Why does Camera require Visibility?

[Liburia]

It doesn't seem to do anything.

[johnsonjo4531]

I am not entirely sure why camera displays its contents when you have visibility set to Hidden. It appears it however will carry down to its children so that they will not show up when set to hidden, so it at least does something to that effect. Here's an example bevy file that will show a circle when Visibility of the Camera is set to Hidden and a square (which is the child of the camera) when the camera is visible. Putting children on camera's is pretty common place for things that need to follow the camera around :). If you want to see the camera in movement with the square inside it I created a demo repo here: https://github.com/johnsonjo4531/bevy_figure_8_demo

use bevy::prelude::*;
use bevy::render::camera::ScalingMode;

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

#[derive(Component, Default)]
#[require(Camera2d)]
struct MainCamera;

fn spawn_camera(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    commands
        .spawn((
            MainCamera,
            Visibility::Hidden, // Since this is hidden you will see the circle toggle this line to be commented out and you should see the children
            Projection::Orthographic(OrthographicProjection {
                scaling_mode: ScalingMode::FixedVertical {
                    viewport_height: 100.0,
                },
                scale: 1.0,
                ..OrthographicProjection::default_2d()
            }),
        ))
        .with_children(|child| {
            child.spawn((
                Mesh2d(meshes.add(Rectangle::new(100., 100.))),
                MeshMaterial2d(
                    materials.add(ColorMaterial::from_color(Color::srgb_u8(255, 255, 255))),
                ),
                Transform::from_translation(Vec3::new(0., 0., -1.)),
                // Visibility::Visible, // Toggle this line as commented in and it should always
                // display a square as the child of the camera regardless of the camera's visibility
            ));
        });
}

fn spawn_ball(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    commands.spawn((
        Mesh2d(meshes.add(Circle::new(50.))),
        MeshMaterial2d(materials.add(ColorMaterial::from_color(Color::srgb_u8(255, 255, 255)))),
    ));
}

[Touma-Kazusa2]

Yes, but if you want an entity to have Visibility, you can add it. why does Camera require it?

[johnsonjo4531]

Sorry, my answer was more towards "It doesn't seem to do anything." It does do something as I showed.

Yes, but if you want an entity to have Visibility, you can add it. why does Camera require it?

I'm kind of new to bevy, but Visibility is basically an enum with Hidden, Inherited, and Visible it's not that much data to have around. I believe every component in bevy that can be visible has a visibility component which makes sense (I could use correction if I'm wrong). What would be the default if it wasn't required? Well if it wasn't required then you would have to define the default in business logic like a system. Well when it is required, we can tell pretty simply by following into Visibility that the default is Inherited. Required Components which are new in bevy 0.15 are automatically added to any bundle, so it's not like you have to require it yourself if you just add a Camera2d.

If an entity is hidden in this way [with visibility], all Children (and all of their children and so on) who are set to Inherited will also be hidden. This is done by the visibility_propagate_system which uses the entity hierarchy and Visibility to set the values of each entity’s InheritedVisibility component.
- from the docs

Here's the implementation of visibility_propagate_system. It's the business logic associated with Visibility's Component. I'm not absolutely sure but it looks like every entity in the view tree atleast needs the InheritedVisibility otherwise all children default to being visible by default if it's inherited? (The reason I believe this is the last link says in a comment, "fall back to true if no parent is found or parent lacks components") So, "if you want an entity to have Visibility, you can add it." may actually be the opposite of how it works?

I've been dabbling in bevy and am still somewhat new to bevy (not to software development in general though), so I could definitely be wrong and I could always use correction.

[Touma-Kazusa2]

Thanks for the great explanation! I think I'm starting to get it now.