From 004c2aeff643e998487233ee6902b5b1d7a11cd2 Mon Sep 17 00:00:00 2001
From: Jackie Chen <46739769+jackiechen73@users.noreply.github.com>
Date: Wed, 27 Jul 2022 00:35:43 -0400
Subject: [PATCH] Implement bezier library's De Casteljau points function
 (#715)

* pre-rebase

* broken wasm

* hull lines

* update rust lib description

* update comments, handle match statement better, pass Point type to vue

* cleanup

* add linear case

* More idiomatic code

* Further simplifications to the algorithm and removal of more heap allocations

* Rename to de_casteljau_points and use colors for the iterations

* Small comment changes

* Improve colors

Co-authored-by: Jackie Chen <jackiechen73>
Co-authored-by: Keavon Chambers <keavon@keavon.com>
Co-authored-by: Hannah Li <hannahli2010@gmail.com>
---
 bezier-rs/docs/interactive-docs/src/App.vue   | 23 ++++++++++++++++++
 .../docs/interactive-docs/wasm/src/lib.rs     | 10 ++++++++
 bezier-rs/lib/src/lib.rs                      | 24 +++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/bezier-rs/docs/interactive-docs/src/App.vue b/bezier-rs/docs/interactive-docs/src/App.vue
index 307d1803c..ed1750ad8 100644
--- a/bezier-rs/docs/interactive-docs/src/App.vue
+++ b/bezier-rs/docs/interactive-docs/src/App.vue
@@ -281,6 +281,29 @@ export default defineComponent({
 						],
 					},
 				},
+				{
+					name: "De Casteljau Points",
+					callback: (canvas: HTMLCanvasElement, bezier: WasmBezierInstance, options: Record<string, number>): void => {
+						const hullPoints: Point[][] = JSON.parse(bezier.de_casteljau_points(options.t));
+						hullPoints.reverse().forEach((iteration: Point[], iterationIndex) => {
+							const colorLight = `hsl(${90 * iterationIndex}, 100%, 50%)`;
+
+							iteration.forEach((point: Point, index) => {
+								// Skip the anchor and handle points which are already drawn in black
+								if (iterationIndex !== hullPoints.length - 1) {
+									drawPoint(getContextFromCanvas(canvas), point, 4, colorLight);
+								}
+
+								if (index !== 0) {
+									const prevPoint: Point = iteration[index - 1];
+									drawLine(getContextFromCanvas(canvas), point, prevPoint, colorLight);
+								}
+							});
+						});
+					},
+					template: markRaw(SliderExample),
+					templateOptions: { sliders: [tSliderOptions] },
+				},
 				{
 					name: "Rotate",
 					callback: (canvas: HTMLCanvasElement, bezier: WasmBezierInstance, options: Record<string, number>): void => {
diff --git a/bezier-rs/docs/interactive-docs/wasm/src/lib.rs b/bezier-rs/docs/interactive-docs/wasm/src/lib.rs
index 1b0445f24..d690a0fbf 100644
--- a/bezier-rs/docs/interactive-docs/wasm/src/lib.rs
+++ b/bezier-rs/docs/interactive-docs/wasm/src/lib.rs
@@ -138,4 +138,14 @@ impl WasmBezier {
 		let bezier_points: Vec<Vec<Point>> = self.0.reduce(None).into_iter().map(bezier_to_points).collect();
 		to_js_value(bezier_points)
 	}
+
+	pub fn de_casteljau_points(&self, t: f64) -> JsValue {
+		let hull = self
+			.0
+			.de_casteljau_points(t)
+			.iter()
+			.map(|level| level.iter().map(|&point| Point { x: point.x, y: point.y }).collect::<Vec<Point>>())
+			.collect::<Vec<Vec<Point>>>();
+		to_js_value(hull)
+	}
 }
diff --git a/bezier-rs/lib/src/lib.rs b/bezier-rs/lib/src/lib.rs
index 71e88acee..1073bb1fd 100644
--- a/bezier-rs/lib/src/lib.rs
+++ b/bezier-rs/lib/src/lib.rs
@@ -631,6 +631,30 @@ impl Bezier {
 			.collect::<Vec<DVec2>>()
 	}
 
+	/// Returns a list of lists of points representing the De Casteljau points for all iterations at the point corresponding to `t` using De Casteljau's algorithm.
+	/// The `i`th element of the list represents the set of points in the `i`th iteration.
+	/// More information on the algorithm can be found in the [De Casteljau section](https://pomax.github.io/bezierinfo/#decasteljau) in Pomax's primer.
+	pub fn de_casteljau_points(&self, t: f64) -> Vec<Vec<DVec2>> {
+		let bezier_points = match self.handles {
+			BezierHandles::Linear => vec![self.start, self.end],
+			BezierHandles::Quadratic { handle } => vec![self.start, handle, self.end],
+			BezierHandles::Cubic { handle_start, handle_end } => vec![self.start, handle_start, handle_end, self.end],
+		};
+		let mut de_casteljau_points = vec![bezier_points];
+		let mut current_points = de_casteljau_points.last().unwrap();
+
+		// Iterate until one point is left, that point will be equal to `evaluate(t)`
+		while current_points.len() > 1 {
+			// Map from every adjacent pair of points to their respective midpoints, which decrements by 1 the number of points for the next iteration
+			let next_points: Vec<DVec2> = current_points.as_slice().windows(2).map(|pair| DVec2::lerp(pair[0], pair[1], t)).collect();
+			de_casteljau_points.push(next_points);
+
+			current_points = de_casteljau_points.last().unwrap();
+		}
+
+		de_casteljau_points
+	}
+
 	/// Determine if it is possible to scale the given curve, using the following conditions:
 	/// 1. All the handles are located on a single side of the curve.
 	/// 2. The on-curve point for `t = 0.5` must occur roughly in the center of the polygon defined by the curve's endpoint normals.