Rendering masks

[schell]

tl;dr adds RenderLayers to link groups of entities to cameras.

This describes which rendering layers an entity belongs to. A layer is a collection of entities that only get rendered by cameras in that layer.

Entities and cameras without a RenderLayers component can be considered to belong to the "default" layer (layer 0), which is the same as entities and cameras with the component RenderLayers::default().

Entities and cameras can belong to multiple layers:

commands
    .spawn(...)
    .with(
        RenderLayers::layer(0) // starts with just layer 0
            .with(1) // also sets layer 1
    )

Using a mask of u32 we have 32 possible rendering layers, 0-31 plus the empty mask RenderLayers::none().

More takeaways:

• by default, entities belong to layer 0
• entities without a mask belong to layer 0
• entities with a mask without any layers (RenderLayers::none()) are invisible, as if they had the component Visible { is_visible: false }

[cart]

I dig this. I only have one comment:

With the current masking approach (empty is default, empty matches empty) I don't think there is a way to encode the intent "draw everything". If you set all of the bits in the mask to 1, it will draw everything but the default empty mask. And if you set all of the bits to 0, it will only draw things that are also 0. I think it makes more sense to default to "group 1" and implicitly assume things without the mask component are also "group 1". This has the added benefit of being the behavior used in Godot, which makes it more familiar. This would mean camera masks would need to default to "group 1" (or maybe all groups, which is what godot does).

[schell]

@cart, @mockersf, @DJMcNab I've updated the PR with your suggestions and then some (docs, tests) and updated the PR desc.

[DJMcNab]

Looks very good, just a few code style nits/duplications for me.

[DJMcNab]

I'm still cautious about making this guarantee, since we still seem to require that every entity has a Visible, which this overlaps with.

That being said, I don't think there is necessarily better behaviour. Maybe we could have a warning log which prints once (i.e. if any entity matches this, then print and set an AtomicBool to true)

[schell]

Yes, it's a bit sketchy but it does help describe the feature. I think this is the best we can do at the moment. I personally plan on using the "invisiblity" of RenderLayers::none() so I don't think we should add a warning.

[cart]

Yeah I thought a bit about just replacing Visible with RenderLayers. But I think we need a "global" visible/invisible toggle:

• That way visibility can still be toggled without needing to know what layers the current entity is visible on
• Visibility will likely eventually propagate down a hierarchy, while RenderLayers might not. Its worth discussing at some point, but imo doesn't need to be settled in this pr.
• I want entity visibility to be easily toggle-able in the editor from the "Entity Inspector" (ex: have an "eye" toggle next to the entity name), which would be much harder for arbitrary layers

[cart]

Just left one last baby comment, then I think this is good to go. Very nicely done!