S2.44: confidence on the Roto Brush span bar

Frames a tracker followed are coloured by its confidence (green, amber, red;
slate where the object was absent) and drawn taller; roto_trust reports them.
Not checked visually: no headless way to track without weights in a snapshot.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/engine/src/analysis/roto.rs

@@ -838,6 +838,25 @@ impl Session {
         changed
     }
 
+    /// How far to trust the frames a tracker followed (SAM 2), by layer frame: its confidence and
+    /// whether the object was absent. Frames the classic engine made have no entry.
+    pub fn roto_trust(&self, comp: ItemId, layer: LayerId, effect: Uid) -> BTreeMap<i64, fx::FrameTrust> {
+        let mut out = BTreeMap::new();
+        let Some(c) = self.project.comp(comp) else { return out };
+        let Some(l) = c.layer(layer) else { return out };
+        let Some(g) = l.props.find_group(effect) else { return out };
+        if is_object_matte(g) {
+            return out;
+        }
+        let ch = fx::Chain::new(&params_static(g), layer_size(&self.project, c, l), c.frame_rate.as_f64(), 1.0);
+        for f in ch.keys.keys() {
+            if let Some(t) = ch.trust_at(*f) {
+                out.insert(*f, t);
+            }
+        }
+        out
+    }
+
     /// The computed frames of an instance (full resolution), by layer frame.
     pub fn roto_computed(&self, comp: ItemId, layer: LayerId, effect: Uid) -> BTreeMap<i64, bool> {
         let mut out = BTreeMap::new();

crates/engine/src/tests/roto.rs

@@ -977,7 +977,9 @@ impl effectcraft_segment::ObjectTracker for Recorder {
 /// corrected frame; the frames before it keep their tracked results.
 #[test]
 fn a_correction_re_tracks_only_the_frames_it_changes() {
-    let (mut s, clip, _) = painted(center, "disk-retrack.mov");
+    let (mut s, clip, uid) = painted(center, "disk-retrack.mov");
+    let cid = s.active_comp_id().unwrap();
+    assert!(s.roto_trust(cid, clip, uid).is_empty(), "no tracker data before tracking");
     let rec = std::sync::Arc::new(Recorder::default());
     effectcraft_effects::roto::set_model(Some(rec.clone()));
     let run = |s: &mut Session| s.execute_checked("roto.propagate", json!({"layer": clip.0, "direction": "forward", "wait": true}));
@@ -1000,4 +1002,8 @@ fn a_correction_re_tracks_only_the_frames_it_changes() {
     let st = s.execute("roto.status", json!({"layer": clip.0, "frame": 3})).unwrap();
     assert_eq!(st["source"], "tracker", "{st}");
     assert_eq!(st["tracker"]["frames"].as_object().unwrap().len(), 21, "{st}");
+    // The span bar's data: the tracker's confidence for every tracked frame.
+    let trust = s.roto_trust(cid, clip, uid);
+    assert_eq!(trust.len(), 21);
+    assert!(trust.values().all(|t| t.confidence > 0.9 && !t.occluded));
 }