Auto-orient: keep the last heading when an expression-driven Position stops (fixes #1650)

crates/render/src/three_d/camera.rs

@@ -241,6 +241,53 @@ fn path_tangent(ctx: &EvalCtx, layer: &Layer) -> Option<Vec3> {
         .filter(|k| **k <= ctx.time)
         .find_map(|&k| dir(pos(k) - pos(k - dt)))
         .or_else(|| keys.iter().filter(|k| **k > ctx.time).find_map(|&k| dir(pos(k + dt) - pos(k))))
+        .or_else(|| {
+            // An expression has no keys to fall back on: sample it instead.
+            let expr = dims.iter().any(|m| tr.get(m).is_some_and(|p| p.has_expression()));
+            expr.then(|| sampled_tangent(ctx, layer, &pos, dt, true).or_else(|| sampled_tangent(ctx, layer, &pos, dt, false))).flatten()
+        })
+}
+
+/// Samples spent on each phase of [`sampled_tangent`]: it runs per layer per frame.
+const MOTION_SEARCH_STEPS: u32 = 12;
+
+/// Direction of the motion nearest to the context time, before it (`earlier`) or after it, for a
+/// position with no keyframes to fall back on (an expression). Steps 1, 2, 4, ... frames away
+/// (within the layer's in/out points) until the position differs from the current one, then
+/// bisects down to `dt` the moment the motion stopped (or started), so the heading kept is the
+/// one at the end of the motion whichever frame is shown.
+fn sampled_tangent(ctx: &EvalCtx, layer: &Layer, pos: &dyn Fn(effectcraft_time::Tick) -> Vec3, dt: effectcraft_time::Tick, earlier: bool) -> Option<Vec3> {
+    let here = pos(ctx.time);
+    let moved = |t| (pos(t) - here).length() > 1e-9;
+    let limit = if earlier { layer.in_point.min(ctx.time) } else { layer.out_point.max(ctx.time) };
+    let mut step = ctx.comp.frame_duration().max(dt);
+    let (mut still, mut away) = (ctx.time, None);
+    for _ in 0..MOTION_SEARCH_STEPS {
+        let t = if earlier { (ctx.time - step).max(limit) } else { (ctx.time + step).min(limit) };
+        if moved(t) {
+            away = Some(t);
+            break;
+        }
+        still = t;
+        if t == limit {
+            break;
+        }
+        step = step + step;
+    }
+    let mut away = away?;
+    for _ in 0..MOTION_SEARCH_STEPS {
+        if (still - away).abs() <= dt {
+            break;
+        }
+        let mid = away + (still - away).mul_ratio(1, 2);
+        if moved(mid) {
+            away = mid;
+        } else {
+            still = mid;
+        }
+    }
+    let d = if earlier { here - pos(away) } else { pos(away) - here };
+    (d.length() > 1e-9 && d.length().is_finite()).then(|| d.normalize())
 }
 
 /// Auto-Orient ▸ Orient Along Path for 2D layers: extra Z rotation in degrees.

crates/render/src/three_d/tests_orient.rs

@@ -8,6 +8,7 @@ use effectcraft_project::{AutoOrient, Comp, ItemId, ItemKind, Layer, LayerSource
 use effectcraft_time::{FrameRate, Tick};
 
 use super::active_camera;
+use super::camera::auto_orient_2d;
 use crate::EvalCtx;
 
 fn setup() -> (Project, ItemId, Comp) {
@@ -78,6 +79,69 @@ fn orient_along_path_keeps_the_last_heading_at_and_after_the_end() {
     }
 }
 
