Objective

Allowing drawing of UI nodes with a gradient instead of a flat color.

Solution

The are three gradient structs corresponding to the three types of gradients supported: LinearGradient, ConicGradient and RadialGradient. These are then wrapped in a Gradient enum discriminator which has Linear, Conic and Radial variants.

Each gradient type consists of the geometric properties for that gradient and a list of color stops.
Color stops consist of a color, a position or angle and an optional hint. If no position is specified for a stop, it's evenly spaced between the previous and following stops. Color stop positions are absolute, if you specify a list of stops:
vec![vec![ColorStop::new(RED, Val::Percent(90.), ColorStop::new(Color::GREEN, Val::Percent(10.))
the colors will be reordered and the gradient will transition from green at 10% to red at 90%.

Colors are interpolated between the stops in SRGB space. The hint is a normalized value that can be used to shift the mid-point where the colors are mixed 50-50. between the stop with the hint and the following stop.

For sharp stops with no interpolated transition, place two stops at the same position.

ConicGradients and RadialGradients have a center which is set using the new Positiontype.Positionconsists of a normalized (relative to the UI node)Vec2` anchor point and a responsive x, y offset.

To draw a UI node with a gradient you insert the components BackgroundGradient and BorderGradient, which both newtype a vector of Gradients. If you set a background color, the background color is drawn first and the gradient(s) are drawn on top.

The implementation is deliberately simple and self contained. The shader draws the gradient in multiple passes which is quite inefficient for gradients with a very large number of color stops. It's simple though and there won't be any compatibility issues. We could make gradients a specialization for UiPipeline but I used a separate pipeline plugin for now to ensure that these changes don't break anything.

Not supported in this PR

• Interpolation in other color spaces besides SRGB.
• Images and text: This would need some breaking changes like a UiColor enum type with Color and Gradient variants, to enable BorderColor, TextColor, BackgroundColor and ImageNode::color to take either a Color or a gradient.
• Repeating gradients

Testing

Includes three examples that can be used for testing:

cargo run --example linear_gradients
cargo run --example stacked_gradients
cargo run --example radial_gradients

Most of the code except the components API is contained within the bevy_ui/src/render/linear_gradients module.
There are no changes to any existing systems or plugins except for the addition of the gradients rendering systems to the render world schedule and the Val changes from #18164 .

Showcase

Conic gradients can be used to draw simple pie charts like in CSS: