From fb79564f2ae2fc020e855bdbdffb5c5b79b8e570 Mon Sep 17 00:00:00 2001
From: MikeeI <MikeeI@users.noreply.github.com>
Date: Mon, 5 Oct 2026 07:00:44 +0200
Subject: [PATCH] perf(compose): remove redundant effect application halos
 (#110)

Keep full-region neighbourhood effect maps while rendering content only for the output rectangle. Compare complete tiled output at U8, U16 and F32 depths rather than three sampled pixels.
---
 crates/compose/src/lib.rs   | 10 +++++-----
 crates/compose/src/tests.rs | 18 +++++++++++++-----
 2 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/crates/compose/src/lib.rs b/crates/compose/src/lib.rs
index d18b1648..c9d866a6 100644
--- a/crates/compose/src/lib.rs
+++ b/crates/compose/src/lib.rs
@@ -915,9 +915,9 @@ fn composite_layer_plain(layer: &Layer, clipped: &[Layer], backdrop: &mut Buffer
     }
 
     if effects::has_effects(layer) {
-        // Effects reach beyond the render rect: render the layer larger.
-        let big = rect.inflate(effects::margin(layer));
-        let Some(mut content) = render_content(layer, big, cx) else { return };
+        // Neighbourhoods are already captured by the full-region effect maps;
+        // content and effect application only need the output rectangle.
+        let Some(mut content) = render_content(layer, rect, cx) else { return };
         for c in clipped.iter().filter(|c| c.visible) {
             composite_atop(c, &mut content, cx);
         }
@@ -1164,8 +1164,8 @@ fn composite_atop_any(layer: &Layer, base: &mut Buffer, cx: &Ctx) {
     if effects::has_effects(layer) {
         // Effects of a clipped layer are clipped to the base too: render
         // them over the base (treated as opaque) and keep the base's alpha.
-        let big = rect.inflate(effects::margin(layer));
-        let Some(content) = render_content(layer, big, cx) else { return };
+        // The full-region effect maps already capture their neighbourhoods.
+        let Some(content) = render_content(layer, rect, cx) else { return };
         let mut opaque = Buffer { rect, px: base.px.iter().map(|p| [p[0], p[1], p[2], 1.0]).collect() };
         let maps = effect_maps(layer, cx);
         effects::composite_with_effects(
diff --git a/crates/compose/src/tests.rs b/crates/compose/src/tests.rs
index 2971b4dc..f451fe8a 100644
--- a/crates/compose/src/tests.rs
+++ b/crates/compose/src/tests.rs
@@ -600,11 +600,19 @@ fn group_effects_apply_to_group_shape() {
 
 #[test]
 fn effects_render_identically_in_tiles() {
-    let d = fx_doc(vec![Effect::DropShadow(Shadow { spread: 0.0, size: 5.0, ..shadow(4.0, 120.0) }), stroke(2.0, StrokePosition::Outside)]);
-    let full = flatten(&d);
-    for (x, y) in [(33, 33), (8, 20), (31, 12)] {
-        let t = render(&d, Rect::from_xywh(x, y, 1, 1)).px[0];
-        assert!(close4(t, full.get(x, y)), "({x},{y})");
+    for depth in [SampleType::U8, SampleType::U16, SampleType::F32] {
+        let mut d = fx_doc(vec![Effect::DropShadow(Shadow { spread: 0.0, size: 5.0, ..shadow(4.0, 120.0) }), stroke(2.0, StrokePosition::Outside)]);
+        d.depth = depth;
+        for layer in &mut d.layers {
+            if let Some(surface) = layer.surface_mut() {
+                *surface = surface.convert(photocraft_color::PixelFormat { sample: depth, ..surface.format() });
+            }
+        }
+        let full = flatten(&d);
+        // Compare every boundary pixel: a few sampled points miss application-halo regressions.
+        for tile in [1, 7, 33] {
+            assert_eq!(render_tiled(&d, d.bounds(), tile), full, "{depth:?}, tile {tile}");
+        }
     }
 }