Stretch the ease length to get back into the right cadence.

Unless the speed change is very small, we can stretch the ease a bit
(from the default 200 ms into anything in the [0,2] second range)
such that we conveniently hit an original frame. This means that if
we go into a speed such as 100% or 200%, we've got a very high
likelyhood of going into a locked cadence, with the associated
quality and performance benefits.

diff --git a/futatabi/player.cpp b/futatabi/player.cpp
index c5581b40a9a6f9462380cac98ac1243b496ff2b1..b905be0f82a1b35dfcf8d0a3bb20c0825bf72afb 100644 (file)
--- a/futatabi/player.cpp
+++ b/futatabi/player.cpp
@@ -149,6 +149,11 @@ public:
        }
 
        void snap_by(int64_t offset) {
+               if (in_easing) {
+                       // Easing will normally aim for a snap at the very end,
+                       // so don't disturb it by jittering during the ease.
+                       return;
+               }
                origin.in_pts += offset;
        }
 
@@ -162,7 +167,9 @@ public:
        // about 200 ms. This is a simple linear ramp; I tried various forms of
        // Bézier curves for more elegant/dramatic changing, but it seemed linear
        // looked just as good in practical video.
-       void start_easing(double new_master_speed, Instant now);
+       void start_easing(double new_master_speed, int64_t length_out_pts, Instant now);
+
+       int64_t find_easing_length(double master_speed_target, int64_t length_out_pts, const vector<FrameOnDisk> &frames, Instant now);
 
 private:
        // Find out how far we are into the easing curve (0..1).
@@ -179,7 +186,7 @@ private:
        bool in_easing = false;
        int64_t ease_started_pts = 0;
        double master_speed_ease_target;
-       static constexpr int64_t ease_length_out_pts = TIMEBASE / 5;  // 200 ms.
+       int64_t ease_length_out_pts = 0;
 };
 
 TimelineTracker::Instant TimelineTracker::advance_to_frame(int64_t frameno)
@@ -220,7 +227,7 @@ void TimelineTracker::change_master_speed(double new_master_speed, Instant now)
        origin = now;
 }
 
-void TimelineTracker::start_easing(double new_master_speed, Instant now)
+void TimelineTracker::start_easing(double new_master_speed, int64_t length_out_pts, Instant now)
 {
        if (in_easing) {
                // Apply whatever we managed to complete of the previous easing.
@@ -231,6 +238,7 @@ void TimelineTracker::start_easing(double new_master_speed, Instant now)
        in_easing = true;
        ease_started_pts = now.out_pts;
        master_speed_ease_target = new_master_speed;
+       ease_length_out_pts = length_out_pts;
 }
 
 double TimelineTracker::find_ease_t(double out_pts) const
@@ -249,6 +257,85 @@ double TimelineTracker::easing_out_pts_adjustment(double out_pts) const
        return val;
 }
 
