From 374c3bf6b9db9a01ce25a93139fc11bbbf4efc06 Mon Sep 17 00:00:00 2001
From: Camille Moine <ki2pixel@gmail.com>
Date: Wed, 21 Oct 2026 14:46:21 +0200
Subject: [PATCH] fix(effects): clear Cut Away 100 opaque edges

The gradient antialias could leave a half-covered pixel where an opaque source reaches h = 1. Add an endpoint guard on CPU and GPU and cover an opaque edge in the regression test.
---
 crates/effects/src/distort3.rs            | 15 +++++++++++++++
 crates/gpu/src/shaders/fx_transition.wgsl |  6 ++++++
 crates/gpu/src/tests_fx_transition.rs     |  1 +
 3 files changed, 22 insertions(+)

diff --git a/crates/effects/src/distort3.rs b/crates/effects/src/distort3.rs
index 88be62e9..cb389d90 100644
--- a/crates/effects/src/distort3.rs
+++ b/crates/effects/src/distort3.rs
@@ -123,6 +123,12 @@ fn dot3f(a: [f32; 3], b: [f32; 3]) -> f32 {
 fn cc_blobbylize(ctx: &EffectCtx, mut b: Buf) -> Buf {
     let soft = ctx.params.f("softness").max(0.0) * b.scale;
     let cut = (ctx.params.f("cutAway") / 100.0).clamp(0.0, 1.0) as f32;
+    if cut >= 1.0 {
+        // Endpoint guard: at Cut Away 100 the gradient antialias must not leave a half-covered
+        // pixel where an opaque source reaches exactly 1.0.
+        b.img.data.fill([0.0; 4]);
+        return b;
+    }
     let blob = layer_or_self(ctx, &b, "blobLayer", true, false);
     let prop = src_at(ctx.params.e("property"));
     let raw = Plane::from_image(&blob, |px| {
@@ -1212,6 +1218,15 @@ mod tests {
         assert!(o.img.data.iter().all(|p| p[3] == 0.0), "Cut Away 100 left opaque pixels");
     }
 
+    #[test]
+    fn blobbylize_cut_away_100_clears_an_opaque_edge() {
+        // An opaque source reaches h = 1 on the high side of an edge; the gradient antialias
+        // used to leave m = 0.5 there, so Cut Away 100 still produced visible pixels.
+        let im = crate::util::gen_image(64, 8, |x, _| if x < 32 { [1.0; 4] } else { [0.0; 4] });
+        let o = run("ec.distort.ccblobbylize", &[("property", Value::Enum(3)), ("softness", num(0.0)), ("cutAway", num(100.0))], im, &[]);
+        assert!(o.img.data.iter().all(|p| p[3] == 0.0), "Cut Away 100 left an opaque edge");
+    }
+
     #[test]
     fn blobbylize_softness_warps_a_step_matte_inward() {
         // AE oracle T2: a self alpha step at x = 960, Softness 20, Cut Away 0 puts the matte
diff --git a/crates/gpu/src/shaders/fx_transition.wgsl b/crates/gpu/src/shaders/fx_transition.wgsl
index ef543633..5037ec9a 100644
--- a/crates/gpu/src/shaders/fx_transition.wgsl
+++ b/crates/gpu/src/shaders/fx_transition.wgsl
@@ -582,6 +582,12 @@ fn ftr_blob(@builtin(global_invocation_id) gid: vec3<u32>) {
         return;
     }
     let cut = P.f[0].x;
+    if (cut >= 1.0) {
+        // Endpoint guard: an opaque source can reach h = 1.0, where the gradient antialias
+        // would otherwise leave a half-covered pixel at Cut Away 100.
+        textureStore(out, p, vec4<f32>(0.0));
+        return;
+    }
     let hv = textureLoad(aux, p, 0).x;
     let g = ftr_grad(p);
     let grad = length(g);
diff --git a/crates/gpu/src/tests_fx_transition.rs b/crates/gpu/src/tests_fx_transition.rs
index b4b3572a..abe9273d 100644
--- a/crates/gpu/src/tests_fx_transition.rs
+++ b/crates/gpu/src/tests_fx_transition.rs
@@ -126,6 +126,7 @@ fn distort() {
         "ec.distort.ccblobbylize",
         &[("property", e(4)), ("softness", n(3.0)), ("cutAway", n(30.0)), ("lightType", e(1)), ("lightPosition", pt(20.0, 10.0))],
     );
+    effect_case("ec.distort.ccblobbylize", &[("cutAway", n(100.0))]);
     effect_case("ec.distort.ccblobbylize", &[("property", e(5)), ("metal", n(30.0)), ("roughness", n(0.2))]);
 }