Add examples for orthographic and perspective zoom (#15092)

# Objective

Add examples for zooming (and orbiting) orthographic and perspective
cameras.

I'm pretty green with 3D, so please treat with suspicion! I note that
if/when #15075 is merged, `.scale` will go away so this example uses
`.scaling_mode`.

Closes #2580

Cargo.toml

@@ -3244,6 +3244,18 @@ description = "A first-person camera that uses a world model and a view model wi
 category = "Camera"
 wasm = true
 
+[[example]]
+name = "projection_zoom"
+path = "examples/camera/projection_zoom.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.projection_zoom]
+name = "Projection Zoom"
+description = "Shows how to zoom and orbit orthographic and perspective projection cameras."
+category = "Camera"
+wasm = true
+
+
 [package.metadata.example.fps_overlay]
 name = "FPS overlay"
 description = "Demonstrates FPS overlay"

examples/README.md

@@ -255,6 +255,7 @@ Example | Description
 --- | ---
 [2D top-down camera](../examples/camera/2d_top_down_camera.rs) | A 2D top-down camera smoothly following player movements
 [First person view model](../examples/camera/first_person_view_model.rs) | A first-person camera that uses a world model and a view model with different field of views (FOV)
+[Projection Zoom](../examples/camera/projection_zoom.rs) | Shows how to zoom and orbit orthographic and perspective projection cameras.
 
