Effects: Time effects see the effects applied before them

Echo, Posterize Time and the other Time effects read the layer at other times with none of
its effects. After Effects renders them with the effects before them: SceneYard "Task
Preparation UI" applies Echo after CC Particle Systems II on black solids, and AE shows the
echoed fireworks over a white page where we echoed the black solids and blacked it out
(Posterize Time after Turbulent Displace, the usual stop-motion setup, depends on the same).
The host already bounds the count to the effects before the one rendering.

Mean abs error vs AE: Task Preparation UI 0.1855 -> 0.0071, Lightbulb Idea 0.0126 -> 0.0102;
the 15 other corpus projects with a Time effect after another effect are pixel-identical.

(cherry picked from commit 46870452db2656c72406f63245e8b1609ceb04d4)

(cherry picked from commit 334d102582e5a64db32e2f3e55836d55ac96bdaa)
(cherry picked from commit 734da3e35dc57c01b73823458bdd87611ac1bf99)

crates/effects/src/lib.rs

@@ -318,8 +318,8 @@ pub trait EffectHost: Sync {
         self.layer(id, false)
     }
     /// The effect's own layer at another **layer time** `layer_time` (seconds): its source with
-    /// masks applied, followed by the first `effects` effects of its stack (0 = none, which is
-    /// what After Effects' Time effects see). `None` when unavailable (no host, recursion
+    /// masks applied, followed by the first `effects` effects of its stack (0 = none; the host
+    /// never goes past the effect being rendered). `None` when unavailable (no host, recursion
     /// limit, adjustment layers). Time effects use this to read neighbouring frames.
     fn self_at(&self, _layer_time: f64, _effects: usize) -> Option<Buf> {
         None

crates/effects/src/time_fx.rs

@@ -1,10 +1,11 @@
 //! Time effects (Effect > Time): Echo, Posterize Time, Time Difference, Time Displacement,
 //! Timewarp, CC Force Motion Blur, CC Wide Time and Pixel Motion Blur.
 //!
-//! These read the layer at *other* layer times through [`EffectHost::self_at`] with zero
-//! preceding effects: like After Effects' Time effects they see the layer's source with its
-//! masks and ignore effects applied before them (precompose to include those). Without a host
-//! (tests, thumbnails) they pass their input through.
+//! These read the layer at *other* layer times through [`EffectHost::self_at`]: its source with
+//! its masks and the effects applied before them, as After Effects renders them (SceneYard:
+//! Echo after CC Particle Systems II echoes the fireworks, not the black solid under them;
+//! Posterize Time after Turbulent Displace steps the displacement). Without a host (tests,
+//! thumbnails) they pass their input through.
 //!
 //! Timewarp's Pixel Motion estimates block-matching optical flow between neighbouring frames
 //! ([`effectcraft_raster::flow`]); Pixel Motion Blur averages sub-frame samples.
@@ -169,7 +170,8 @@ fn fetch(ctx: &EffectCtx, times: &[f64]) -> Option<Frames> {
     let mut uniq: Vec<i64> = times.iter().map(|t| (t * 1e6).round() as i64).collect();
     uniq.sort_unstable();
     uniq.dedup();
-    let bufs: Vec<(i64, Option<Buf>)> = uniq.iter().map(|&k| (k, host.self_at(k as f64 / 1e6, 0))).collect();
+    // Every effect before this one (the host bounds the count to them).
+    let bufs: Vec<(i64, Option<Buf>)> = uniq.iter().map(|&k| (k, host.self_at(k as f64 / 1e6, usize::MAX))).collect();
     let first = bufs.iter().find_map(|(_, b)| b.as_ref())?;
     let scale = first.scale;
     // Union of every frame's extent in layer pixels (offset = where layer (0,0) sits).
@@ -948,7 +950,8 @@ mod tests {
             None
         }
         fn self_at(&self, t: f64, effects: usize) -> Option<Buf> {
-            assert_eq!(effects, 0);
+            // Time effects see every effect before them.
+            assert_eq!(effects, usize::MAX);
             let v = if self.static_layer { 0.4 } else { t as f32 };
             Some(Buf { img: Image::filled(6, 4, [v, v, v, 1.0]), offset: [0.0; 2], scale: 1.0 })
         }

crates/render/src/tests_time.rs

@@ -64,6 +64,22 @@ fn echo_blends_neighbouring_frames() {
     assert!((a - want).abs() < 0.01, "{a} vs {want}");
 }
 
+/// Time effects see the effects applied before them, as After Effects renders them (SceneYard
+/// "Task Preparation UI": Echo after CC Particle Systems II echoed the black solid under the
+/// fireworks and blacked out the white page). Fill (red) then Echo: the echoes are red.
+#[test]
+fn time_effects_see_the_effects_before_them() {
+    let (mut p, cid) = scene(Some(("ec.generate.fill", &[])));
+    let spec = effectcraft_effects::find("ec.time.echo").unwrap();
+    let mut next = p.next_id;
+    let mut g = effectcraft_effects::instantiate(spec, &mut Ids(&mut next), spec.name, [200.0, 100.0]);
+    g.prop_mut("numberOfEchoes").unwrap().value = Value::Scalar(0.0);
+    p.next_id = next;
+    p.comp_mut(cid).unwrap().layers[0].props.sub_mut("effects").unwrap().children.push(g.into());
+    let c = render(&p, cid, 1.5, None).get(100, 50);
+    assert!(c[0] > 0.99 && c[1] < 0.01 && c[2] < 0.01 && c[3] > 0.99, "{c:?}");
+}
+
 #[test]
 fn time_difference_of_static_layer_is_black() {
     let mut p = Project::default();