Reels: clips play through the beat instead of freezing early

A clip beat capped playback at CLIP_SECONDS and filled the rest of the
narration with a tpad freeze-frame, so a clip stopped dead on its last
frame for a second or two before the transition — a glitchy pause that
stills don't have. Extract clip_beat_plan: the clip now plays for as
much of its beat as the source footage covers, and we freeze only when
the source is genuinely shorter than the narration. Bump RENDER_VERSION.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/reels/mod.rs

@@ -207,7 +207,7 @@ fn finish_job(
 
 /// Render version: bump to invalidate every cached reel after a rendering /
 /// scripting change that should produce a fresh result.
-const RENDER_VERSION: u32 = 6;
+const RENDER_VERSION: u32 = 7;
 
 /// Narration expressiveness — Chatterbox's `exaggeration` knob. A slight bump
 /// over the ~0.5 default warms up otherwise-flat narration without over-acting;

src/reels/render.rs

@@ -41,8 +41,10 @@ const NARRATION_TAIL_SECONDS: f64 = 0.6;
 const SINGLE_FADE_SECONDS: f64 = 0.35;
 const BURST_FADE_SECONDS: f64 = 0.12;
 
-/// Video-clip framing. A clip plays at most this long, with its live audio
-/// ducked to `CLIP_DUCK_VOLUME` under the narration.
+/// Video-clip framing. Fallback cap on how much of a clip we read when the
+/// source length can't be probed; with a known length, a clip instead plays for
+/// as much of its beat as its footage allows (see [`clip_beat_plan`]). Its live
+/// audio is ducked to `CLIP_DUCK_VOLUME` under the narration.
 pub const CLIP_SECONDS: f64 = 5.0;
 const CLIP_DUCK_VOLUME: f64 = 0.35;
 
@@ -316,6 +318,28 @@ pub async fn render_beat(
 
 // --- Video-clip beats --------------------------------------------------------
 
+/// Decide how long the clip plays and how long the whole beat lasts, from the
+/// source video's length (if known) and the narration length. Returns
+/// `(clip_dur, beat_total)`.
+///
+/// The beat always lasts long enough for the full narration. The clip plays for
+/// as much of that beat as its footage covers — so the motion fills the screen
+/// time rather than stopping early. We only freeze the last frame (the
+/// `beat_total - clip_dur` gap, handled by `tpad` in [`clip_video_filter`]) when
+/// the source video is genuinely shorter than the narration. Capping clip
+/// playback at a fixed length while the narration ran longer was what produced
+/// the second-or-two freeze that read as a glitchy pause before the transition.
+pub fn clip_beat_plan(source_dur: Option<f64>, narration_secs: f64) -> (f64, f64) {
+    let want = segment_duration(narration_secs);
+    let clip_dur = match source_dur {
+        // Known length: play up to the whole beat, but never past the source.
+        Some(d) if d > 0.0 => d.min(want),
+        // Unknown length: read up to the fallback cap; tpad covers any shortfall.
+        _ => want.min(CLIP_SECONDS),
+    };
+    (clip_dur, want.max(clip_dur))
+}
+
 /// Video chain for a clip beat: fill the clip to the portrait canvas (blurred
 /// backdrop, same look as photos), normalize fps, hold the last frame if the
 /// narration outlasts the clip (`tpad`), then fade. Produces `[v]`.
@@ -446,16 +470,13 @@ pub async fn render_clip_beat(
     opts: &SegmentOpts,
 ) -> Result<()> {
     let clip_str = clip_path.to_string_lossy().to_string();
-    // Clamp the clip to its own length so a short video isn't padded to the cap.
+    // Play the clip for as much of the beat as its footage covers; freeze only
+    // when the source is genuinely shorter than the narration (see clip_beat_plan).
     let source_dur = crate::video::ffmpeg::get_duration_seconds(&clip_str)
         .await
         .ok()
         .flatten();
-    let clip_dur = match source_dur {
-        Some(d) if d > 0.0 && d < CLIP_SECONDS => d,
-        _ => CLIP_SECONDS,
-    };
-    let beat_total = clip_dur.max(segment_duration(narration_secs));
+    let (clip_dur, beat_total) = clip_beat_plan(source_dur, narration_secs);
     let has_audio = has_audio_stream(&clip_str).await;
 
     let args = build_clip_beat_args(
@@ -630,6 +651,33 @@ mod tests {
         assert!(g.contains("overlay=(W-w)/2:(H-h)/2"));
     }
 
+    #[test]
+    fn clip_beat_plan_plays_clip_through_the_whole_beat_when_source_is_long() {
+        // 30s source, 4s narration → beat is narration+tail (4.6), and the clip
+        // plays that whole 4.6s of motion: no freeze (clip_dur == beat_total).
+        let (clip_dur, beat_total) = clip_beat_plan(Some(30.0), 4.0);
+        assert!((beat_total - 4.6).abs() < 1e-9);
+        assert!((clip_dur - 4.6).abs() < 1e-9);
+        assert!((beat_total - clip_dur).abs() < 1e-9); // no hold
+    }
+
+    #[test]
+    fn clip_beat_plan_freezes_only_when_source_shorter_than_narration() {
+        // 2s source under a 4s narration → play all 2s, freeze the remainder.
+        let (clip_dur, beat_total) = clip_beat_plan(Some(2.0), 4.0);
+        assert!((clip_dur - 2.0).abs() < 1e-9);
+        assert!((beat_total - 4.6).abs() < 1e-9);
+        assert!(beat_total - clip_dur > 2.0); // unavoidable freeze gap
+    }
+
+    #[test]
+    fn clip_beat_plan_caps_read_when_source_length_unknown() {
+        // Probe failed: read up to the fallback cap, beat still covers narration.
+        let (clip_dur, beat_total) = clip_beat_plan(None, 8.0);
+        assert!((clip_dur - CLIP_SECONDS).abs() < 1e-9);
+        assert!((beat_total - 8.6).abs() < 1e-9);
+    }
+
     #[test]
     fn clip_filter_no_tpad_when_clip_covers_the_beat() {
         // Clip at least as long as the beat → no freeze.