Keyframes: a linear side next to an eased side has 16.67 % influence, as After Effects

In a segment that is Bezier on one side and linear on the other, the linear
side keeps the segment's average speed, but with the 16.67 % influence After
Effects shows for linear keys in the Keyframe Velocity dialog (and reports to
scripts), not 33.3 %. Measured on an After Effects 2026 time-remap render (a
square at x = 50 + 455 * remap): out of a key at 2 s (3.5, speed 0, influence
50 %) into a linear key at 4 s (0.5), AE plays 3.290 at 2.5 s and 2.657 at
3 s; EffectCraft played 3.308 and 2.629 (8 and 13 px off), now the same
square positions to the pixel.

LINEAR_SIDE_INFLUENCE is used by evaluation (scalar, per-dimension, paths and
spatial), side_ease (Graph Editor, velocity dialog, keyIn/OutTemporalEase) and
the Lottie export, so exported easing plays as EffectCraft does. Segments
linear on both sides stay straight lines.

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

crates/keyframe/src/lib.rs

@@ -56,6 +56,12 @@ impl Ease {
     pub const EASY: Ease = Ease { speed: 0.0, influence: 1.0 / 3.0 };
 }
 
+/// Influence of a linear key side in a segment whose other side is eased: the side keeps the
+/// segment's average speed with a 16.67 % influence, as After Effects shows it in the Keyframe
+/// Velocity dialog and evaluates it (time-remap read-backs: a bezier-out (speed 0, 50 %) into a
+/// linear-in from 3.5 to 0.5 over 2 s gives 3.290 at a quarter and 2.657 at half of the segment).
+pub const LINEAR_SIDE_INFLUENCE: f64 = 1.0 / 6.0;
+
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Keyframe {
     pub time: Tick,
@@ -389,7 +395,7 @@ fn effective_ease(keys: &[Keyframe], i: usize, d: usize, spatial: bool, out: boo
 
 /// The ease a key side presents in the Graph Editor and the Keyframe Velocity dialog: stored
 /// Bezier eases, auto-Bezier slopes, or the equivalent of a linear side (speed of the straight
-/// segment, 33.3 % influence). `None` for hold sides and sides without a neighbour.
+/// segment, [`LINEAR_SIDE_INFLUENCE`]). `None` for hold sides and sides without a neighbour.
 pub fn side_ease(keys: &[Keyframe], i: usize, d: usize, spatial: bool, out: bool) -> Option<Ease> {
     let k = keys.get(i)?;
     let j = if out { i + 1 } else { i.checked_sub(1)? };
@@ -410,7 +416,7 @@ pub fn side_ease(keys: &[Keyframe], i: usize, d: usize, spatial: bool, out: bool
             let dv = b.get(d).unwrap_or(&0.0) - a.get(d).unwrap_or(&0.0);
             if out { dv } else { -dv }
         };
-        return Some(Ease { speed: dv / dur, influence: 1.0 / 3.0 });
+        return Some(Ease { speed: dv / dur, influence: LINEAR_SIDE_INFLUENCE });
     }
     Some(effective_ease(keys, i, if is_spatial { 0 } else { d }, is_spatial, out))
 }
@@ -596,8 +602,8 @@ pub fn segment_value(keys: &[Keyframe], i: usize, t: Tick, spatial: bool) -> Val
         let f = if lin_out && lin_in || len <= 1e-12 {
             x
         } else {
-            let eo = if lin_out { Ease { speed: len / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i, 0, true, true) };
-            let ei = if lin_in { Ease { speed: len / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i + 1, 0, true, false) };
+            let eo = if lin_out { Ease { speed: len / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i, 0, true, true) };
+            let ei = if lin_in { Ease { speed: len / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i + 1, 0, true, false) };
             ease_progress(eo.speed * dur / len, eo.influence, ei.speed * dur / len, ei.influence, x)
         };
         // A straight path (both handles on the chord, as auto-Bezier keys on a line have) is the
@@ -616,16 +622,16 @@ pub fn segment_value(keys: &[Keyframe], i: usize, t: Tick, spatial: bool) -> Val
     let cb = b.value.components();
     if ca.is_empty() || ca.len() != cb.len() {
         // Paths, gradients: ease on dimension 0 as overall progress.
-        let eo = if lin_out { Ease { speed: 1.0 / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i, 0, false, true) };
-        let ei = if lin_in { Ease { speed: 1.0 / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i + 1, 0, false, false) };
+        let eo = if lin_out { Ease { speed: 1.0 / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i, 0, false, true) };
+        let ei = if lin_in { Ease { speed: 1.0 / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i + 1, 0, false, false) };
         let f = ease_progress(eo.speed * dur, eo.influence, ei.speed * dur, ei.influence, x);
         return a.value.lerp(&b.value, f);
     }
     let mut out = Vec::with_capacity(ca.len());
     for d in 0..ca.len() {
         let dv = cb[d] - ca[d];
-        let eo = if lin_out { Ease { speed: dv / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i, d, false, true) };
-        let ei = if lin_in { Ease { speed: dv / dur, influence: 1.0 / 3.0 } } else { effective_ease(keys, i + 1, d, false, false) };
+        let eo = if lin_out { Ease { speed: dv / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i, d, false, true) };
+        let ei = if lin_in { Ease { speed: dv / dur, influence: LINEAR_SIDE_INFLUENCE } } else { effective_ease(keys, i + 1, d, false, false) };
         // Bezier in (time, value) space directly: P1 = (i0, v0 + s0*i0*dur), P2 = (1-i1, v1 - s1*i1*dur).
         let (i0, i1) = (eo.influence.clamp(0.0, 1.0), ei.influence.clamp(0.0, 1.0));
         let u = solve_u(0.0, i0, 1.0 - i1, 1.0, x);
@@ -927,6 +933,31 @@ mod tests {
         }
     }
 
