Review fixes for #765: the glow outside a shape keeps the edge's colour

The output multiplied the glow's colour by its opacity a second time, so outside the shape (where
the glow sets the opacity) it darkened as it faded and showed as a grey rim over a light
background. The test now checks that the fading glow keeps the edge's colour.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/effects/src/pixel/edges.rs

@@ -74,11 +74,13 @@ pub(super) fn glow(px: &mut [[u8; 4]], w: usize, h: usize, width: f64, brightnes
         let glow = if peak > 0.0 { (edge / peak * brightness as f32 / 6.0).clamp(0.0, 1.0) } else { 0.0 };
         let colour_weight = colours[0][index].max(colours[1][index]).max(colours[2][index]);
         let straight = if colour_weight > 1e-6 { std::array::from_fn(|channel| colours[channel][index] / colour_weight) } else { [0.0; 3] };
+        // Premultiplied: the glow is light of the edge's colour, so outside the shape it keeps that
+        // colour at the glow's opacity instead of darkening as it fades.
         let a = alpha.max(glow);
         *out = [
-            (straight[0] * glow * a * 255.0).round().clamp(0.0, 255.0) as u8,
-            (straight[1] * glow * a * 255.0).round().clamp(0.0, 255.0) as u8,
-            (straight[2] * glow * a * 255.0).round().clamp(0.0, 255.0) as u8,
+            (straight[0] * glow * 255.0).round().clamp(0.0, 255.0) as u8,
+            (straight[1] * glow * 255.0).round().clamp(0.0, 255.0) as u8,
+            (straight[2] * glow * 255.0).round().clamp(0.0, 255.0) as u8,
             (a * 255.0).round() as u8,
         ];
     }

crates/effects/src/pixel/tests.rs

@@ -155,6 +155,8 @@ fn glowing_edges_defaults_keep_the_outline_bright_and_use_source_colour() {
     let outside = at(&out, w, 3, 8);
     assert_eq!(outside[1], 0, "transparent halo must not be grey: {outside:?}");
     assert_eq!(outside[2], 0, "transparent halo must retain source hue: {outside:?}");
+    // The glow outside keeps the edge's colour as it fades, rather than darkening towards grey.
+    assert!(outside[3] > 0 && u32::from(outside[0]) * 255 >= 240 * u32::from(outside[3]), "{outside:?}");
     assert!(out.as_chunks::<4>().0.iter().all(|p| p[0] <= p[3] && p[1] <= p[3] && p[2] <= p[3]));
 }