start to a shader??

assets/shaders/bezier.wgsl

@@ -0,0 +1,61 @@
+struct Material {
+    p0: vec2<f32>,
+    c0: vec2<f32>,
+    c1: vec2<f32>,
+    p1: vec2<f32>,
+    width: f32,
+    color: vec4<f32>,
+};
+
+@group(2) @binding(0)
+var<uniform> mat: Material;
+
+fn bezier(p0: vec2<f32>, c0: vec2<f32>, c1: vec2<f32>, p1: vec2<f32>, t: f32) -> vec2<f32> {
+    let u = 1.0 - t;
+    return
+        u*u*u*p0 +
+        3.0*u*u*t*c0 +
+        3.0*u*t*t*c1 +
+        t*t*t*p1;
+}
+
+fn distance_to_bezier(p: vec2<f32>) -> f32 {
+    var min_d = 1e9;
+    let steps = 32;
+
+    var prev = mat.p0;
+    for (var i = 1; i <= steps; i++) {
+        let t = f32(i) / f32(steps);
+        let cur = bezier(mat.p0, mat.c0, mat.c1, mat.p1, t);
+
+        // distance to segment
+        let v = cur - prev;
+        let w = p - prev;
+        let t_seg = clamp(dot(w, v) / dot(v, v), 0.0, 1.0);
+        let proj = prev + t_seg * v;
+
+        min_d = min(min_d, distance(p, proj));
+        prev = cur;
+    }
+
+    return min_d;
+}
+
+@fragment
+fn fragment(
+    @location(0) world_pos: vec3<f32>,
+) -> @location(0) vec4<f32> {
+    let d = distance_to_bezier(world_pos.xy);
+
+    let half_w = mat.width * 0.5;
+    let aa = fwidth(d);
+
+    let alpha = smoothstep(half_w + aa, half_w - aa, d);
+
+    if (alpha <= 0.001) {
+        discard;
+    }
+
+    return vec4(mat.color.rgb, mat.color.a * alpha);
+}
+

src/main.rs

@@ -1,4 +1,7 @@
-use crate::avian::{CharacterControllerBundle, CharacterControllerPlugin};
+use crate::{
+    avian::{CharacterControllerBundle, CharacterControllerPlugin},
+    shaders::BezierMaterial,
+};
 use avian2d::{
     PhysicsPlugins,
     math::Vector,
@@ -6,12 +9,14 @@ use avian2d::{
 };
 use bevy::{
     camera::ScalingMode,
-    color::palettes::css::{GREEN, RED},
+    color::palettes::css::{GREEN, RED, WHITE},
     input_focus::InputFocus,
     prelude::*,
+    sprite_render::Material2dPlugin,
 };
 
 pub mod avian;
+pub mod shaders;
 
 const NORMAL_BUTTON: Color = Color::srgb(0.15, 0.15, 0.15);
 const HOVERED_BUTTON: Color = Color::srgb(0.25, 0.25, 0.25);
@@ -24,12 +29,12 @@ fn main() {
             // Add physics plugins and specify a units-per-meter scaling factor, 1 meter = 20 pixels.
             // The unit allows the engine to tune its parameters for the scale of the world, improving stability.
             PhysicsPlugins::default().with_length_unit(20.0),
+            Material2dPlugin::<BezierMaterial>::default(),
             CharacterControllerPlugin,
         ))
         .init_resource::<InputFocus>()
-        .add_systems(Startup, setup)
-        .add_systems(Update, debug_border)
-        .add_systems(Update, do_menu)
+        .add_systems(Startup, (setup, setup_ui))
+        .add_systems(Update, (debug_border, do_menu, move_bezier))
         .insert_resource(Gravity(Vector::ZERO))
         .run();
 }
@@ -81,8 +86,41 @@ fn do_menu(
     }
 }
 
