+spiral curve

src/ex.rs

@@ -139,3 +139,13 @@ pub fn cassini_astroid(ca: f64, cc: f64, aa: f64) -> impl Fn(f64) -> [f64; 2] {
         [ap[0] + cs[0], ap[1] + cs[1]]
     }
 }
+
+pub fn spiral(k: u32) -> impl Fn(f64) -> [f64; 2] {
+    let x2pi = (k as f64) * core::f64::consts::TAU;
+    move |t: f64| {
+        let tstretch = t - 0.5;
+        let tstretch2 = tstretch * tstretch;
+        let (ttxsin, ttxcos) = (t * x2pi).sin_cos();
+        [tstretch2 * ttxcos, tstretch2 * ttxsin]
+    }
+}

src/lib.rs

@@ -17,6 +17,8 @@ pub enum SelectExample {
 
     Lemniskate { a: f64 },
 
+    Spiral { k: u32 },
+
     Circle,
     Intersect8,
     Intersect8pp,
@@ -42,7 +44,7 @@ impl SelectExample {
             Self::Lemniskate { a } => {
                 *a += l;
             }
-            Self::Circle | Self::Intersect8 | Self::Intersect8pp => {
+            Self::Spiral { .. } | Self::Circle | Self::Intersect8 | Self::Intersect8pp => {
                 panic!("offsetting not supported for this example")
             }
         }
@@ -59,6 +61,7 @@ impl SelectExample {
             },
             Self::Ellipse { a, b } => Box::new(ex::ellipse(a, b)),
             Self::Lemniskate { a } => Box::new(move |t: f64| ex::lemniskate(t, a)),
+            Self::Spiral { k } => Box::new(ex::spiral(k)),
             Self::Circle => Box::new(ex::circle),
             Self::Intersect8 => Box::new(ex::intersect_eight),
             Self::Intersect8pp => Box::new(ex::intersect_8pp),