+int64_t TimelineTracker::find_easing_length(double master_speed_target, int64_t desired_length_out_pts, const vector<FrameOnDisk> &frames, Instant now)
+{
+       // Find out what frame we would have hit (approximately) with the given ease length.
+       double in_pts_length = 0.5 * (master_speed_target + master_speed) * desired_length_out_pts * clip->speed;
+       const int input_frame_num = distance(
+               frames.begin(),
+               find_first_frame_at_or_after(frames, lrint(now.in_pts + in_pts_length)));
+
+       // Round length_out_pts to the nearest amount of whole frames.
+       const double frame_length = TIMEBASE / global_flags.output_framerate;
+       const int length_out_frames = lrint(desired_length_out_pts / frame_length);
+
+       // Time the easing so that we aim at 200 ms (or whatever length_out_pts
+       // was), but adjust it so that we hit exactly on a frame. Unless we are
+       // somehow unlucky and run in the middle of a bad fade, this should
+       // lock us nicely into a cadence where we hit original frames (of course
+       // assuming the new speed is a reasonable ratio).
+       //
+       // Assume for a moment that we are easing into a slowdown, and that
+       // we're slightly too late to hit the frame we want to. This means that
+       // we can shorten the ease a bit; this chops some of the total integrated
+       // velocity and arrive at the frame a bit sooner. Solve for the time
+       // we want to shorten the ease by (let's call it x, where the original
+       // length of the ease is called len) such that we hit exactly the in
+       // pts at the right time:
+       //
+       //   0.5 * (mst + ms) * (len - x) * cs + mst * x * cs = desired_len_in_pts
+       //
+       // gives
+       //
+       //   x = (2 * desired_len_in_pts / cs - (mst + ms) * len) / (mst - ms)
+       //
+       // Conveniently, this holds even if we are too early; a negative x
+       // (surprisingly!) gives a lenghtening such that we don't hit the desired
+       // frame, but hit one slightly later. (x larger than len means that
+       // it's impossible to hit the desired frame, even if we dropped the ease
+       // altogether and just changed speeds instantly.) We also have sign invariance,
+       // so that these properties hold even if we are speeding up, not slowing
+       // down. Together, these two properties mean that we can cast a fairly
+       // wide net, trying various input and output frames and seeing which ones
+       // can be matched up with a minimal change to easing time. (This lets us
+       // e.g. end the ease close to the midpoint between two endpoint frames
+       // even if we don't know the frame rate, or deal fairly robustly with
+       // dropped input frames.) Many of these will give us the same answer,
+       // but that's fine, because the ease length is the only output.
+       int64_t best_length_out_pts = TIMEBASE * 10;  // Infinite.
+       for (int output_frame_offset = -2; output_frame_offset <= 2; ++output_frame_offset) {
+               int64_t aim_length_out_pts = lrint((length_out_frames + output_frame_offset) * frame_length);
+               if (aim_length_out_pts < 0) {
+                       continue;
+               }
+
+               for (int input_frame_offset = -2; input_frame_offset <= 2; ++input_frame_offset) {
+                       if (input_frame_num + input_frame_offset < 0 ||
+                           input_frame_num + input_frame_offset >= int(frames.size())) {
+                               continue;
+                       }
+                       const int64_t in_pts = frames[input_frame_num + input_frame_offset].pts;
+                       double shorten_by_out_pts = (2.0 * (in_pts - now.in_pts) / clip->speed - (master_speed_target + master_speed) * aim_length_out_pts) / (master_speed_target - master_speed);
+                       int64_t length_out_pts = lrint(aim_length_out_pts - shorten_by_out_pts);
+
+                       if (length_out_pts >= 0 &&
+                           abs(length_out_pts - desired_length_out_pts) < abs(best_length_out_pts - desired_length_out_pts)) {
+                               best_length_out_pts = length_out_pts;
+                       }
+               }
+       }
+
+       // If we need more than two seconds of easing, we give up --
+       // this can happen if we're e.g. going from 101% to 100%.
+       // If so, it would be better to let other mechanisms, such as the switch
+       // to the next clip, deal with getting us back into sync.
+       if (best_length_out_pts > TIMEBASE * 2) {
+               return desired_length_out_pts;
+       } else {
+               return best_length_out_pts;
+       }
+}
+
 }  // namespace
 
 void Player::play_playlist_once()
@@ -339,7 +426,9 @@ void Player::play_playlist_once()
 
                        float new_master_speed = change_master_speed.exchange(0.0f / 0.0f);
                        if (!std::isnan(new_master_speed) && !timeline.in_master_speed(new_master_speed)) {
-                               timeline.start_easing(new_master_speed, instant);
+                               int64_t ease_length_out_pts = TIMEBASE / 5;  // 200 ms.
+                               int64_t recommended_pts_length = timeline.find_easing_length(new_master_speed, ease_length_out_pts, frames[clip->stream_idx], instant);
+                               timeline.start_easing(new_master_speed, recommended_pts_length, instant);
                        }
 
                        if (should_skip_to_next.exchange(false)) {  // Test and clear.