Merge branch 'feat/gas-dispersion'

Cargo.lock

@@ -644,7 +644,9 @@ checksum = "c36f4d3af0cda50c07e2010d0351ab79594681116edd280592ca394db73ef32b"
 
 [[package]]
 name = "bevy_rapier2d"
-version = "0.18.0"
+version = "0.19.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5fe53a4fefd2d8bade79f2e5260bd206643bfcc951a3c5f86b3b7a0273eb19e0"
 dependencies = [
  "bevy",
  "bitflags",
@@ -2082,6 +2084,7 @@ dependencies = [
  "bevy-inspector-egui",
  "bevy_prototype_debug_lines",
  "bevy_rapier2d",
+ "fastrand",
  "lazy_static",
  "noise",
 ]

Cargo.toml

@@ -9,7 +9,8 @@ edition = "2021"
 bevy = { version = "0.9.0", features = ["dynamic"] }
 bevy-inspector-egui = "0.14.0"
 bevy_prototype_debug_lines = "0.9.0"
-bevy_rapier2d = { path = "../bevy_rapier/bevy_rapier2d" }
+bevy_rapier2d = "0.19.0"
+fastrand = "1.8.0"
 lazy_static = "1.4.0"
 noise = "0.8.2"
 

src/game.rs

@@ -1,53 +1,46 @@
-use bevy::{input::mouse::MouseWheel, prelude::*};
-use bevy_inspector_egui::*;
-use bevy_prototype_debug_lines::DebugLinesPlugin;
+use bevy::prelude::*;
 use bevy_rapier2d::prelude::*;
 
 use crate::{
-    terrain2d::{Chunk2D, Terrain2D, Terrain2DPlugin, TerrainGen2D},
-    util::Vector2I,
+    terrain2d::*,
+    util::{frame_counter::FrameCounterPlugin, Vector2I},
 };
 
 use self::{
-    camera::{CameraFollow, GameCameraPlugin, WORLD_WIDTH},
-    kinematic::KinematicPlugin,
-    player::PlayerPlugin,
+    camera::GameCameraPlugin, debug::DebugPlugin, kinematic::KinematicPlugin, player::PlayerPlugin,
 };
 
 pub mod camera;
+pub mod debug;
 pub mod kinematic;
 pub mod player;
 
 pub fn init() {
     App::new()
         .add_plugins(DefaultPlugins)
-        .add_plugin(DebugLinesPlugin::default())
+        .add_plugin(FrameCounterPlugin)
         .add_plugin(RapierPhysicsPlugin::<NoUserData>::default())
-        .add_plugin(RapierDebugRenderPlugin::default())
-        .add_plugin(WorldInspectorPlugin::new())
+        .add_plugin(Terrain2DPlugin)
+        .add_plugin(DebugPlugin)
         .add_plugin(KinematicPlugin)
         .add_plugin(GameCameraPlugin)
-        .add_plugin(Terrain2DPlugin)
         .add_plugin(PlayerPlugin)
-        .add_startup_system(setup_debug_terrain)
+        .add_startup_system(setup_terrain)
+        .add_startup_system(setup_window)
         .run();
 }
 
-fn debug_controls(
-    mut query: Query<&mut Transform, With<CameraFollow>>,
-    mut events: EventReader<MouseWheel>,
-) {
-    for event in events.iter() {
-        for mut transform in query.iter_mut() {
-            transform.translation += Vec3::new(0.0, event.y, 0.0) * 30.0;
-        }
+fn setup_window(mut windows: ResMut<Windows>) {
+    if let Some(window) = windows.get_primary_mut() {
+        window.set_resolution(1280.0, 720.0);
+        window.set_title("Kuilu".to_string());
     }
 }
 
-fn setup_debug_terrain(mut commands: Commands, mut terrain: ResMut<Terrain2D>) {
+fn setup_terrain(mut commands: Commands, mut terrain: ResMut<Terrain2D>) {
     let terrain_gen = TerrainGen2D::new(432678);
-    for y in 0..(WORLD_WIDTH / Chunk2D::SIZE_Y as i32) {
-        for x in 0..(WORLD_WIDTH / Chunk2D::SIZE_X as i32) {
+    for y in 0..(Terrain2D::WORLD_HEIGHT / Chunk2D::SIZE_Y as i32) {
+        for x in 0..(Terrain2D::WORLD_WIDTH / Chunk2D::SIZE_X as i32) {
             let position = Vector2I { x, y };
             terrain.add_chunk(position, terrain_gen.gen_chunk(&position));
         }
@@ -64,6 +57,6 @@ fn setup_debug_terrain(mut commands: Commands, mut terrain: ResMut<Terrain2D>) {
         .spawn(Name::new("Right wall"))
         .insert(Collider::halfspace(Vec2::NEG_X).unwrap())
         .insert(TransformBundle::from_transform(
-            Transform::from_translation(Vec3::new(WORLD_WIDTH as f32, 0.0, 0.0)),
+            Transform::from_translation(Vec3::new(Terrain2D::WORLD_WIDTH as f32, 0.0, 0.0)),
         ));
 }

src/game/camera.rs

@@ -4,9 +4,10 @@ use bevy::{
 };
 use bevy_inspector_egui::{Inspectable, RegisterInspectable};
 
