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, 13 insertions, 1 deletions

diff --git a/video/out/gl_video_shaders.glsl b/video/out/gl_video_shaders.glsl
index e40a94e185..c9398c8cd5 100644
--- a/video/out/gl_video_shaders.glsl
+++ b/video/out/gl_video_shaders.glsl
@@ -190,6 +190,7 @@ uniform float filter_param1_l;
 uniform float filter_param1_c;
 uniform float antiring_factor;
 uniform vec2 dither_size;
+uniform float inter_coeff;
 
 in vec2 texcoord;
 DECLARE_FRAGPARMS
@@ -374,7 +375,18 @@ void main() {
 #ifndef USE_CONV
 #define USE_CONV 0
 #endif
-#if USE_CONV == CONV_PLANAR
+#ifndef USE_LINEAR_INTERPOLATION
+#define USE_LINEAR_INTERPOLATION 0
+#endif
+#if USE_LINEAR_INTERPOLATION == 1
+    vec4 acolor = mix(
+        texture(texture0, texcoord),
+        texture(texture1, texcoord),
+        inter_coeff);
+    // debug code to visually check the interpolation amount
+    // vec4 acolor = texture(texture0, texcoord) -
+    //               inter_coeff * texture(texture1, texcoord);
+#elif USE_CONV == CONV_PLANAR
     vec4 acolor = vec4(SAMPLE(texture0, textures_size[0], texcoord).r,
                        SAMPLE_C(texture1, textures_size[1], chr_texcoord).r,
                        SAMPLE_C(texture2, textures_size[2], chr_texcoord).r,