Follow-up to #2687: an even sample count keeps the Vegas midpoint

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/effects/src/generate3/paths.rs

@@ -388,10 +388,12 @@ pub(super) fn vegas(ctx: &EffectCtx, mut b: Buf) -> Buf {
 }
 
 /// Sub-samples per Vegas segment: at least 12 (opacity ramp), more for long segments so each straight piece stays
-/// short compared with the stroke width and the segment follows rounded corners smoothly.
+/// short compared with the stroke width and the segment follows rounded corners smoothly. Always even.
 pub(super) fn vegas_substeps(each: f64, width: f64) -> usize {
     let step = width.max(2.0);
-    ((each / step).ceil().max(12.0) as usize).min(512)
+    let n = ((each / step).ceil().max(12.0) as usize).min(512);
+    // Even, so the segment's midpoint (0.5) is always one of the samples.
+    n + (n & 1)
 }
 
 fn vegas_plan(ctx: &EffectCtx, b: &Buf) -> PaintPlan {

crates/effects/src/generate3/tests.rs

@@ -401,7 +401,8 @@ fn vegas_long_segments_are_sampled_finely() {
     // A short segment keeps the 12 samples; a long one gets more so it bends smoothly round rounded corners.
     assert_eq!(super::paths::vegas_substeps(10.0, 4.0), 12);
     let n = super::paths::vegas_substeps(400.0, 4.0);
-    assert!(n > 12, "{n}");
+    assert!(n > 12 && n % 2 == 0, "{n}");
+    assert_eq!(super::paths::vegas_substeps(13.0 * 4.0, 4.0) % 2, 0);
     assert!(400.0 / n as f64 <= 4.0 + 1e-9);
     assert_eq!(super::paths::vegas_substeps(1.0e9, 4.0), 512);
 }