# Diffuse Transmission

There are some ways to make a light going around an object.

We use [diffuse_transmission](https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html#structfield.diffuse_transmission) in [StandardMaterial](https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html).
This field makes a light transmitted to the back of the object, which results in the backside of the object brighter.

```rust
commands.spawn(PbrBundle {
    material: materials.add(StandardMaterial {
        diffuse_transmission: 1.,
        ..default()
    }),
    ..default()
});
```

The value of [diffuse_transmission](https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html#structfield.diffuse_transmission) is between `0` and `1`.
The larger the value, the more it transmits a light to the back of the object.
Note that this field does not make the object transparent.

In the following example, we create three spheres.
From the left to right, their [diffuse_transmission](https://docs.rs/bevy/latest/bevy/pbr/struct.StandardMaterial.html#structfield.diffuse_transmission) are `0`, `0.5` and `1` respectively.

The full code is as follows:

```rust
use bevy::{
    app::{App, Startup},
    asset::Assets,
    core_pipeline::core_3d::Camera3dBundle,
    ecs::system::{Commands, ResMut},
    math::Vec3,
    pbr::{DirectionalLight, DirectionalLightBundle, PbrBundle, StandardMaterial},
    render::mesh::{
        shape::{Plane, UVSphere},
        Mesh,
    },
    transform::components::Transform,
    utils::default,
    DefaultPlugins,
};

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

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    commands.spawn(Camera3dBundle {
        transform: Transform::from_xyz(0., 2., 3.).looking_at(Vec3::new(0., 0.5, 0.), Vec3::Y),
        ..default()
    });

    // left
    commands.spawn(PbrBundle {
        mesh: meshes.add(
            UVSphere {
                radius: 0.5,
                ..default()
            }
            .into(),
        ),
        material: materials.add(StandardMaterial {
            diffuse_transmission: 0.,
            ..default()
        }),
        transform: Transform::from_xyz(-1.25, 0.5, 0.),
        ..default()
    });

    // middle
    commands.spawn(PbrBundle {
        mesh: meshes.add(
            UVSphere {
                radius: 0.5,
                ..default()
            }
            .into(),
        ),
        material: materials.add(StandardMaterial {
            diffuse_transmission: 0.5,
            ..default()
        }),
        transform: Transform::from_xyz(0., 0.5, 0.),
        ..default()
    });

    // right
    commands.spawn(PbrBundle {
        mesh: meshes.add(
            UVSphere {
                radius: 0.5,
                ..default()
            }
            .into(),
        ),
        material: materials.add(StandardMaterial {
            diffuse_transmission: 1.,
            ..default()
        }),
        transform: Transform::from_xyz(1.25, 0.5, 0.),
        ..default()
    });

    commands.spawn(PbrBundle {
        mesh: meshes.add(Plane::from_size(5.).into()).into(),
        material: materials.add(StandardMaterial::default()).into(),
        ..default()
    });

    commands.spawn(DirectionalLightBundle {
        directional_light: DirectionalLight {
            illuminance: 20000.,
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::default().looking_to(Vec3::new(-1., -1., -1.), Vec3::Y),
        ..default()
    });
}
```

Result:

![Diffuse Transmission](./pic/diffuse_transmission.png)

:arrow_right:  Next: [Specular Transmission](./specular_transmission.md)

:blue_book: Back: [Table of contents](./../README.md)