Add star model

models/star/Cargo.toml

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+[package]
+name = "star"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies.fj]
+path = "../../fj"

models/star/README.md

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+# Fornjot - Star
+
+A model of a star that demonstrates sweeping a sketch to create a 3D shape, concave sketches, and how to generate sketches from code.
+
+To display this model, run the following from the repository root (model parameters are optional):
+``` sh
+cargo run -- --model start --parameters num_points=5 r1=1.0 r2=2.0 h=1.0
+```
+
+![Screenshot of the star model](star.png)

models/star/src/lib.rs

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+use std::{collections::HashMap, f64::consts::PI};
+
+#[no_mangle]
+pub extern "C" fn model(args: &HashMap<String, String>) -> fj::Shape {
+ // Number of points of the star
+ //
+ // "Points" in the sense of "pointy ends", not in the sense of geometrical
+ // points, or vertices.
+ let num_points: u64 = args
+ .get("num_points")
+ .map(|arg| arg.parse().unwrap())
+ .unwrap_or(5);
+
+ // Radius of the circle that all the vertices between the pointy ends are on
+ let r1: f64 = args
+ .get("r1")
+ .map(|arg| arg.parse().unwrap())
+ .unwrap_or(1.0);
+
+ // Radius of the circle that all the pointy ends are on
+ let r2: f64 = args
+ .get("r2")
+ .map(|arg| arg.parse().unwrap())
+ .unwrap_or(2.0);
+
+ // The height of the star
+ let h: f64 = args.get("h").map(|arg| arg.parse().unwrap()).unwrap_or(1.0);
+
+ // We need to figure out where to generate vertices, depending on the number
+ // of points the star is supposed to have. Let's generate an iterator that
+ // gives us the angle and radius for each vertex.
+ let num_vertices = num_points * 2;
+ let vertex_iter = (0..num_vertices).map(|i| {
+ let angle = 2. * PI / num_vertices as f64 * i as f64;
+ let radius = if i % 2 == 0 { r1 } else { r2 };
+ (angle, radius)
+ });
+
+ // Now that we got that iterator prepared, generating the vertices is just a
+ // bit of trigonometry.
+ let mut outer = Vec::new();
+ let mut inner = Vec::new();
+ for (angle, radius) in vertex_iter {
+ let (sin, cos) = angle.sin_cos();
+
+ let x = cos * radius;
+ let y = sin * radius;
+
+ outer.push([x, y]);
+ inner.push([x / 2., y / 2.]);
+ }
+
+ let outer = fj::Sketch::from_points(outer);
+ let inner = fj::Sketch::from_points(inner);
+
+ let footprint = fj::Difference2d {
+ a: outer.into(),
+ b: inner.into(),
+ };
+
+ let star = fj::Sweep {
+ shape: footprint.into(),
+ length: h,
+ };
+
+ star.into()
+}