Follow-up to #2919: Match Grain matches no grain without a clean sample box

A source with no box-sized area clear of transparent pixels (a small
layer inside a blur's padding) got no sample boxes, and
grain_stats_boxes then measured the whole image, edge included. With no
clean box Match Grain now matches no grain, as After Effects applies
none when its samples hold transparent areas. Compensate for Existing
Noise took the same route through the layer's own boxes, so a layer
with no clean box now has nothing taken off. grain_stats_boxes keeps
its empty-list fallback.

crates/effects/src/noise2.rs

@@ -190,15 +190,14 @@ fn match_grain(ctx: &EffectCtx, mut b: Buf) -> Buf {
         b.img = img;
         return b;
     }
-    let src_stats = grain_stats_boxes(&source, &src_boxes);
+    // No box with an opaque surround (a small layer inside transparent pixels) means no clean
+    // sample: no grain to match, as After Effects applies none when its samples hold transparent
+    // areas. The whole-image fallback of `grain_stats_boxes` would measure the edge again.
+    let clean_stats = |img: &Image, boxes: &[(usize, usize, usize)]| if boxes.is_empty() { GrainStats::default() } else { grain_stats_boxes(img, boxes) };
+    let src_stats = clean_stats(&source, &src_boxes);
     let comp = (ctx.params.f("compensateForExistingNoise") / 100.0) as f32;
     // The own-layer boxes only matter for the compensation term; skip them when it is off.
-    let own = if comp > 0.0 {
-        let own_boxes = sampling_boxes(ctx, &b.img, b.scale);
-        grain_stats_boxes(&b.img, &own_boxes)
-    } else {
-        GrainStats::default()
-    };
+    let own = if comp > 0.0 { clean_stats(&b.img, &sampling_boxes(ctx, &b.img, b.scale)) } else { GrainStats::default() };
     let orig = (view == 0).then(|| b.img.clone());
     let look = GrainLook::from_params(ctx);
     let amp: [f32; 3] = [0, 1, 2].map(|k| {
@@ -471,27 +470,45 @@ mod tests {
     /// edge, and fine grain on the others.
     #[test]
     fn match_grain_ignores_the_edge_of_a_transparent_border() {
-        // A flat, noiseless sky inside the 2 px transparent frame of a padded buffer.
-        let (w, h) = (64u32, 48u32);
-        let mut sky = Image::filled(w, h, [0.4, 0.6, 0.9, 1.0]);
-        for y in 0..h {
-            for x in 0..w {
-                if x < 2 || y < 2 || x >= w - 2 || y >= h - 2 {
-                    sky.set(x, y, [0.0; 4]);
+        // A flat colour inside the 2 px transparent frame of a padded buffer.
+        let framed = |w: u32, h: u32, c: [f32; 4]| {
+            let mut img = Image::filled(w, h, c);
+            for y in 0..h {
+                for x in 0..w {
+                    if x < 2 || y < 2 || x >= w - 2 || y >= h - 2 {
+                        img.set(x, y, [0.0; 4]);
+                    }
                 }
             }
+            img
+        };
+        let most_change = |a: &Image, b: &Image| a.data.iter().zip(&b.data).flat_map(|(p, q)| (0..3).map(move |k| (p[k] - q[k]).abs())).fold(0.0f32, f32::max);
+        // A noiseless sky. At 16 px no 9 px box has an opaque 3 px surround, so there is no
+        // clean sample at all (the whole-image fallback would measure the edge).
+        for (w, h, clean_boxes) in [(64u32, 48u32, true), (16, 16, false)] {
+            let sky = framed(w, h, [0.4, 0.6, 0.9, 1.0]);
+            let boxes = crate::noise::sample_boxes(&sky, 80, 9);
+            assert_eq!(!boxes.is_empty(), clean_boxes, "{w}x{h}: {boxes:?}");
+            if clean_boxes {
+                let st = grain_stats_boxes(&sky, &boxes);
+                assert!(st.std.iter().all(|s| *s < 1e-5), "{w}x{h}: {st:?} from {boxes:?}");
+            }
+            // So Match Grain finds no grain to add.
+            let host = FakeHost(sky);
+            let env = EffectEnv { host: Some(&host), ..Default::default() };
+            let target = Image::filled(w, h, [0.2, 0.3, 0.5, 1.0]);
+            let out = run_fx("ec.noise.matchgrain", &[("noiseSourceLayer", Value::Layer(Some(7)))], target.clone(), 0.5, env);
+            let most = most_change(&out.img, &target);
+            assert!(most < 1e-5, "{w}x{h}: Match Grain added {most}");
         }
-        let boxes = crate::noise::sample_boxes(&sky, 80, 9);
-        assert!(!boxes.is_empty());
-        let st = grain_stats_boxes(&sky, &boxes);
-        assert!(st.std.iter().all(|s| *s < 1e-5), "{st:?} from {boxes:?}");
-        // So Match Grain finds no grain to add.
-        let host = FakeHost(sky);
+        // Compensate for Existing Noise has no clean sample of such a layer either, so its edge
+        // doesn't cancel the source's grain.
+        let host = FakeHost(noisy(16, 16, 0.1, 3));
         let env = EffectEnv { host: Some(&host), ..Default::default() };
-        let target = Image::filled(w, h, [0.2, 0.3, 0.5, 1.0]);
-        let out = run_fx("ec.noise.matchgrain", &[("noiseSourceLayer", Value::Layer(Some(7)))], target.clone(), 0.5, env);
-        let most = out.img.data.iter().zip(&target.data).flat_map(|(a, b)| (0..3).map(move |k| (a[k] - b[k]).abs())).fold(0.0f32, f32::max);
-        assert!(most < 1e-5, "Match Grain added {most}");
+        let target = framed(16, 16, [0.9, 0.9, 0.9, 1.0]);
+        let params = [("noiseSourceLayer", Value::Layer(Some(7))), ("compensateForExistingNoise", crate::num(100.0))];
+        let out = run_fx("ec.noise.matchgrain", &params, target.clone(), 0.5, env);
+        assert!(most_change(&out.img, &target) > 0.01, "the layer's own edge cancelled the grain");
     }
 
     #[test]