display refresh rate (similar to MadVR's smoothmotion,
+        thus the naming).
+
+    ``smoothmotion-threshold=<threshold>``
+        Mix threshold at which interpolation is skipped (default: 0.0 – never
+        skip).
+
 ``opengl-hq``
     Same as ``opengl``, but with default settings for high quality rendering.