//! Demonstrates using a custom extension to the `StandardMaterial` to modify the results of the builtin pbr shader.

use bevy::{
    color::palettes::basic::RED,
    pbr::{ExtendedMaterial, MaterialExtension, OpaqueRendererMethod},
    prelude::*,
    render::render_resource::*,
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_plugins(MaterialPlugin::<
            ExtendedMaterial<StandardMaterial, MyExtension>,
        >::default())
        .add_systems(Startup, setup)
        .add_systems(Update, rotate_things)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ExtendedMaterial<StandardMaterial, MyExtension>>>,
) {
    // sphere
    commands.spawn(MaterialMeshBundle {
        mesh: meshes.add(Sphere::new(1.0)),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        material: materials.add(ExtendedMaterial {
            base: StandardMaterial {
                base_color: RED.into(),
                // can be used in forward or deferred mode.
                opaque_render_method: OpaqueRendererMethod::Auto,
                // in deferred mode, only the PbrInput can be modified (uvs, color and other material properties),
                // in forward mode, the output can also be modified after lighting is applied.
                // see the fragment shader `extended_material.wgsl` for more info.
                // Note: to run in deferred mode, you must also add a `DeferredPrepass` component to the camera and either
                // change the above to `OpaqueRendererMethod::Deferred` or add the `DefaultOpaqueRendererMethod` resource.
                ..Default::default()
            },
            extension: MyExtension { quantize_steps: 3 },
        }),
        ..default()
    });

    // light
    commands.spawn((
        DirectionalLightBundle {
            transform: Transform::from_xyz(1.0, 1.0, 1.0).looking_at(Vec3::ZERO, Vec3::Y),
            ..default()
        },
        Rotate,
    ));

    // camera
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });
}

#[derive(Component)]
struct Rotate;

fn rotate_things(mut q: Query<&mut Transform, With<Rotate>>, time: Res<Time>) {
    for mut t in &mut q {
        t.rotate_y(time.delta_seconds());
    }
}

#[derive(Asset, AsBindGroup, Reflect, Debug, Clone)]
struct MyExtension {
    // We need to ensure that the bindings of the base material and the extension do not conflict,
    // so we start from binding slot 100, leaving slots 0-99 for the base material.
    #[uniform(100)]
    quantize_steps: u32,
}

impl MaterialExtension for MyExtension {
    fn fragment_shader() -> ShaderRef {
        "shaders/extended_material.wgsl".into()
    }

    fn deferred_fragment_shader() -> ShaderRef {
        "shaders/extended_material.wgsl".into()
    }
}

#import bevy_pbr::{
    pbr_fragment::pbr_input_from_standard_material,
    pbr_functions::alpha_discard,
}

#ifdef PREPASS_PIPELINE
#import bevy_pbr::{
    prepass_io::{VertexOutput, FragmentOutput},
    pbr_deferred_functions::deferred_output,
}
#else
#import bevy_pbr::{
    forward_io::{VertexOutput, FragmentOutput},
    pbr_functions::{apply_pbr_lighting, main_pass_post_lighting_processing},
}
#endif

struct MyExtendedMaterial {
    quantize_steps: u32,
}

@group(2) @binding(100)
var<uniform> my_extended_material: MyExtendedMaterial;

@fragment
fn fragment(
    in: VertexOutput,
    @builtin(front_facing) is_front: bool,
) -> FragmentOutput {
    // generate a PbrInput struct from the StandardMaterial bindings
    var pbr_input = pbr_input_from_standard_material(in, is_front);

    // we can optionally modify the input before lighting and alpha_discard is applied
    pbr_input.material.base_color.b = pbr_input.material.base_color.r;

    // alpha discard
    pbr_input.material.base_color = alpha_discard(pbr_input.material, pbr_input.material.base_color);

#ifdef PREPASS_PIPELINE
    // in deferred mode we can't modify anything after that, as lighting is run in a separate fullscreen shader.
    let out = deferred_output(in, pbr_input);
#else
    var out: FragmentOutput;
    // apply lighting
    out.color = apply_pbr_lighting(pbr_input);

    // we can optionally modify the lit color before post-processing is applied
    out.color = vec4<f32>(vec4<u32>(out.color * f32(my_extended_material.quantize_steps))) / f32(my_extended_material.quantize_steps);

    // apply in-shader post processing (fog, alpha-premultiply, and also tonemapping, debanding if the camera is non-hdr)
    // note this does not include fullscreen postprocessing effects like bloom.
    out.color = main_pass_post_lighting_processing(pbr_input, out.color);

    // we can optionally modify the final result here
    out.color = out.color * 2.0;
#endif

    return out;
}