+/// #1650: the same end-of-path flip when Position has no keyframes of its own and is driven by
+/// an expression (e.g. `pointOnPath()` fed by a keyframed slider on another layer): once the
+/// motion stops there are no Position keys to fall back on, and the heading snapped to 0 degrees.
+#[test]
+fn orient_along_path_keeps_the_last_heading_when_position_is_an_expression() {
+    /// Stand-in for an expression: holds, moves 100 px left over 0.5..1 s, then holds again (like
+    /// a slider going from 0% to 100%).
+    struct LeftThenHold;
+    impl crate::ExprHost for LeftThenHold {
+        fn eval(&self, ctx: &EvalCtx, _: &Layer, prop: &effectcraft_project::Property, value: &Value) -> Result<Value, String> {
+            if prop.match_id != "position" {
+                return Ok(value.clone());
+            }
+            let s = ((ctx.time.seconds() - 0.5) * 2.0).clamp(0.0, 1.0);
+            Ok(Value::Vec3([150.0 - 100.0 * s, 50.0, 0.0]))
+        }
+    }
+    for three_d in [false, true] {
+        let (mut p, cid, comp) = setup();
+        let mut l = solid(&mut p, &comp, three_d, [150.0, 50.0, 0.0]);
+        l.auto_orient = AutoOrient::AlongPath;
+        let pos = l.props.prop_mut("transform/position").unwrap();
+        assert!(pos.keys.is_empty(), "the fixture must have no Position keyframes");
+        pos.expr = Some(effectcraft_project::Expression { text: "left then hold".into(), enabled: true });
+        p.comp_mut(cid).unwrap().layers.push(l);
+        let c = p.comp(cid).unwrap();
+        for secs in [0.2, 0.75, 1.0, 1.0 + 1.0 / 30.0, 1.2, 1.9] {
+            let ctx = EvalCtx { expr: Some(&LeftThenHold), ..EvalCtx::new(&p, cid, c, Tick::from_seconds_f64(secs)) };
+            let x_axis = ctx.world_matrix(&c.layers[0]).apply_vec(vec3(1.0, 0.0, 0.0));
+            // Moving left (-x): the layer's x axis points left before the motion starts and after it stops.
+            assert!((x_axis.x + 1.0).abs() < 1e-6 && x_axis.y.abs() < 1e-6 && x_axis.z.abs() < 1e-6, "3d={three_d} t={secs}: {x_axis:?}");
+        }
+    }
+}
+
+/// #1650 on a curved path: the heading kept once an expression stops moving is the one at the end
+/// of the motion, the same on every later frame, not whatever earlier point a coarse search lands on.
+#[test]
+fn orient_along_a_curved_expression_path_keeps_the_end_heading() {
+    /// Quarter circle over 0..1 s: heading 0 degrees at the start, 90 degrees (down) at the end.
+    struct Arc;
+    impl crate::ExprHost for Arc {
+        fn eval(&self, ctx: &EvalCtx, _: &Layer, prop: &effectcraft_project::Property, value: &Value) -> Result<Value, String> {
+            if prop.match_id != "position" {
+                return Ok(value.clone());
+            }
+            let a = ctx.time.seconds().clamp(0.0, 1.0) * std::f64::consts::FRAC_PI_2;
+            Ok(Value::Vec3([100.0 * a.sin(), 100.0 - 100.0 * a.cos(), 0.0]))
+        }
+    }
+    let (mut p, cid, comp) = setup();
+    let mut l = solid(&mut p, &comp, false, [0.0, 0.0, 0.0]);
+    l.auto_orient = AutoOrient::AlongPath;
+    l.props.prop_mut("transform/position").unwrap().expr = Some(effectcraft_project::Expression { text: "arc".into(), enabled: true });
+    p.comp_mut(cid).unwrap().layers.push(l);
+    let c = p.comp(cid).unwrap();
+    for frame in [31, 33, 37, 45, 59] {
+        let ctx = EvalCtx { expr: Some(&Arc), ..EvalCtx::new(&p, cid, c, Tick::from_seconds_f64(frame as f64 / 30.0)) };
+        let deg = auto_orient_2d(&ctx, &c.layers[0]);
+        assert!((deg - 90.0).abs() < 0.5, "frame {frame}: {deg}");
+    }
+}
+
 #[test]
 fn layers_parented_to_a_camera_follow_its_rotation() {
     let (mut p, cid, comp) = setup();