//! Shows how to render to a texture. Useful for mirrors, UI, or exporting images.

use bevy::{
    core_pipeline::clear_color::ClearColorConfig,
    prelude::*,
    render::{
        camera::RenderTarget,
        render_resource::{
            Extent3d, TextureDescriptor, TextureDimension, TextureFormat, TextureUsages,
        },
        view::RenderLayers,
    },
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup)
        .add_system(cube_rotator_system)
        .add_system(rotator_system)
        .run();
}

// Marks the first pass cube (rendered to a texture.)
#[derive(Component)]
struct FirstPassCube;

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

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,
        },
        ..default()
    };

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

    let image_handle = images.add(image);

    let cube_handle = meshes.add(Mesh::from(shape::Cube { size: 4.0 }));
    let cube_material_handle = materials.add(StandardMaterial {
        base_color: Color::rgb(0.8, 0.7, 0.6),
        reflectance: 0.02,
        unlit: false,
        ..default()
    });

    // This specifies the layer used for the first pass, which will be attached to the first pass camera and cube.
    let first_pass_layer = RenderLayers::layer(1);

    // The cube that will be rendered to the texture.
    commands
        .spawn_bundle(PbrBundle {
            mesh: cube_handle,
            material: cube_material_handle,
            transform: Transform::from_translation(Vec3::new(0.0, 0.0, 1.0)),
            ..default()
        })
        .insert(FirstPassCube)
        .insert(first_pass_layer);

    // Light
    // NOTE: Currently lights are shared between passes - see https://github.com/bevyengine/bevy/issues/3462
    commands.spawn_bundle(PointLightBundle {
        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 10.0)),
        ..default()
    });

    commands
        .spawn_bundle(Camera3dBundle {
            camera_3d: Camera3d {
                clear_color: ClearColorConfig::Custom(Color::WHITE),
                ..default()
            },
            camera: Camera {
                // render before the "main pass" camera
                priority: -1,
                target: RenderTarget::Image(image_handle.clone()),
                ..default()
            },
            transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
                .looking_at(Vec3::default(), Vec3::Y),
            ..default()
        })
        .insert(first_pass_layer);

    let cube_size = 4.0;
    let cube_handle = meshes.add(Mesh::from(shape::Box::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()
    });

    // Main pass cube, with material containing the rendered first pass texture.
    commands
        .spawn_bundle(PbrBundle {
            mesh: cube_handle,
            material: material_handle,
            transform: Transform {
                translation: Vec3::new(0.0, 0.0, 1.5),
                rotation: Quat::from_rotation_x(-std::f32::consts::PI / 5.0),
                ..default()
            },
            ..default()
        })
        .insert(MainPassCube);

    // The main pass camera.
    commands.spawn_bundle(Camera3dBundle {
        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 15.0))
            .looking_at(Vec3::default(), Vec3::Y),
        ..default()
    });
}

/// Rotates the inner cube (first pass)
fn rotator_system(time: Res<Time>, mut query: Query<&mut Transform, With<FirstPassCube>>) {
    for mut transform in query.iter_mut() {
        transform.rotation *= Quat::from_rotation_x(1.5 * time.delta_seconds());
        transform.rotation *= Quat::from_rotation_z(1.3 * time.delta_seconds());
    }
}

/// Rotates the outer cube (main pass)
fn cube_rotator_system(time: Res<Time>, mut query: Query<&mut Transform, With<MainPassCube>>) {
    for mut transform in query.iter_mut() {
        transform.rotation *= Quat::from_rotation_x(1.0 * time.delta_seconds());
        transform.rotation *= Quat::from_rotation_y(0.7 * time.delta_seconds());
    }
}