Improve frame snapping in difficult cases.

diff --git a/futatabi/player.cpp b/futatabi/player.cpp
index 8aac5777cc7ef24cdc87db6112cf33b41d4dd1c7..c4ed8972135e24df4e817f47dcd001ac77cffdec 100644 (file)
--- a/futatabi/player.cpp
+++ b/futatabi/player.cpp
@@ -108,8 +108,6 @@ got_clip:
                        }
                }
 
-               // TODO: Lock the speed to a rational multiple of the frame rate if possible.
-
                int64_t in_pts_start_next_clip = -1;
                steady_clock::time_point next_frame_start;
                for (int frameno = 0; !should_quit; ++frameno) {  // Ends when the clip ends.
@@ -260,10 +258,10 @@ got_clip:
                        // Snap to input frame: If we can do so with less than 1% jitter
                        // (ie., move less than 1% of an _output_ frame), do so.
                        // TODO: Snap secondary (fade-to) clips in the same fashion.
+                       double pts_snap_tolerance = 0.01 * double(TIMEBASE) / global_flags.output_framerate;
                        bool snapped = false;
                        for (FrameOnDisk snap_frame : { frame_lower, frame_upper }) {
-                               double snap_pts_as_frameno = (snap_frame.pts - in_pts_origin) * global_flags.output_framerate / TIMEBASE / clip.speed;
-                               if (fabs(snap_pts_as_frameno - frameno) < 0.01) {
+                               if (fabs(snap_frame.pts - in_pts) < pts_snap_tolerance) {
                                        auto display_func = [this, primary_stream_idx, snap_frame, secondary_frame, fade_alpha]{
                                                if (destination != nullptr) {
                                                        destination->setFrame(primary_stream_idx, snap_frame, secondary_frame, fade_alpha);
@@ -292,6 +290,31 @@ got_clip:
                                continue;
                        }
 
+                       // The snapping above makes us lock to the input framerate, even in the presence
+                       // of pts drift, for most typical cases where it's needed, like converting 60 → 2x60
+                       // or 60 → 2x59.94. However, there are some corner cases like 25 → 2x59.94, where we'd
+                       // get a snap very rarely (in the given case, once every 24 output frames), and by
+                       // that time, we'd have drifted out. We could have solved this by changing the overall
+                       // speed ever so slightly, but it requires that we know the actual frame rate (which
+                       // is difficult in the presence of jitter and missed frames), or at least do some kind
+                       // of matching/clustering. Instead, we take the opportunity to lock to in-between rational
+                       // points if we can. E.g., if we are converting 60 → 2x60, we would not only snap to
+                       // an original frame every other frame; we would also snap to exactly alpha=0.5 every
+                       // in-between frame. Of course, we will still need to interpolate, but we get a lot
+                       // closer when we actually get close to an original frame. In other words: Snap more
+                       // often, but snap less each time. Unless the input and output frame rates are completely
+                       // decorrelated with no common factor, of course (e.g. 12.345 → 34.567, which we should
+                       // really never see in practice).
+                       for (double fraction : { 1.0 / 2.0, 1.0 / 3.0, 2.0 / 3.0, 1.0 / 4.0, 3.0 / 4.0,
+                                                1.0 / 5.0, 2.0 / 5.0, 3.0 / 5.0, 4.0 / 5.0 }) {
+                               double subsnap_pts = frame_lower.pts + fraction * (frame_upper.pts - frame_lower.pts);
+                               if (fabs(subsnap_pts - in_pts) < pts_snap_tolerance) {
+                                       in_pts_origin += lrint(subsnap_pts) - in_pts;
+                                       in_pts = lrint(subsnap_pts);
+                                       break;
+                               }
+                       }
+
                        if (stream_output != FILE_STREAM_OUTPUT && time_behind >= milliseconds(100)) {
                                fprintf(stderr, "WARNING: %ld ms behind, dropping an interpolated frame.\n",
                                        lrint(1e3 * duration<double>(time_behind).count()));