+    /// A linear side next to an eased one keeps the segment's average speed at 16.67 %
+    /// influence. After Effects time-remap read-backs (a square at x = 50 + 455 * remap): out of
+    /// a key at 2 s (3.5, speed 0, influence 50 %) into a linear key at 4 s (0.5) the remap is
+    /// 3.290 at 2.5 s and 2.657 at 3 s (33.3 % gave 3.308 and 2.629).
+    #[test]
+    fn a_linear_side_next_to_an_eased_side_matches_after_effects() {
+        let mut a = Keyframe::new(s(2.0), Value::Scalar(3.5));
+        a.out_interp = Interp::Bezier;
+        a.out_ease = vec![Ease { speed: 0.0, influence: 0.5 }];
+        let keys = vec![a, Keyframe::new(s(4.0), Value::Scalar(0.5))];
+        for (t, ae) in [(2.5, 3.290), (3.0, 2.657)] {
+            let v = evaluate(&keys, s(t), false).unwrap().as_f64();
+            // AE's square edge is read to the pixel: 1 px = 1 / 455 of remap.
+            assert!((v - ae).abs() < 2.0 / 455.0, "{t}: {v} vs {ae}");
+        }
+        // The mirror case: a linear out into an eased in.
+        let mut b = Keyframe::new(s(4.0), Value::Scalar(0.5));
+        b.in_interp = Interp::Bezier;
+        b.in_ease = vec![Ease { speed: 0.0, influence: 0.5 }];
+        let mirror = vec![Keyframe::new(s(2.0), Value::Scalar(3.5)), b];
+        let v = evaluate(&mirror, s(3.5), false).unwrap().as_f64();
+        assert!((v - (4.0 - 3.290)).abs() < 2.0 / 455.0, "{v}");
+        assert_eq!(side_ease(&keys, 1, 0, false, false).map(|e| e.influence), Some(LINEAR_SIDE_INFLUENCE));
+    }
+
     #[test]
     fn spatial_linear_constant_speed() {
         let keys = vec![Keyframe::new(s(0.0), Value::Vec2([0.0, 0.0])), Keyframe::new(s(1.0), Value::Vec2([300.0, 400.0]))];

