//! This example shows how to create a custom render pass that runs after the main pass
//! and reads the texture generated by the main pass.
//!
//! The example shader is a very simple implementation of chromatic aberration.
//! To adapt this example for 2D, replace all instances of 3D structures (such as `Core3D`, etc.) with their corresponding 2D counterparts.
//!
//! This is a fairly low level example and assumes some familiarity with rendering concepts and wgpu.

use bevy::{
    core_pipeline::{
        core_3d::graph::{Core3d, Node3d},
        fullscreen_vertex_shader::fullscreen_shader_vertex_state,
    },
    ecs::query::QueryItem,
    prelude::*,
    render::{
        extract_component::{
            ComponentUniforms, DynamicUniformIndex, ExtractComponent, ExtractComponentPlugin,
            UniformComponentPlugin,
        },
        render_graph::{
            NodeRunError, RenderGraphApp, RenderGraphContext, RenderLabel, ViewNode, ViewNodeRunner,
        },
        render_resource::{
            binding_types::{sampler, texture_2d, uniform_buffer},
            *,
        },
        renderer::{RenderContext, RenderDevice},
        view::ViewTarget,
        RenderApp,
    },
};

/// This example uses a shader source file from the assets subdirectory
const SHADER_ASSET_PATH: &str = "shaders/post_processing.wgsl";

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

/// It is generally encouraged to set up post processing effects as a plugin
struct PostProcessPlugin;

impl Plugin for PostProcessPlugin {
    fn build(&self, app: &mut App) {
        app.add_plugins((
            // The settings will be a component that lives in the main world but will
            // be extracted to the render world every frame.
            // This makes it possible to control the effect from the main world.
            // This plugin will take care of extracting it automatically.
            // It's important to derive [`ExtractComponent`] on [`PostProcessingSettings`]
            // for this plugin to work correctly.
            ExtractComponentPlugin::<PostProcessSettings>::default(),
            // The settings will also be the data used in the shader.
            // This plugin will prepare the component for the GPU by creating a uniform buffer
            // and writing the data to that buffer every frame.
            UniformComponentPlugin::<PostProcessSettings>::default(),
        ));

        // We need to get the render app from the main app
        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app
            // Bevy's renderer uses a render graph which is a collection of nodes in a directed acyclic graph.
            // It currently runs on each view/camera and executes each node in the specified order.
            // It will make sure that any node that needs a dependency from another node
            // only runs when that dependency is done.
            //
            // Each node can execute arbitrary work, but it generally runs at least one render pass.
            // A node only has access to the render world, so if you need data from the main world
            // you need to extract it manually or with the plugin like above.
            // Add a [`Node`] to the [`RenderGraph`]
            // The Node needs to impl FromWorld
            //
            // The [`ViewNodeRunner`] is a special [`Node`] that will automatically run the node for each view
            // matching the [`ViewQuery`]
            .add_render_graph_node::<ViewNodeRunner<PostProcessNode>>(
                // Specify the label of the graph, in this case we want the graph for 3d
                Core3d,
                // It also needs the label of the node
                PostProcessLabel,
            )
            .add_render_graph_edges(
                Core3d,
                // Specify the node ordering.
                // This will automatically create all required node edges to enforce the given ordering.
                (
                    Node3d::Tonemapping,
                    PostProcessLabel,
                    Node3d::EndMainPassPostProcessing,
                ),
            );
    }

    fn finish(&self, app: &mut App) {
        // We need to get the render app from the main app
        let Some(render_app) = app.get_sub_app_mut(RenderApp) else {
            return;
        };

        render_app
            // Initialize the pipeline
            .init_resource::<PostProcessPipeline>();
    }
}

#[derive(Debug, Hash, PartialEq, Eq, Clone, RenderLabel)]
struct PostProcessLabel;

// The post process node used for the render graph
#[derive(Default)]
struct PostProcessNode;

// The ViewNode trait is required by the ViewNodeRunner
impl ViewNode for PostProcessNode {
    // The node needs a query to gather data from the ECS in order to do its rendering,
    // but it's not a normal system so we need to define it manually.
    //
    // This query will only run on the view entity
    type ViewQuery = (
        &'static ViewTarget,
        // This makes sure the node only runs on cameras with the PostProcessSettings component
        &'static PostProcessSettings,
        // As there could be multiple post processing components sent to the GPU (one per camera),
        // we need to get the index of the one that is associated with the current view.
        &'static DynamicUniformIndex<PostProcessSettings>,
    );

