+ // If there's nothing to interpolate between, or if interpolation is turned off,
+ // or we're a preview, then just display the frame.
+ if (frame_lower.pts == frame_upper.pts || global_flags.interpolation_quality == 0 || video_stream == nullptr) {
+ display_single_frame(primary_stream_idx, frame_lower, secondary_stream_idx,
+ secondary_frame, fade_alpha, next_frame_start, /*snapped=*/false,
+ subtitle, play_audio);
+ 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;
+ }
+ }
+