box shadow

[ickshonpe]

Objective

UI box shadow support

Adds a new component BoxShadow:

pub struct BoxShadow {
    /// The shadow's color
    pub color: Color,
    /// Horizontal offset
    pub x_offset: Val,
    /// Vertical offset
    pub y_offset: Val,
    /// Horizontal difference in size from the occluding uninode
    pub spread_radius: Val,
    /// Blurriness of the shadow
    pub blur_radius: Val,
}

To use BoxShadow, add the component to any Bevy UI node and a shadow will be drawn beneath that node.
Also adds a resource BoxShadowSamples that can be used to adjust the shadow quality.

Notes

• I'm not super happy with the field names. Maybe we need a struct Size { width: Val, height: Val } type or something.
• The shader isn't very optimised but I don't see that it's too important for now as the number of shadows being rendered is not going to be massive most of the time. I think it's more important to get the API and geometry correct with this PR.
• I didn't implement an inset property, it's not essential and can easily be added in a follow up.
• Shadows are only rendered for uinodes, not for images or text.
• Batching isn't supported, it would need out-of-the-scope-of-this-pr changes to the way the UI handles z-ordering for it to be effective.

Showcase

cargo run --example box_shadow -- --samples 4

cargo run --example box_shadow -- --samples 10

[JMS55]

If you need a reference, there's linebender/vello#665, although idk if it's suitable for games/is fast or not.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[ickshonpe]

Does this work for boxes with transparent backgrounds? (shadow should not render beneath content)

No, atm this is very simple it just draws a shadow below any node with the BoxShadow component, not around the node. It's not difficult to support only drawing around and I agree that it's desirable, but adds a lot of extra complexity to the shader so I'd rather get this basic version merged first and address that in a follow up.

[ickshonpe]

If you need a reference, there's linebender/vello#665, although idk if it's suitable for games/is fast or not.

I think the approach they are using isn't that different, except it has to fit into their big unified shader. I'm not sure if a unified UI shader would be right for bevy because game UI tends to be much more diverse etc.

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1721 if you'd like to help out.