Flickering with transparency and identical 'depth'

[BorisBoutillier]

Bevy version

v0.8.0

Relevant system information

AdapterInfo { name: "NVIDIA GeForce GTX 1080", vendor: 4318, device: 7040, device_type: DiscreteGpu, backend: Vulkan }
Ubuntu 22.04

What you did

I have modified the 3d_scene.rs example to give alpha = 0.3 to the cube and add another cube inside, itself with an alpha!=1.
Full code :

//! A simple 3D scene with light shining over a cube sitting on a plane.

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup)
        .run();
}

/// set up a simple 3D scene
fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<StandardMaterial>>,
) {
    // plane
    commands.spawn_bundle(PbrBundle {
        mesh: meshes.add(Mesh::from(shape::Plane { size: 5.0 })),
        material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
        ..default()
    });
    // cube
    commands
        .spawn_bundle(PbrBundle {
            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
            material: materials.add(Color::rgba(0.8, 0.7, 0.6, 0.3).into()),
            transform: Transform::from_xyz(0.0, 0.5, 0.0),
            ..default()
        })
        .with_children(|cube| {
            cube.spawn_bundle(PbrBundle {
                mesh: meshes.add(Mesh::from(shape::Cube { size: 0.1 })),
                material: materials.add(Color::rgba(0.8, 0.7, 0.6, 0.5).into()),
                ..default()
            });
        });
    // light
    commands.spawn_bundle(PointLightBundle {
        point_light: PointLight {
            intensity: 1500.0,
            shadows_enabled: true,
            ..default()
        },
        transform: Transform::from_xyz(4.0, 8.0, 4.0),
        ..default()
    });
    // camera
    commands.spawn_bundle(Camera3dBundle {
        transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
        ..default()
    });

What went wrong

When running this modified example, the cube inside the main cube is 'flickering', its render color is changing each frame.
Because nothing other is changing in the scene, I did expect he rendered color to be stable across frames.
See short video:
https://user-images.githubusercontent.com/18518188/184179259-a0f4cd2c-88fb-4e91-b799-112ba4f9ceb2.mp4

Additional information

1. If the internal cube has alpha=1 there is no flickering issue, only if internal cube 0.0 < alpha < 1.0
2. If the internal cube has alpha=0 there is no issue either, the internal cube is properly always invisible.

[superdump]

For correct transparency, drawing needs to be done back to front. However, both cubes are at the same z so the order is not guaranteed to be the same between frames. There is a depth bias on a material somewhere as I recall, which adds a fake offset to the z so that you can adjust the order of entities with the same z. @robtfm knows as they implemented it. I think maybe it’s in StandardMaterial?

[BorisBoutillier]

I can confirm that adding an infinitesimal Z translation stabilize rendering.

[BorisBoutillier]

Also, there is effectively a depth_bias attribute on the StandardMaterial and setting it to non-0 for one of the two materials will also solve the flickering.

So is it expected behavior of the bevy-render that the z-conflict resolution is 'unstable' and can provide different outcome at each frame, thus creating flickering.
I suspect this is probably the case, as stabilizing the conflict would need to 'order' on something else, and would need the rendering to be stable, with probable performance impact.

If somebody can confirm this is expected behavior, I'll close the issue.

[aevyrie]

This is solved by the recently added OIT.