Sprites do not respect Z layering properly.

[StarArawn]

Bevy version

0.8.1

What you did

Attempted to render Text2D on top of a separate non-batched Transparent2D phase item.

What went wrong

The order of draw calls is random and a Text2D can draw under a lower z axis Transparent2D entity.

Additional information

I believe this is due to how batches are handled within the sprite renderer which is what Text2D uses.

bevy_sprite sorts extracted sprites by transform.translation.z value twice. Once during the phase sort and once here:
https://github.com/bevyengine/bevy/blob/main/crates/bevy_sprite/src/render/mod.rs#L412

Sorting extracted_sprites with the z axis value is fine if all you are rendering is sprites, but when you have other Transparent2D calls you can't guarantee the correct order of draw calls without batching by z axis as well. A fix would be to batch by z axis. This can be done by adding the sort key to SpriteBatch.

I've tested this fix with a local copy of bevy and this resolved my issue.

[rparrett]

#5942 was merged recently and seems potentially related. Could you test on main?

[StarArawn]

Here is a reproducible example:

//! Displays a single [`Sprite`], created from an image.

use bevy::{prelude::*, sprite::MaterialMesh2dBundle};

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

fn setup(
    mut commands: Commands,
    asset_server: Res<AssetServer>,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    let font = asset_server.load("fonts/FiraSans-Bold.ttf");
    let text_style = TextStyle {
        font,
        font_size: 60.0,
        color: Color::WHITE,
    };
    let text_alignment = TextAlignment::CENTER;

    commands.spawn_bundle(Camera2dBundle::default());
    commands.spawn_bundle(MaterialMesh2dBundle {
        mesh: meshes.add(Mesh::from(shape::Quad::default())).into(),
        transform: Transform::from_xyz(0.0, 0.0, 10.0).with_scale(Vec3::splat(128.)),
        material: materials.add(ColorMaterial::from(Color::PURPLE)),
        ..default()
    });
    // Demonstrate changing translation
    commands
    .spawn_bundle(Text2dBundle {
        text: Text::from_section("translation", text_style.clone())
            .with_alignment(text_alignment),
        transform: Transform::from_xyz(0.0, 0.0, 12.0),
        ..default()
    });

    commands
    .spawn_bundle(Text2dBundle {
        text: Text::from_section("Hello!", text_style.clone())
            .with_alignment(text_alignment),
        transform: Transform::from_xyz(-100.0, 0.0, 0.0),
        ..default()
    });
}

[StarArawn]

#5942 was merged recently and seems potentially related. Could you test on main?

Yeah I'll take a look with the latest bevy.

[StarArawn]

@rparrett Yes that does appear to also fix the issue.

I think if that's the case then the Z sort here is pointless:
https://github.com/bevyengine/bevy/blob/main/crates/bevy_sprite/src/render/mod.rs#L412
As the phase sort will already be sorting correctly.