Merge pull request #1558 from marcoarnulfo/fix/zigzag-regular-waveform

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

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 +244,62 @@ 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:?}");
+        }
+    }
+}
+
+/// #1554 on curves: a circle's ridges sit at equal angles, at the radius plus and minus the size in
+/// turn all the way round.
+#[test]
+fn zigzag_on_a_circle_is_regular() {
+    let circle = shapes::ellipse(Rect::new(-50.0, -50.0, 50.0, 50.0));
+    let out = run("distort.zigZag", json!({"ridges": 3, "points": "corner", "size": 10}), &circle);
+    let pts: Vec<Point> = out.subpaths[0].anchors.iter().map(|a| a.p).collect();
+    assert_eq!(pts.len(), 16);
+    let start = pts[0].y.atan2(pts[0].x);
+    let mut radii = vec![];
+    for (i, q) in pts.iter().enumerate() {
+        let turn = (q.y.atan2(q.x) - start).to_degrees().abs() / 22.5;
+        assert!(close(turn, turn.round(), 0.02), "point {i} at {turn} sixteenths");
+        radii.push(q.to_vec2().hypot().round());
+    }
+    assert!(radii.iter().all(|r| *r == 60.0 || *r == 40.0) && (0..16).all(|i| radii[i] != radii[(i + 1) % 16]), "{radii:?}");
+}
+
+/// #1554 on an uneven curve: the ridges divide its length, not its parameter, into equal parts.
+#[test]
+fn zigzag_spaces_ridges_by_length_on_an_uneven_curve() {
+    use vectorcraft_geom::kurbo::{ParamCurve, ParamCurveArclen, ParamCurveNearest};
+    let (a, b) = (Point::new(0.0, 0.0), Point::new(300.0, 0.0));
+    let curve = PathData::single(SubPath::new(
+        vec![
+            vectorcraft_geom::Anchor::with_handles(a, a, Point::new(10.0, 40.0)),
+            vectorcraft_geom::Anchor::with_handles(b, Point::new(100.0, 200.0), b),
+        ],
+        false,
+    ));
+    let c = curve.subpaths[0].segment(0);
+    let len = c.arclen(1e-6);
+    let out = run("distort.zigZag", json!({"ridges": 3, "points": "corner", "size": 0}), &curve);
+    let along: Vec<f64> = out.subpaths[0].anchors.iter().map(|q| c.subsegment(0.0..c.nearest(q.p, 1e-9).t).arclen(1e-6) / len).collect();
+    assert_eq!(along.len(), 5);
+    for (k, share) in along.iter().enumerate() {
+        assert!(close(*share, k as f64 / 4.0, 1e-3), "{along:?}");
+    }
+}
+
 #[test]
 fn offset_path_grows_and_shrinks() {
     let grow = run("path.offsetPath", json!({"offset": 10, "joins": "miter"}), &square()).bounds().unwrap();