Zig Zag: ridges are spaced evenly along each segment's length, so a straight line gets a regular wave (fixes #1554)

crates/effects/src/distort.rs

@@ -1,6 +1,7 @@
 //! Distort & Transform effects.
 
 use serde_json::Value;
+use vectorcraft_geom::kurbo::ParamCurveArclen;
 use vectorcraft_geom::{Affine, Anchor, ParamCurve, PathData, Point, Rect, SubPath, Vec2};
 
 use crate::util::*;
@@ -194,7 +195,10 @@ pub fn twist(path: &PathData, b: Rect, angle: f64) -> PathData {
     map_nonlinear(path, pieces, f)
 }
 
-/// Zig Zag: `ridges` extra points per segment, alternately displaced along the normal.
+const ZIGZAG_ARCLEN_ACCURACY: f64 = 1e-4;
+
+/// Zig Zag: `ridges` extra points per segment, evenly spaced along its length and alternately
+/// displaced along the normal.
 pub fn zig_zag(path: &PathData, b: Rect, p: &Value) -> PathData {
     let size = num(p, "size", 10.0).clamp(-1e4, 1e4);
     let amp = if flag(p, "relative", false) { size / 100.0 * mean_size(b) } else { size };
@@ -207,8 +211,12 @@ pub fn zig_zag(path: &PathData, b: Rect, p: &Value) -> PathData {
         let mut k = 0usize;
         for i in 0..segs {
             let c = sp.segment(i);
+            // Ridges are spaced by length, not by `t`: a line's retracted handles bunch `t` up
+            // near the anchors, and so does any uneven curve.
+            let len = c.arclen(ZIGZAG_ARCLEN_ACCURACY);
             for j in 0..=ridges {
-                let t = j as f64 / (ridges + 1) as f64;
+                let s = j as f64 / (ridges + 1) as f64;
+                let t = if j > 0 && len > 1e-9 { c.inv_arclen(s * len, ZIGZAG_ARCLEN_ACCURACY).clamp(0.0, 1.0) } else { s };
                 let nrm = if j == 0 {
                     // Average the normals of the segments meeting at the anchor.
                     let prev = if i > 0 {

crates/effects/src/tests.rs

@@ -244,6 +244,22 @@ fn zigzag_ridge_count_and_amplitude() {
     assert!(b.width() > 100.0 && b.width() <= 120.0 + 1e-6, "{b:?}");
 }
 
+#[test]
+fn zigzag_spaces_ridges_evenly_along_a_straight_line() {
+    // #1554: on a straight line every ridge is the same distance from the line and as far from
+    // the next one, whether the points are corners or smooth.
+    let line = PathData::single(SubPath::polyline(&[Point::new(0.0, 0.0), Point::new(0.0, 500.0)], false));
+    for points in ["corner", "smooth"] {
+        let out = run("distort.zigZag", json!({"ridges": 4, "points": points, "size": 20}), &line);
+        let ps: Vec<Point> = out.subpaths[0].anchors.iter().map(|a| a.p).collect();
+        assert_eq!(ps.len(), 6, "{points}");
+        for (k, q) in ps.iter().enumerate() {
+            assert!(close(q.x.abs(), 20.0, 1e-6), "{points} {k}: {q:?}");
+            assert!(close(q.y, 100.0 * k as f64, 1e-3), "{points} {k}: {q:?}");
+        }
+    }
+}
+
 #[test]
 fn offset_path_grows_and_shrinks() {
     let grow = run("path.offsetPath", json!({"offset": 10, "joins": "miter"}), &square()).bounds().unwrap();