`DepthPrepass` doesn't respect changing `alpha_mode` from `Opaque` to `Blend`.

[eero-lehtinen]

Bevy version

main 95470df

SystemInfo { os: "Windows 11 Pro", kernel: "26100", cpu: "AMD Ryzen 9 9950X 16-Core Processor", core_count: "16", memory: "31.2 GiB" }

AdapterInfo { name: "NVIDIA GeForce RTX 3070 Ti", vendor: 4318, device: 9346, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "576.28", backend: Vulkan }

What you did

I added a depth prepass to the camera and changed the alpha mode of an entity with StandardMaterial from Opaque to Blend in the 3d_scene example.

Example:

use bevy::{core_pipeline::prepass::DepthPrepass, prelude::*};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .insert_resource(ClearColor(Color::BLACK))
        .add_systems(Startup, setup)
        .add_systems(Update, update)
        .run();
}

#[derive(Component)]
struct Shape;

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // circular base
    commands.spawn((
        Mesh3d(meshes.add(Circle::new(4.0))),
        MeshMaterial3d(materials.add(Color::WHITE)),
        Transform::from_rotation(Quat::from_rotation_x(-std::f32::consts::FRAC_PI_2)),
    ));
    // cube
    commands.spawn((
        Shape,
        Mesh3d(meshes.add(Cuboid::new(1.0, 1.0, 1.0))),
        MeshMaterial3d(materials.add(Color::srgba_u8(124, 144, 255, 50))),
        Transform::from_xyz(0.0, 0.5, 0.0),
    ));
    // light
    commands.spawn((
        PointLight {
            shadows_enabled: true,
            ..default()
        },
        Transform::from_xyz(4.0, 8.0, 4.0),
    ));
    // camera
    commands.spawn((
        Camera3d::default(),
        Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
        DepthPrepass,
    ));
}

fn update(
    keyboard_input: Res<ButtonInput<KeyCode>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut mat_q: Query<&mut MeshMaterial3d<StandardMaterial>, With<Shape>>,
) {
    if keyboard_input.just_pressed(KeyCode::Space) {
        if let Ok(mat) = mat_q.single_mut() {
            let m = materials.get_mut(&mat.0).unwrap();
            m.alpha_mode = if m.alpha_mode == AlphaMode::Opaque {
                AlphaMode::Blend
            } else {
                AlphaMode::Opaque
            };
        };
    }
}

What went wrong

At first the alpha mode is blend, and looks transparent as it should.

Then pressing space makes it opaque, it looks opaque as it should.

Then once more pressing space the material goes back to blend, but you can see through the white circle, which is wrong. I assume the reason is that DepthPrepass doesn't "realize" that the object is not opaque anymore and doesn't bother to draw anything behind it. Might be a retained rendering issue.

Additional information

[NWPlayer123]

This seems like a similar bug to my issue, #18928