render_ui_to_texture.rs

//! Shows how to render UI to a texture. Useful for displaying UI in 3D space.

use std::f32::consts::PI;

use bevy::{
    prelude::*,
    render::{
        camera::RenderTarget,
        render_resource::{
            Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages,
        },
    },
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, rotator_system)
        .run();
}

// Marks the cube, to which the UI texture is applied.
#[derive(Component)]
struct Cube;

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut images: ResMut<Assets<Image>>,
) {
    let size = Extent3d {
        width: 512,
        height: 512,
        ..default()
    };

    // This is the texture that will be rendered to.
    let mut image = Image {
        texture_descriptor: TextureDescriptor {
            label: None,
            size,
            dimension: TextureDimension::D2,
            format: TextureFormat::Bgra8UnormSrgb,
            mip_level_count: 1,
            sample_count: 1,
            usage: TextureUsages::TEXTURE_BINDING
                | TextureUsages::COPY_DST
                | TextureUsages::RENDER_ATTACHMENT,
            view_formats: &[],
        },
        ..default()
    };

    // fill image.data with zeroes
    image.resize(size);

    let image_handle = images.add(image);

    // Light
    commands.spawn(DirectionalLightBundle::default());

    let texture_camera = commands
        .spawn(Camera2dBundle {
            camera: Camera {
                // render before the "main pass" camera
                order: -1,
                target: RenderTarget::Image(image_handle.clone()),
                ..default()
            },
            ..default()
        })
        .id();

    commands
        .spawn((
            NodeBundle {
                style: Style {
                    // Cover the whole image
                    width: Val::Percent(100.),
                    height: Val::Percent(100.),
                    flex_direction: FlexDirection::Column,
                    justify_content: JustifyContent::Center,
                    align_items: AlignItems::Center,
                    ..default()
                },
                background_color: LegacyColor::GOLD.into(),
                ..default()
            },
            TargetCamera(texture_camera),
        ))
        .with_children(|parent| {
            parent.spawn(TextBundle::from_section(
                "This is a cube",
                TextStyle {
                    font_size: 40.0,
                    color: LegacyColor::BLACK,
                    ..default()
                },
            ));
        });

    let cube_size = 4.0;
    let cube_handle = meshes.add(Cuboid::new(cube_size, cube_size, cube_size));

    // This material has the texture that has been rendered.
    let material_handle = materials.add(StandardMaterial {
        base_color_texture: Some(image_handle),
        reflectance: 0.02,
        unlit: false,

        ..default()
    });

    // Cube with material containing the rendered UI texture.
    commands.spawn((
        PbrBundle {
            mesh: cube_handle,
            material: material_handle,
            transform: Transform::from_xyz(0.0, 0.0, 1.5)
                .with_rotation(Quat::from_rotation_x(-PI / 5.0)),
            ..default()
        },
        Cube,
    ));

    // The main pass camera.
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(0.0, 0.0, 15.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

const ROTATION_SPEED: f32 = 0.5;

fn rotator_system(time: Res<Time>, mut query: Query<&mut Transform, With<Cube>>) {
    for mut transform in &mut query {
        transform.rotate_x(1.0 * time.delta_seconds() * ROTATION_SPEED);
        transform.rotate_y(0.7 * time.delta_seconds() * ROTATION_SPEED);
    }
}