Add trace image generator

src/bin/trace.rs

@@ -0,0 +1,58 @@
+use symplectic_double_pendulum::*;
+
+use tiny_skia::*;
+
+const WIDTH: u32 = 800;
+const HEIGHT: u32 = 800;
+
+const MIN: f32 = -2.0;
+const MAX: f32 = 2.0;
+const MARGIN: f32 = 0.2;
+
+fn system_xy(q: &DoublePendulum) -> [f64; 2] {
+	[
+		q.theta.sin() + (q.theta + q.phi).sin(),
+		q.theta.cos() + (q.theta + q.phi).cos()
+	]
+}
+
+fn canvas_xy(q: &DoublePendulum) -> [f32; 2] {
+	system_xy(q).map(|x| (x as f32 * (1.0 - MARGIN) - MIN) * WIDTH as f32 / (MAX - MIN))
+}
+
+fn main() {
+	let omega = 500.0;
+	let h = 1e-3;
+	let t = 20.0;
+
+	let mut state = Augmented::from(State {
+		q: DoublePendulum { theta: std::f64::consts::PI, phi: 0.1 },
+		p: DoublePendulum { theta: 0.0, phi: 0.0 },
+	}, omega);
+
+	let path = {
+		let mut pb = PathBuilder::new();
+
+		let [x, y] = canvas_xy(&state.average().q);
+		pb.move_to(x, y);
+
+		let n = (t / h) as u32;
+		for _ in 0..n {
+			state.evolve(h);
+			let [x, y] = canvas_xy(&state.average().q);
+			pb.line_to(x, y);
+		}
+
+		pb.finish().unwrap()
+	};
+
+	let mut paint = Paint::default();
+	paint.set_color_rgba8(0, 0, 0, 0xff);
+	paint.anti_alias = true;
+	let mut stroke = Stroke::default();
+	stroke.width = 4.0;
+	stroke.line_cap = LineCap::Round;
+	let mut pixmap = Pixmap::new(WIDTH, HEIGHT).unwrap();
+	pixmap.stroke_path(&path, &paint, &stroke, Transform::identity(), None);
+	pixmap.save_png("trace.png").unwrap();
+}