    // Runs the node logic
    // This is where you encode draw commands.
    //
    // This will run on every view on which the graph is running.
    // If you don't want your effect to run on every camera,
    // you'll need to make sure you have a marker component as part of [`ViewQuery`]
    // to identify which camera(s) should run the effect.
    fn run(
        &self,
        _graph: &mut RenderGraphContext,
        render_context: &mut RenderContext,
        (view_target, _post_process_settings, settings_index): QueryItem<Self::ViewQuery>,
        world: &World,
    ) -> Result<(), NodeRunError> {
        // Get the pipeline resource that contains the global data we need
        // to create the render pipeline
        let post_process_pipeline = world.resource::<PostProcessPipeline>();

        // The pipeline cache is a cache of all previously created pipelines.
        // It is required to avoid creating a new pipeline each frame,
        // which is expensive due to shader compilation.
        let pipeline_cache = world.resource::<PipelineCache>();

        // Get the pipeline from the cache
        let Some(pipeline) = pipeline_cache.get_render_pipeline(post_process_pipeline.pipeline_id)
        else {
            return Ok(());
        };

        // Get the settings uniform binding
        let settings_uniforms = world.resource::<ComponentUniforms<PostProcessSettings>>();
        let Some(settings_binding) = settings_uniforms.uniforms().binding() else {
            return Ok(());
        };

        // This will start a new "post process write", obtaining two texture
        // views from the view target - a `source` and a `destination`.
        // `source` is the "current" main texture and you _must_ write into
        // `destination` because calling `post_process_write()` on the
        // [`ViewTarget`] will internally flip the [`ViewTarget`]'s main
        // texture to the `destination` texture. Failing to do so will cause
        // the current main texture information to be lost.
        let post_process = view_target.post_process_write();

        // The bind_group gets created each frame.
        //
        // Normally, you would create a bind_group in the Queue set,
        // but this doesn't work with the post_process_write().
        // The reason it doesn't work is because each post_process_write will alternate the source/destination.
        // The only way to have the correct source/destination for the bind_group
        // is to make sure you get it during the node execution.
        let bind_group = render_context.render_device().create_bind_group(
            "post_process_bind_group",
            &post_process_pipeline.layout,
            // It's important for this to match the BindGroupLayout defined in the PostProcessPipeline
            &BindGroupEntries::sequential((
                // Make sure to use the source view
                post_process.source,
                // Use the sampler created for the pipeline
                &post_process_pipeline.sampler,
                // Set the settings binding
                settings_binding.clone(),
            )),
        );

        // Begin the render pass
        let mut render_pass = render_context.begin_tracked_render_pass(RenderPassDescriptor {
            label: Some("post_process_pass"),
            color_attachments: &[Some(RenderPassColorAttachment {
                // We need to specify the post process destination view here
                // to make sure we write to the appropriate texture.
                view: post_process.destination,
                resolve_target: None,
                ops: Operations::default(),
            })],
            depth_stencil_attachment: None,
            timestamp_writes: None,
            occlusion_query_set: None,
        });

        // This is mostly just wgpu boilerplate for drawing a fullscreen triangle,
        // using the pipeline/bind_group created above
        render_pass.set_render_pipeline(pipeline);
        // By passing in the index of the post process settings on this view, we ensure
        // that in the event that multiple settings were sent to the GPU (as would be the
        // case with multiple cameras), we use the correct one.
        render_pass.set_bind_group(0, &bind_group, &[settings_index.index()]);
        render_pass.draw(0..3, 0..1);

        Ok(())
    }
}

// This contains global data used by the render pipeline. This will be created once on startup.
#[derive(Resource)]
struct PostProcessPipeline {
    layout: BindGroupLayout,
    sampler: Sampler,
    pipeline_id: CachedRenderPipelineId,
}

impl FromWorld for PostProcessPipeline {
    fn from_world(world: &mut World) -> Self {
        let render_device = world.resource::<RenderDevice>();

        // We need to define the bind group layout used for our pipeline
        let layout = render_device.create_bind_group_layout(
            "post_process_bind_group_layout",
            &BindGroupLayoutEntries::sequential(
                // The layout entries will only be visible in the fragment stage
                ShaderStages::FRAGMENT,
                (
                    // The screen texture
                    texture_2d(TextureSampleType::Float { filterable: true }),
                    // The sampler that will be used to sample the screen texture
                    sampler(SamplerBindingType::Filtering),
                    // The settings uniform that will control the effect
                    uniform_buffer::<PostProcessSettings>(true),
                ),
            ),
        );

        // We can create the sampler here since it won't change at runtime and doesn't depend on the view
        let sampler = render_device.create_sampler(&SamplerDescriptor::default());

