From 68d928aff325c9af3682c840b4ef7e14a015b97a Mon Sep 17 00:00:00 2001
From: arnav-hiwarkar <asharnav2008@gmail.com>
Date: Wed, 14 Oct 2026 13:42:59 +0530
Subject: [PATCH] fix(render): preserve preceding effects in GPU Echo (#1120)

---
 crates/gpu/src/tests_fx_noise.rs | 22 ++++++++++++++++++++++
 crates/render/src/lib.rs         | 26 +++++++++++++++++---------
 2 files changed, 39 insertions(+), 9 deletions(-)

diff --git a/crates/gpu/src/tests_fx_noise.rs b/crates/gpu/src/tests_fx_noise.rs
index 0fc384425..476f08642 100644
--- a/crates/gpu/src/tests_fx_noise.rs
+++ b/crates/gpu/src/tests_fx_noise.rs
@@ -203,6 +203,28 @@ fn echo_and_posterize_time() {
     }
 }
 
+#[test]
+fn echo_after_star_burst_preserves_the_stars() {
+    for depth in [BitDepth::Bpc8, BitDepth::Bpc32] {
+        for operator in 0..7 {
+            for posterize in [false, true] {
+                let mut s = Scene::new(depth);
+                let mut l = s.solid([1.0; 3], 97, 61);
+                s.effect(&mut l, "ec.sim.ccstarburst", &[("gridSpacing", n(8.0)), ("size", n(40.0))]);
+                s.effect(&mut l, "ec.time.echo", &[("numberOfEchoes", n(2.0)), ("echoTime", n(-0.1)), ("decay", n(0.6)), ("echoOperator", e(operator))]);
+                if posterize {
+                    s.effect(&mut l, "ec.time.posterizetime", &[("frameRate", n(4.0))]);
+                }
+                s.push(l);
+                let t = Tick::from_seconds_f64(0.6);
+                let label = format!("Star Burst + Echo {operator} posterize={posterize} {depth:?}");
+                check(&label, compare_at(&s, opts(), t), 0.0);
+                check(&format!("{label} half"), compare_at(&s, RenderOpts { scale: 0.5, ..opts() }, t), 0.0);
+            }
+        }
+    }
+}
+
 #[test]
 fn median_levels_match_cpu_division() {
     let Some(g) = crate::tests::gpu() else { return };
diff --git a/crates/render/src/lib.rs b/crates/render/src/lib.rs
index 41036fe6b..808356f12 100644
--- a/crates/render/src/lib.rs
+++ b/crates/render/src/lib.rs
@@ -216,7 +216,7 @@ struct FxHost<'r, 'a, 'c> {
     /// whose effects run on the comp below).
     origin: [f64; 2],
     /// Index of the effect being rendered (bounds `self_at` to effects before it).
-    index: std::sync::atomic::AtomicUsize,
+    index: usize,
     original: Option<&'c Buf>,
 }
 
@@ -307,7 +307,7 @@ impl EffectHost for FxHost<'_, '_, '_> {
         if self.r.depth > MAX_FX_DEPTH || self.layer.switches.adjustment {
             return None;
         }
-        let n = effects.min(self.index.load(std::sync::atomic::Ordering::Relaxed));
+        let n = effects.min(self.index);
         let t = self.layer.comp_time(Tick::from_seconds_f64(layer_time));
         let sub = self.r.nested();
         sub.layer_input(&self.ctx.at(t), self.layer, n).map(|b| (*b).clone())
@@ -337,8 +337,7 @@ impl EffectHost for FxHost<'_, '_, '_> {
     }
 
     fn params_at(&self, layer_time: f64) -> Option<Params> {
-        let i = self.index.load(std::sync::atomic::Ordering::Relaxed);
-        let g = self.layer.effects()?.groups().nth(i)?;
+        let g = self.layer.effects()?.groups().nth(self.index)?;
         let ctx = self.ctx.at(self.layer.comp_time(Tick::from_seconds_f64(layer_time)));
         Some(effectcraft_effects::flatten_params(g, &mut |pr| ctx.value(self.layer, pr)))
     }
@@ -1158,7 +1157,7 @@ impl<'a> Renderer<'a> {
                 buf
             });
         }
-        let host = FxHost { r: self, ctx, layer, origin, index: Default::default(), original: original.as_ref() };
+        let host = FxHost { r: self, ctx, layer, origin, index: 0, original: original.as_ref() };
         let env = EffectEnv {
             masks: &mask_shapes,
             host: Some(&host),
@@ -1192,12 +1191,21 @@ impl<'a> Renderer<'a> {
             {
                 let run: Vec<_> = stack[k..].iter().take_while(|(_, _, s)| s.gpu && a.supports_effect(s.id)).collect();
                 let params: Vec<Params> = run.iter().map(|(_, g, _)| self.effect_params(ctx, layer, g)).collect();
+                let hosts: Vec<_> = run.iter().map(|(i, _, _)| FxHost { index: *i, ..host }).collect();
                 let steps: Vec<FxStep> = run
                     .iter()
                     .zip(&params)
-                    .map(|((i, g, s), params)| FxStep {
+                    .zip(&hosts)
+                    .map(|(((i, g, s), params), host)| FxStep {
                         spec: s,
-                        ctx: EffectCtx { params, time: lt.seconds(), layer_size, seed: g.uid as u32, adjustment, env: EffectEnv { effect_index: *i, ..env } },
+                        ctx: EffectCtx {
+                            params,
+                            time: lt.seconds(),
+                            layer_size,
+                            seed: g.uid as u32,
+                            adjustment,
+                            env: EffectEnv { host: Some(host), effect_index: *i, ..env },
+                        },
                     })
                     .collect();
                 let t0 = web_time::Instant::now();
@@ -1212,8 +1220,8 @@ impl<'a> Renderer<'a> {
             }
             k += 1;
             let params = self.effect_params(ctx, layer, g);
-            host.index.store(i, std::sync::atomic::Ordering::Relaxed);
-            let env = EffectEnv { effect_index: i, ..env };
+            let host = FxHost { index: i, ..host };
+            let env = EffectEnv { host: Some(&host), effect_index: i, ..env };
             let ectx = EffectCtx { params: &params, time: lt.seconds(), layer_size, seed: g.uid as u32, adjustment, env };
             let t0 = web_time::Instant::now();
             target.cpu(&mut |buf| {