 ## Dev tools
 

examples/camera/projection_zoom.rs

@@ -0,0 +1,245 @@
+//! Shows how to zoom and orbit orthographic and perspective projection cameras.
+
+use std::{
+    f32::consts::{FRAC_PI_2, PI},
+    ops::Range,
+};
+
+use bevy::{input::mouse::AccumulatedMouseScroll, prelude::*, render::camera::ScalingMode};
+
+#[derive(Debug, Default, Resource)]
+struct CameraSettings {
+    pub orbit_distance: f32,
+    // Multiply keyboard inputs by this factor
+    pub orbit_speed: f32,
+    // Clamp fixed vertical scale to this range
+    pub orthographic_zoom_range: Range<f32>,
+    // Multiply mouse wheel inputs by this factor
+    pub orthographic_zoom_speed: f32,
+    // Clamp field of view to this range
+    pub perspective_zoom_range: Range<f32>,
+    // Multiply mouse wheel inputs by this factor
+    pub perspective_zoom_speed: f32,
+    // Clamp pitch to this range
+    pub pitch_range: Range<f32>,
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .init_resource::<CameraSettings>()
+        .add_systems(Startup, (setup, instructions))
+        .add_systems(Update, (orbit, switch_projection, zoom))
+        .run();
+}
+
+/// Set up a simple 3D scene
+fn setup(
+    mut camera_settings: ResMut<CameraSettings>,
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Perspective projections use field of view, expressed in radians. We would
+    // normally not set it to more than π, which represents a 180° FOV.
+    let min_fov = PI / 5.;
+    let max_fov = PI - 0.2;
+
+    // In orthographic projections, we specify sizes in world units. The below values
+    // are very roughly similar to the above FOV settings, in terms of how "far away"
+    // the subject will appear when used with FixedVertical scaling mode.
+    let min_zoom = 5.0;
+    let max_zoom = 150.0;
+
+    // Limiting pitch stops some unexpected rotation past 90° up or down.
+    let pitch_limit = FRAC_PI_2 - 0.01;
+
+    camera_settings.orbit_distance = 10.0;
+    camera_settings.orbit_speed = 1.0;
+    camera_settings.orthographic_zoom_range = min_zoom..max_zoom;
+    camera_settings.orthographic_zoom_speed = 1.0;
+    camera_settings.perspective_zoom_range = min_fov..max_fov;
+    // Changes in FOV are much more noticeable due to its limited range in radians
+    camera_settings.perspective_zoom_speed = 0.05;
+    camera_settings.pitch_range = -pitch_limit..pitch_limit;
+
+    commands.spawn((
+        Name::new("Camera"),
+        Camera3dBundle {
+            projection: OrthographicProjection {
+                scaling_mode: ScalingMode::FixedVertical(
+                    camera_settings.orthographic_zoom_range.start,
+                ),
+                ..OrthographicProjection::default_3d()
+            }
+            .into(),
+            transform: Transform::from_xyz(5.0, 5.0, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
+            ..default()
+        },
+    ));
+
+    commands.spawn((
+        Name::new("Plane"),
+        PbrBundle {
+            mesh: meshes.add(Plane3d::default().mesh().size(5.0, 5.0)),
+            material: materials.add(StandardMaterial {
+                base_color: Color::srgb(0.3, 0.5, 0.3),
+                // Turning off culling keeps the plane visible when viewed from beneath.
+                cull_mode: None,
+                ..default()
+            }),
+            ..default()
+        },
+    ));
+
+    commands.spawn((
+        Name::new("Cube"),
+        PbrBundle {
+            mesh: meshes.add(Cuboid::default()),
+            material: materials.add(Color::srgb(0.8, 0.7, 0.6)),
+            transform: Transform::from_xyz(1.5, 0.51, 1.5),
+            ..default()
+        },
+    ));
+
+    commands.spawn((
+        Name::new("Light"),
+        PointLightBundle {
+            transform: Transform::from_xyz(3.0, 8.0, 5.0),
+            ..default()
+        },
+    ));
+}
+
+fn instructions(mut commands: Commands) {
+    commands
+        .spawn((
+            Name::new("Instructions"),
+            NodeBundle {
+                style: Style {
+                    align_items: AlignItems::Start,
+                    flex_direction: FlexDirection::Column,
+                    justify_content: JustifyContent::Start,
+                    width: Val::Percent(100.),
+                    ..default()
+                },
+                ..default()
+            },
+        ))
+        .with_children(|parent| {
+            parent.spawn(TextBundle::from_section(
+                "Scroll mouse wheel to zoom in/out",
+                TextStyle::default(),
+            ));
+            parent.spawn(TextBundle::from_section(
+                "W or S: pitch",
+                TextStyle::default(),
+            ));
+            parent.spawn(TextBundle::from_section(
+                "A or D: yaw",
+                TextStyle::default(),
+            ));
+        });
+}
+
+fn orbit(
+    mut camera: Query<&mut Transform, With<Camera>>,
+    camera_settings: Res<CameraSettings>,
+    keyboard_input: Res<ButtonInput<KeyCode>>,
+    time: Res<Time>,
+) {
+    let mut transform = camera.single_mut();
+
+    let mut delta_pitch = 0.0;
+    let mut delta_yaw = 0.0;
+
+    if keyboard_input.pressed(KeyCode::KeyW) {
+        delta_pitch += camera_settings.orbit_speed;
+    }
+    if keyboard_input.pressed(KeyCode::KeyA) {
+        delta_yaw -= camera_settings.orbit_speed;
+    }
+    if keyboard_input.pressed(KeyCode::KeyS) {
+        delta_pitch -= camera_settings.orbit_speed;
+    }
+    if keyboard_input.pressed(KeyCode::KeyD) {
+        delta_yaw += camera_settings.orbit_speed;
+    }
+
+    // Incorporating the delta time between calls prevents this from being framerate-bound.
+    delta_pitch *= time.delta_seconds();
+    delta_yaw *= time.delta_seconds();
+
+    // Obtain the existing pitch, yaw, and roll values from the transform.
+    let (yaw, pitch, roll) = transform.rotation.to_euler(EulerRot::YXZ);
+
+    // Establish the new yaw and pitch, preventing the pitch value from exceeding our limits.
+    let pitch = (pitch + delta_pitch).clamp(
+        camera_settings.pitch_range.start,
+        camera_settings.pitch_range.end,
+    );
+    let yaw = yaw + delta_yaw;
+    transform.rotation = Quat::from_euler(EulerRot::YXZ, yaw, pitch, roll);
+
+    // Adjust the translation to maintain the correct orientation toward the orbit target.
+    transform.translation = Vec3::ZERO - transform.forward() * camera_settings.orbit_distance;
+}
+
+fn switch_projection(
+    mut camera: Query<&mut Projection, With<Camera>>,
+    camera_settings: Res<CameraSettings>,
+    keyboard_input: Res<ButtonInput<KeyCode>>,
+) {
+    let mut projection = camera.single_mut();
+
+    if keyboard_input.just_pressed(KeyCode::Space) {
+        // Switch projection type
+        *projection = match *projection {
+            Projection::Orthographic(_) => Projection::Perspective(PerspectiveProjection {
+                fov: camera_settings.perspective_zoom_range.start,
+                ..default()
+            }),
+            Projection::Perspective(_) => Projection::Orthographic(OrthographicProjection {
+                scaling_mode: ScalingMode::FixedVertical(
+                    camera_settings.orthographic_zoom_range.start,
+                ),
+                ..OrthographicProjection::default_3d()
+            }),
+        }
+    }
+}
+
+fn zoom(
+    mut camera: Query<&mut Projection, With<Camera>>,
+    camera_settings: Res<CameraSettings>,
+    mouse_wheel_input: Res<AccumulatedMouseScroll>,
+) {
+    let projection = camera.single_mut();
+
+    // Usually, you won't need to handle both types of projection. This is by way of demonstration.
+    match projection.into_inner() {
+        Projection::Orthographic(ref mut orthographic) => {
+            // Get the current scaling_mode value to allow clamping the new value to our zoom range.
+            let ScalingMode::FixedVertical(current) = orthographic.scaling_mode else {
+                return;
+            };
+            // Set a new ScalingMode, clamped to a limited range.
+            let zoom_level = (current
+                + camera_settings.orthographic_zoom_speed * mouse_wheel_input.delta.y)
+                .clamp(
+                    camera_settings.orthographic_zoom_range.start,
+                    camera_settings.orthographic_zoom_range.end,
+                );
+            orthographic.scaling_mode = ScalingMode::FixedVertical(zoom_level);
+        }
+        Projection::Perspective(ref mut perspective) => {
+            // Adjust the field of view, but keep it within our stated range.
+            perspective.fov = (perspective.fov
+                + camera_settings.perspective_zoom_speed * mouse_wheel_input.delta.y)
+                .clamp(
+                    camera_settings.perspective_zoom_range.start,
+                    camera_settings.perspective_zoom_range.end,
+                );
+        }
+    }
+}