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//! A shader that uses the GLSL shading language.

use bevy::{
    pbr::{MaterialPipeline, MaterialPipelineKey},
    prelude::*,
    reflect::TypePath,
    render::{
        mesh::MeshVertexBufferLayoutRef,
        render_resource::{
            AsBindGroup, RenderPipelineDescriptor, ShaderRef, SpecializedMeshPipelineError,
        },
    },
};

fn main() {
    App::new()
        .add_plugins((DefaultPlugins, MaterialPlugin::<CustomMaterial>::default()))
        .add_systems(Startup, setup)
        .run();
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<CustomMaterial>>,
    asset_server: Res<AssetServer>,
) {
    // cube
    commands.spawn(MaterialMeshBundle {
        mesh: meshes.add(Cuboid::default()),
        transform: Transform::from_xyz(0.0, 0.5, 0.0),
        material: materials.add(CustomMaterial {
            color: LinearRgba::BLUE,
            color_texture: Some(asset_server.load("branding/icon.png")),
            alpha_mode: AlphaMode::Blend,
        }),
        ..default()
    });

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

// This is the struct that will be passed to your shader
#[derive(Asset, TypePath, AsBindGroup, Clone)]
struct CustomMaterial {
    #[uniform(0)]
    color: LinearRgba,
    #[texture(1)]
    #[sampler(2)]
    color_texture: Option<Handle<Image>>,
    alpha_mode: AlphaMode,
}

/// The Material trait is very configurable, but comes with sensible defaults for all methods.
/// You only need to implement functions for features that need non-default behavior. See the Material api docs for details!
/// When using the GLSL shading language for your shader, the specialize method must be overridden.
impl Material for CustomMaterial {
    fn vertex_shader() -> ShaderRef {
        "shaders/custom_material.vert".into()
    }

    fn fragment_shader() -> ShaderRef {
        "shaders/custom_material.frag".into()
    }

    fn alpha_mode(&self) -> AlphaMode {
        self.alpha_mode
    }

    // Bevy assumes by default that vertex shaders use the "vertex" entry point
    // and fragment shaders use the "fragment" entry point (for WGSL shaders).
    // GLSL uses "main" as the entry point, so we must override the defaults here
    fn specialize(
        _pipeline: &MaterialPipeline<Self>,
        descriptor: &mut RenderPipelineDescriptor,
        _layout: &MeshVertexBufferLayoutRef,
        _key: MaterialPipelineKey<Self>,
    ) -> Result<(), SpecializedMeshPipelineError> {
        descriptor.vertex.entry_point = "main".into();
        descriptor.fragment.as_mut().unwrap().entry_point = "main".into();
        Ok(())
    }
}