-fn button(asset_server: &AssetServer) -> impl Bundle {
-    (
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    let mut projection = OrthographicProjection::default_2d();
+    projection.scaling_mode = ScalingMode::AutoMin {
+        min_width: 1920.,
+        min_height: 1080.,
+    };
+    commands.spawn((Camera2d, Projection::Orthographic(projection)));
+
+    // player
+    commands.spawn((
+        Sprite::from_image(asset_server.load("player.png")),
+        Transform::from_xyz(0.0, 0.0, 0.0),
+        CharacterControllerBundle::new(Collider::capsule(15., 27.5)),
+    ));
+
+    // A cube to move around
+    commands.spawn((
+        Sprite {
+            color: Color::srgb(0.0, 0.4, 0.7),
+            custom_size: Some(Vec2::new(30.0, 30.0)),
+            ..default()
+        },
+        Transform::from_xyz(50.0, -100.0, 0.0),
+        RigidBody::Dynamic,
+        Collider::rectangle(30.0, 30.0),
+    ));
+}
+
+fn setup_ui(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<BezierMaterial>>,
+) {
+    commands.spawn((
         Node {
             width: percent(100),
             height: percent(100),
@@ -116,37 +154,47 @@ fn button(asset_server: &AssetServer) -> impl Bundle {
                 TextShadow::default(),
             )]
         )],
-    )
+    ));
+
+    // Curve points (world-space)
+    let p0 = Vec2::new(-200.0, -100.0);
+    let p1 = Vec2::new(200.0, 100.0);
+    let c0 = p0 + Vec2::new(150.0, 0.0);
+    let c1 = p1 + Vec2::new(-150.0, 0.0);
+
+    // Bounding quad (must contain entire curve + width)
+    let center = (p0 + p1) * 0.5;
+    let size = Vec2::new(500.0, 300.0);
+
+    commands.spawn((
+        Mesh2d(meshes.add(Rectangle::new(size.x, size.y))),
+        MeshMaterial2d(materials.add(BezierMaterial {
+            p0,
+            c0,
+            c1,
+            p1,
+            width: 12.0,
+            color: WHITE.into(),
+        })),
+        Transform::from_translation(center.extend(0.0)),
+    ));
 }
 
-fn setup(mut commands: Commands, asset_server: Res<AssetServer>, assets: Res<AssetServer>) {
-    let mut projection = OrthographicProjection::default_2d();
-    projection.scaling_mode = ScalingMode::AutoMin {
-        min_width: 1920.,
-        min_height: 1080.,
+fn move_bezier(mut materials: ResMut<Assets<BezierMaterial>>, windows: Query<&Window>) {
+    let Ok(window) = windows.single() else {
+        return;
+    };
+    let Some(mouse_pos) = window.cursor_position() else {
+        return;
     };
-    commands.spawn((Camera2d, Projection::Orthographic(projection)));
 
-    // player
-    commands.spawn((
-        Sprite::from_image(asset_server.load("player.png")),
-        Transform::from_xyz(0.0, 0.0, 0.0),
-        CharacterControllerBundle::new(Collider::capsule(15., 27.5)),
-    ));
+    for (_, mat) in materials.iter_mut() {
+        mat.p0 = mouse_pos;
+    }
 
-    // A cube to move around
-    commands.spawn((
-        Sprite {
-            color: Color::srgb(0.0, 0.4, 0.7),
-            custom_size: Some(Vec2::new(30.0, 30.0)),
-            ..default()
-        },
-        Transform::from_xyz(50.0, -100.0, 0.0),
-        RigidBody::Dynamic,
-        Collider::rectangle(30.0, 30.0),
-    ));
-
-    commands.spawn(button(&assets));
+    // pos: Vec2, in logical pixels
+    // (0, 0) is bottom-left of the window
+    println!("Mouse window position: {:?}", mouse_pos);
 }
 
 fn debug_border(mut gizmos: Gizmos) {

src/shaders/mod.rs

@@ -0,0 +1,33 @@
+use bevy::prelude::*;
+use bevy::render::render_resource::*;
+use bevy::shader::ShaderRef;
+use bevy::sprite_render::{AlphaMode2d, Material2d};
+
+// This is the struct that will be passed to your shader
+#[derive(Asset, TypePath, AsBindGroup, Debug, Clone)]
+pub struct BezierMaterial {
+    #[uniform(0)]
+    pub p0: Vec2,
+    #[uniform(0)]
+    pub c0: Vec2,
+    #[uniform(0)]
+    pub c1: Vec2,
+    #[uniform(0)]
+    pub p1: Vec2,
+    #[uniform(0)]
+    pub width: f32,
+    #[uniform(0)]
+    pub color: LinearRgba,
+}
+
+/// The Material2d trait is very configurable, but comes with sensible defaults for all methods.
+/// You only need to implement functions for features that need non-default behavior. See the Material2d api docs for details!
+impl Material2d for BezierMaterial {
+    fn fragment_shader() -> ShaderRef {
+        "shaders/bezier.wgsl".into()
+    }
+
+    fn alpha_mode(&self) -> AlphaMode2d {
+        AlphaMode2d::Blend
+    }
+}