crates/lottie/src/anim.rs

@@ -139,11 +139,15 @@ fn segment_ease(keys: &[Keyframe], i: usize, spatial: bool) -> (Vec<f64>, Vec<f6
             let side = |k: &Keyframe, out: bool| {
                 let interp = if out { k.out_interp } else { k.in_interp };
                 let list = if out { &k.out_ease } else { &k.in_ease };
-                if interp == Interp::Bezier { list.first().copied().unwrap_or_default() } else { Ease { speed: 1.0 / dur, influence: 1.0 / 3.0 } }
+                if interp == Interp::Bezier {
+                    list.first().copied().unwrap_or_default()
+                } else {
+                    Ease { speed: 1.0 / dur, influence: kf::LINEAR_SIDE_INFLUENCE }
+                }
             };
             (1.0, Some(side(a, true)), Some(side(b, false)))
         };
-        let lin = Ease { speed: dv / dur, influence: 1.0 / 3.0 };
+        let lin = Ease { speed: dv / dur, influence: kf::LINEAR_SIDE_INFLUENCE };
         let (eo, ei) = (eo.unwrap_or(lin), ei.unwrap_or(lin));
         // Crossing time handles must retain the independently specified influences.
         let (i0, i1) = (eo.influence.clamp(0.0, 1.0), ei.influence.clamp(0.0, 1.0));

crates/script/src/tests_ease_presets.rs

@@ -90,7 +90,7 @@ fn per_key_methods_keep_the_key_selection_and_report_eases_as_they_play() {
     let r = ok(
         &mut s,
         r#"var comp = app.project.activeItem, p = comp.layer(1).transform.opacity;
-        // A linear side reports the speed it plays at (33.3 % influence).
+        // A linear side reports the speed it plays at (16.67 % influence, as After Effects).
         var lin = p.keyInTemporalEase(3)[0];
         p.setTemporalEaseAtKey(2, [new KeyframeEase(0, 50)], [new KeyframeEase(0, 75)]);
         p.setInterpolationTypeAtKey(3, KeyframeInterpolationType.HOLD, KeyframeInterpolationType.HOLD);
@@ -98,7 +98,7 @@ fn per_key_methods_keep_the_key_selection_and_report_eases_as_they_play() {
         var sel = comp.selectedProperties;
         [lin.speed, lin.influence, p.selectedKeys, sel.length, sel.length ? sel[0].name : ""]"#,
     );
-    assert!(close(r[0].as_f64().unwrap(), -100.0, 1e-9) && close(r[1].as_f64().unwrap(), 100.0 / 3.0, 1e-9), "{r}");
+    assert!(close(r[0].as_f64().unwrap(), -100.0, 1e-9) && close(r[1].as_f64().unwrap(), 100.0 / 6.0, 1e-9), "{r}");
     assert_eq!(r[2], json!([1, 3]), "the selection is the user's");
     assert_eq!((r[3].clone(), r[4].clone()), (json!(1), json!("Opacity")));
     assert_eq!(s.state.selected_keys, before);

docs/fidelity/README.md

@@ -71,6 +71,18 @@ After the fix, all ten tangents equal AE's to the printed 0.001 px, and the two
 test `spatial_auto_bezier_tangents_follow_after_effects` checks the rule on other original keys,
 including a timing change. AE's output stays local; only the script and these scores are tracked.
 
+## Linear side next to an eased side
+
+Measured on 19 October 2026 with After Effects 2026 (26.2) through a time-remapped precomposition
+(a square moving at 455 px per second of source time, read off rendered frames to the pixel): a
+segment from a Bezier out side (speed 0, influence 50 %) at 2 s, value 3.5, into a linear key at
+4 s, value 0.5, plays 3.290 at 2.5 s and 2.657 at 3 s. EffectCraft gave the linear side the
+segment's average speed at 33.3 % influence (3.308 and 2.629, 8 and 13 px off); AE uses the average
+speed at 16.67 %, the influence its Keyframe Velocity dialog shows for linear keys. Now the same
+square positions to the pixel. `LINEAR_SIDE_INFLUENCE` holds the value for evaluation, the Graph
+Editor / velocity dialog sides and the Lottie export; segments that are linear on both sides stay
+straight. Regression test: `a_linear_side_next_to_an_eased_side_matches_after_effects`.
+
 ## Still unmeasured
 
 Rendered frame comparisons now have a deterministic `effectcraft_effects::fidelity::compare_frames`