        // Get the shader handle
        let shader = world.load_asset(SHADER_ASSET_PATH);

        let pipeline_id = world
            .resource_mut::<PipelineCache>()
            // This will add the pipeline to the cache and queue its creation
            .queue_render_pipeline(RenderPipelineDescriptor {
                label: Some("post_process_pipeline".into()),
                layout: vec![layout.clone()],
                // This will setup a fullscreen triangle for the vertex state
                vertex: fullscreen_shader_vertex_state(),
                fragment: Some(FragmentState {
                    shader,
                    shader_defs: vec![],
                    // Make sure this matches the entry point of your shader.
                    // It can be anything as long as it matches here and in the shader.
                    entry_point: "fragment".into(),
                    targets: vec![Some(ColorTargetState {
                        format: TextureFormat::bevy_default(),
                        blend: None,
                        write_mask: ColorWrites::ALL,
                    })],
                }),
                // All of the following properties are not important for this effect so just use the default values.
                // This struct doesn't have the Default trait implemented because not all fields can have a default value.
                primitive: PrimitiveState::default(),
                depth_stencil: None,
                multisample: MultisampleState::default(),
                push_constant_ranges: vec![],
                zero_initialize_workgroup_memory: false,
            });

        Self {
            layout,
            sampler,
            pipeline_id,
        }
    }
}

// This is the component that will get passed to the shader
#[derive(Component, Default, Clone, Copy, ExtractComponent, ShaderType)]
struct PostProcessSettings {
    intensity: f32,
    // WebGL2 structs must be 16 byte aligned.
    #[cfg(feature = "webgl2")]
    _webgl2_padding: Vec3,
}

/// Set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // camera
    commands.spawn((
        Camera3d::default(),
        Transform::from_translation(Vec3::new(0.0, 0.0, 5.0)).looking_at(Vec3::default(), Vec3::Y),
        Camera {
            clear_color: Color::WHITE.into(),
            ..default()
        },
        // Add the setting to the camera.
        // This component is also used to determine on which camera to run the post processing effect.
        PostProcessSettings {
            intensity: 0.02,
            ..default()
        },
    ));

    // cube
    commands.spawn((
        Mesh3d(meshes.add(Cuboid::default())),
        MeshMaterial3d(materials.add(Color::srgb(0.8, 0.7, 0.6))),
        Transform::from_xyz(0.0, 0.5, 0.0),
        Rotates,
    ));
    // light
    commands.spawn(DirectionalLight {
        illuminance: 1_000.,
        ..default()
    });
}

#[derive(Component)]
struct Rotates;

/// Rotates any entity around the x and y axis
fn rotate(time: Res<Time>, mut query: Query<&mut Transform, With<Rotates>>) {
    for mut transform in &mut query {
        transform.rotate_x(0.55 * time.delta_secs());
        transform.rotate_z(0.15 * time.delta_secs());
    }
}

// Change the intensity over time to show that the effect is controlled from the main world
fn update_settings(mut settings: Query<&mut PostProcessSettings>, time: Res<Time>) {
    for mut setting in &mut settings {
        let mut intensity = ops::sin(time.elapsed_secs());
        // Make it loop periodically
        intensity = ops::sin(intensity);
        // Remap it to 0..1 because the intensity can't be negative
        intensity = intensity * 0.5 + 0.5;
        // Scale it to a more reasonable level
        intensity *= 0.015;

        // Set the intensity.
        // This will then be extracted to the render world and uploaded to the GPU automatically by the [`UniformComponentPlugin`]
        setting.intensity = intensity;
    }
}

// This shader computes the chromatic aberration effect

// Since post processing is a fullscreen effect, we use the fullscreen vertex shader provided by bevy.
// This will import a vertex shader that renders a single fullscreen triangle.
//
// A fullscreen triangle is a single triangle that covers the entire screen.
// The box in the top left in that diagram is the screen. The 4 x are the corner of the screen
//
// Y axis
//  1 |  x-----x......
//  0 |  |  s  |  . ´
// -1 |  x_____x´
// -2 |  :  .´
// -3 |  :´
//    +---------------  X axis
//      -1  0  1  2  3
//
// As you can see, the triangle ends up bigger than the screen.
//
// You don't need to worry about this too much since bevy will compute the correct UVs for you.
#import bevy_core_pipeline::fullscreen_vertex_shader::FullscreenVertexOutput

@group(0) @binding(0) var screen_texture: texture_2d<f32>;
@group(0) @binding(1) var texture_sampler: sampler;
struct PostProcessSettings {
    intensity: f32,
#ifdef SIXTEEN_BYTE_ALIGNMENT
    // WebGL2 structs must be 16 byte aligned.
    _webgl2_padding: vec3<f32>
#endif
}
@group(0) @binding(2) var<uniform> settings: PostProcessSettings;

@fragment
fn fragment(in: FullscreenVertexOutput) -> @location(0) vec4<f32> {
    // Chromatic aberration strength
    let offset_strength = settings.intensity;

    // Sample each color channel with an arbitrary shift
    return vec4<f32>(
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(offset_strength, -offset_strength)).r,
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(-offset_strength, 0.0)).g,
        textureSample(screen_texture, texture_sampler, in.uv + vec2<f32>(0.0, offset_strength)).b,
        1.0
    );
}