Dummy scene with mostly-working camera

Cargo.toml

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+[package]
+name = "terroir"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+bevy = { version = "0.14.1", features = ["dynamic_linking"] }
+
+[profile.dev]
+opt-level = 1
+
+[profile.dev.package."*"]
+opt-level = 3
+
+[profile.release]
+codegen-units = 1
+lto = "thin"

src/camera.rs

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+use {
+    bevy::{input::mouse::MouseMotion, prelude::*, window::CursorGrabMode},
+    std::f32::consts::{FRAC_PI_2, FRAC_PI_4, PI, TAU},
+};
+
+#[derive(Bundle, Default)]
+pub struct PanOrbitCameraBundle {
+    pub camera: Camera3dBundle,
+    pub state: PanOrbitState,
+    pub settings: PanOrbitSettings,
+}
+
+#[derive(Component)]
+pub struct PanOrbitState {
+    pub center: Vec3,
+    pub radius: f32,
+    pub pitch: f32,
+    pub yaw: f32,
+}
+
+impl Default for PanOrbitState {
+    fn default() -> Self {
+        PanOrbitState {
+            center: Vec3::ZERO,
+            radius: 2.0,
+            pitch: -FRAC_PI_4,
+            yaw: 0.0,
+        }
+    }
+}
+
+/// The configuration of the pan-orbit controller
+#[derive(Component)]
+pub struct PanOrbitSettings {
+    /// World units per pixel of mouse motion
+    pub pan_sensitivity: f32,
+    /// Radians per pixel of mouse motion
+    pub orbit_sensitivity: f32,
+    /// Exponent per pixel of mouse motion
+    pub zoom_sensitivity: f32,
+}
+
+impl Default for PanOrbitSettings {
+    fn default() -> Self {
+        PanOrbitSettings {
+            pan_sensitivity: 0.001,                 // 1000 pixels per world unit
+            orbit_sensitivity: 0.1f32.to_radians(), // 0.1 degree per pixel
+            zoom_sensitivity: 0.01,
+        }
+    }
+}
+
+pub fn update(
+    mut evr_motion: EventReader<MouseMotion>,
+    mouse: Res<ButtonInput<MouseButton>>,
+    mut q_camera: Query<(&PanOrbitSettings, &mut PanOrbitState, &mut Transform)>,
+    mut q_window: Query<&mut Window>,
+) {
+    let mouse_motion: Vec2 = evr_motion.read().map(|ev| ev.delta).sum();
+
+    for (settings, mut state, mut transform) in &mut q_camera {
+        if mouse.any_pressed([MouseButton::Left, MouseButton::Right]) {
+            q_window.single_mut().cursor.grab_mode = CursorGrabMode::Locked;
+        } else {
+            q_window.single_mut().cursor.grab_mode = CursorGrabMode::None;
+        }
+
+        if mouse.pressed(MouseButton::Right) {
+            state.pitch = (state.pitch - settings.orbit_sensitivity * mouse_motion.y)
+                .clamp(-FRAC_PI_2, FRAC_PI_2);
+            state.yaw += settings.orbit_sensitivity * mouse_motion.x;
+            if state.yaw > PI {
+                state.yaw -= TAU;
+            }
+            if state.yaw < -PI {
+                state.yaw += TAU;
+            }
+        }
+
+        if mouse.pressed(MouseButton::Left) {
+            // TODO this needs to take the current rotation into account
+            state.center += (settings.pan_sensitivity * mouse_motion * -1.0)
+                .extend(0.0)
+                .xzy();
+        }
+
+        if mouse.pressed(MouseButton::Middle) {
+            state.radius = (state.radius + settings.zoom_sensitivity * mouse_motion.y).max(0.0);
+        }
+
+        transform.rotation = Quat::from_euler(EulerRot::YXZ, state.yaw, state.pitch, 0.0);
+        transform.translation = state.center + transform.back() * state.radius;
+    }
+}

src/main.rs

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+use bevy::prelude::*;
+
+mod camera;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_systems(Startup, setup)
+        .add_systems(Update, camera::update)
+        .run();
+}
+
+/// set up a simple 3D scene
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(Cuboid::new(1.0, 0.2, 1.0)),
+        material: materials.add(Color::srgb_u8(124, 144, 255)),
+        transform: Transform::from_xyz(0.0, -0.2, 0.0),
+        ..default()
+    });
+    commands.spawn(PointLightBundle {
+        point_light: PointLight {
+            shadows_enabled: true,
+            ..default()
+        },
+        transform: Transform::from_xyz(4.0, 8.0, 4.0),
+        ..default()
+    });
+    commands.spawn(camera::PanOrbitCameraBundle::default());
+}