Projection keeps free-form geometry exact: tilted circles project to ellipses and tilted arcs to conics (rational quadratic arcs), other sketches' ellipses, splines and conics in parallel planes map exactly, and smooth edges that are neither lines nor arcs become fit splines instead of line chains

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/sketch_project_tests.rs

@@ -298,3 +298,55 @@ fn isoparametric_curves_of_a_cylinder() {
     assert!(zs.iter().cloned().fold(f64::MIN, f64::max) > 19.9 && zs.iter().cloned().fold(f64::MAX, f64::min) < 0.1, "{zs:?}");
     assert!(w.pts.iter().all(|p| (p.x - 6.0).abs() < 1e-3 && p.y.abs() < 1e-3));
 }
+
+#[test]
+fn tilted_circles_project_to_exact_ellipses_and_splines_stay_splines() {
+    let mut s = Session::default();
+    run(&mut s, "PrimitiveCylinder", json!({"base": [0, 0, 0], "axis": [1, 0, 1], "radius": 5, "height": 20}));
+    run(&mut s, "SketchCreate", json!({"plane": "XY", "offset": -10}));
+    // The base circle (centred at the origin) seen from above.
+    let r = run(&mut s, "ProjectNewCmd", json!({"refs": [{"edge": [0, 5, 0]}]}));
+    // One seam half: exact conics (rational quadratic arcs).
+    let si = inspect(&mut s);
+    let c = si["curves"].as_array().unwrap().iter().find(|c| c["id"] == r["links"][0]["curves"][0]).unwrap().clone();
+    assert_eq!(c["type"], "conic", "{c}");
+    let st = s.model.state();
+    let sk = &st.sketch(s.active_sketch.unwrap()).unwrap().sketch;
+    for ci in sk.link_curves(r["links"][0]["link"].as_str().unwrap()) {
+        for q in sk.polyline(ci) {
+            // On the ellipse x²/(5/√2)² + y²/5² = 1.
+            let e = (q.x / (5.0 * std::f64::consts::FRAC_1_SQRT_2)).powi(2) + (q.y / 5.0).powi(2);
+            assert!((e - 1.0).abs() < 1e-9, "{q:?}");
+        }
+    }
+    // The whole face (both halves of its rim, plus its seam) gives the full ellipse.
+    let r = run(&mut s, "ProjectNewCmd", json!({"refs": [{"face": [0, 0, 0]}]}));
+    let si = inspect(&mut s);
+    let types: Vec<String> = r["links"][0]["curves"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|id| si["curves"].as_array().unwrap().iter().find(|c| c["id"] == *id).unwrap()["type"].as_str().unwrap().to_string())
+        .collect();
+    assert!(types.iter().all(|t| t == "conic" || t == "ellipse"), "{types:?}");
+    let r = run(&mut s, "ProjectNewCmd", json!({"refs": [{"edge": [0, -5, 0]}]}));
+    let si = inspect(&mut s);
+    let c = si["curves"].as_array().unwrap().iter().find(|c| c["id"] == r["links"][0]["curves"][0]).unwrap().clone();
+    if c["type"] == "ellipse" {
+        assert!((c["minor_radius"].as_f64().unwrap() - 5.0 * std::f64::consts::FRAC_1_SQRT_2).abs() < 1e-6, "{c}");
+        let major = &c["major_at"];
+        assert!(((major[0].as_f64().unwrap().powi(2) + major[1].as_f64().unwrap().powi(2)).sqrt() - 5.0).abs() < 1e-6, "{c}");
+    }
+    run(&mut s, "SketchStop", json!({}));
+    // A spline of a parallel sketch projects as the same spline.
+    run(&mut s, "SketchCreate", json!({"plane": "XY", "offset": 30}));
+    let sp = run(&mut s, "DrawSpline", json!({"points": [[0, 0], [10, 5], [20, 0]]}))["curves"][0].clone();
+    run(&mut s, "SketchStop", json!({}));
+    let sk1 = s.doc.features.last().unwrap().id;
+    run(&mut s, "SketchCreate", json!({"plane": "XY"}));
+    let r = run(&mut s, "ProjectNewCmd", json!({"refs": [{"sketch": sk1, "curve": sp}]}));
+    let si = inspect(&mut s);
+    let c = si["curves"].as_array().unwrap().iter().find(|c| c["id"] == r["links"][0]["curves"][0]).unwrap().clone();
+    assert_eq!(c["type"], "spline", "{c}");
+    assert_eq!(c["points"].as_array().unwrap().len(), 3);
+}