From 774551728b93c551c1150d2df11f3d978dbd3a0e Mon Sep 17 00:00:00 2001
From: Rouven Spreckels <rs@qu1x.dev>
Date: Tue, 20 Feb 2024 17:56:17 +0100
Subject: [PATCH] Implement `track` operation.

---
 src/clamp.rs | 32 ++++++++++++++++++++++++++++++++
 src/delta.rs | 11 +++++++++++
 src/frame.rs | 12 +++++++++---
 3 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/src/clamp.rs b/src/clamp.rs
index 66c492c..84c0b4c 100644
--- a/src/clamp.rs
+++ b/src/clamp.rs
@@ -122,6 +122,38 @@ pub trait Clamp<N: Copy + RealField>: Send + Sync + Debug + 'static {
 				}
 				(min_delta != *delta).then_some((min_delta, loops))
 			}
+			Delta::Track { vec: _ } => {
+				let old_frame = frame;
+				let old_target = frame.target();
+				let old_rot_inverse = frame.view().rotation.inverse();
+				let mut min_delta = *delta;
+				let mut loops = 0;
+				loop {
+					let frame = min_delta.transform(old_frame);
+					let mut bound = false;
+					if let Some(plane) = self.target(&frame) {
+						bound = true;
+						let new_target = plane.project_point(frame.target());
+						let vec = old_rot_inverse * (new_target - old_target);
+						min_delta = Delta::Track { vec };
+					}
+					let frame = min_delta.transform(old_frame);
+					if let Some(_plane) = self.up(&frame) {
+						bound = true;
+						// TODO Implement gliding.
+						min_delta = Delta::Frame;
+					}
+					if bound {
+						if loops == self.loops() {
+							break;
+						}
+						loops += 1;
+					} else {
+						break;
+					}
+				}
+				(min_delta != *delta).then_some((min_delta, loops))
+			}
 			&Delta::Orbit { rot: _, pos } => {
 				if pos != Point3::origin() {
 					return Some((Delta::Frame, 0));
diff --git a/src/delta.rs b/src/delta.rs
index 74d6b2c..fe534f1 100644
--- a/src/delta.rs
+++ b/src/delta.rs
@@ -19,6 +19,13 @@ pub enum Delta<N: Copy + RealField> {
 		/// Yaw axis.
 		yaw_axis: Unit<Vector3<N>>,
 	},
+	/// Tracks target which slides by vector in world space.
+	///
+	/// Preserves eye position inclusive its roll attitude.
+	Track {
+		/// Vector in world space of a sliding target to track.
+		vec: Vector3<N>,
+	},
 	/// Orbits eye by rotation in camera space around point in camera space.
 	///
 	/// See [`Frame::local_orbit_around()`].
@@ -58,6 +65,7 @@ impl<N: Copy + RealField> Delta<N> {
 				yaw,
 				ref yaw_axis,
 			} => frame.look_around(*pitch, *yaw, yaw_axis),
+			Self::Track { vec } => frame.set_target(frame.target() + vec),
 			Self::Orbit { rot, pos } => frame.local_orbit_around(rot, pos),
 			Self::Slide { vec } => frame.local_slide(vec),
 			Self::Scale { rat, pos } => frame.local_scale_around(*rat, pos),
@@ -80,6 +88,7 @@ impl<N: Copy + RealField> Delta<N> {
 				yaw: -yaw,
 				yaw_axis,
 			},
+			Self::Track { vec } => Self::Track { vec: -vec },
 			Self::Orbit { rot, pos } => Self::Orbit {
 				rot: rot.inverse(),
 				pos,
@@ -111,6 +120,7 @@ impl<N: Copy + RealField> Delta<N> {
 				yaw: yaw * t,
 				yaw_axis,
 			},
+			Self::Track { vec } => Self::Track { vec: vec * t },
 			Self::Orbit { rot, pos } => Self::Orbit {
 				rot: rot.powf(t),
 				pos,
@@ -139,6 +149,7 @@ impl<N: Copy + RealField> Delta<N> {
 				yaw: yaw.to_superset(),
 				yaw_axis: yaw_axis.cast(),
 			},
+			Self::Track { vec } => Delta::Track { vec: vec.cast() },
 			Self::Orbit { rot, pos } => Delta::Orbit {
 				rot: rot.cast(),
 				pos: pos.cast(),
diff --git a/src/frame.rs b/src/frame.rs
index a2cd2df..5e5e829 100644
--- a/src/frame.rs
+++ b/src/frame.rs
@@ -40,10 +40,16 @@ impl<N: Copy + RealField> Frame<N> {
 		&self.pos
 	}
 	/// Sets target position in world space preserving eye position inclusive its roll attitude.
-	pub fn set_target(&mut self, target: Point3<N>) {
+	///
+	/// Allows to track a moving object.
+	pub fn set_target(&mut self, pos: Point3<N>) {
 		let eye = self.eye();
-		self.pos = target;
-		self.zat = (self.pos - eye).norm();
+		let (new_dir, zat) = Unit::new_and_get(pos - eye);
+		let old_dir = Unit::new_normalize(self.pos - eye);
+		let rot = UnitQuaternion::rotation_between_axis(&old_dir, &new_dir)
+			.unwrap_or_else(|| UnitQuaternion::from_axis_angle(&self.yaw_axis(), N::pi()));
+		let rot = rot * self.rot;
+		*self = Self { pos, rot, zat }
 	}
 	/// Distance between eye and target.
 	#[must_use]