-use crate::util::{move_towards_vec3, vec3_lerp};
-
-pub const WORLD_WIDTH: i32 = 512;
+use crate::{
+    terrain2d::Terrain2D,
+    util::{move_towards_vec3, vec3_lerp},
+};
 
 pub struct GameCameraPlugin;
 
@@ -48,7 +49,7 @@ fn camera_setup(mut commands: Commands) {
         Name::new("Camera"),
         Camera2dBundle {
             projection: OrthographicProjection {
-                scaling_mode: ScalingMode::FixedHorizontal(WORLD_WIDTH as f32),
+                scaling_mode: ScalingMode::FixedHorizontal(Terrain2D::WORLD_WIDTH as f32),
                 window_origin: WindowOrigin::Center,
                 scale: 1.0 / 2.0,
                 ..default()
@@ -77,10 +78,9 @@ fn camera_system(
         None => return,
     };
 
-    // let offset = Vec3::new(WORLD_WIDTH as f32 / 2.0, 0.0, 999.9);
     for (mut camera_transform, projection) in camera_query.iter_mut() {
         let left_limit = 0.0;
-        let right_limit = WORLD_WIDTH as f32;
+        let right_limit = Terrain2D::WORLD_WIDTH as f32;
         let offset = Vec3::new(0.0, 0.0, 999.9);
         match follow.movement {
             FollowMovement::Instant => {
@@ -101,6 +101,7 @@ fn camera_system(
                 );
             }
         }
+        // horizontal boundaries
         let camera_x = camera_transform.translation.x;
         camera_transform.translation += Vec3::new(
             (left_limit - (projection.left * projection.scale + camera_x)).max(0.0),

src/game/debug.rs

@@ -0,0 +1,17 @@
+use bevy::prelude::*;
+use bevy_prototype_debug_lines::DebugLinesPlugin;
+
+pub mod terrain;
+
+use terrain::TerrainDebugPlugin;
+
+pub struct DebugPlugin;
+
+impl Plugin for DebugPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_plugin(DebugLinesPlugin::default())
+            // .add_plugin(bevy_rapier2d::prelude::RapierDebugRenderPlugin::default())
+            // .add_plugin(bevy_inspector_egui::WorldInspectorPlugin::new())
+            .add_plugin(TerrainDebugPlugin);
+    }
+}

src/game/debug/terrain.rs

@@ -0,0 +1,220 @@
+use std::collections::HashMap;
+
+use crate::{game::camera::GameCamera, terrain2d::*, util::Vector2I};
+use bevy::{input::mouse::MouseWheel, prelude::*, render::camera::RenderTarget};
+use bevy_prototype_debug_lines::DebugLines;
+
+pub struct TerrainDebugPlugin;
+
+impl Plugin for TerrainDebugPlugin {
+    fn build(&self, app: &mut App) {
+        app.insert_resource(TerrainBrush2D::default())
+            // .add_system_to_stage(TerrainStages::EventHandler, dirty_rect_visualizer)
+            // .add_system_to_stage(CoreStage::Last, chunk_debugger)
+            .add_system(debug_painter);
+    }
+}
+
+#[derive(Resource)]
+struct TerrainBrush2D {
+    pub radius: i32,
+    pub tile: TexelID,
+}
+
+impl Default for TerrainBrush2D {
+    fn default() -> Self {
+        TerrainBrush2D {
+            radius: 40,
+            tile: 8,
+        }
+    }
+}
+
+// REM: Dirty and hopefully temporary
+fn debug_painter(
+    mut terrain: ResMut<Terrain2D>,
+    mut debug_draw: ResMut<DebugLines>,
+    mut brush: ResMut<TerrainBrush2D>,
+    windows: Res<Windows>,
+    mouse_input: Res<Input<MouseButton>>,
+    key_input: Res<Input<KeyCode>>,
+    mut mouse_wheel: EventReader<MouseWheel>,
+    camera_query: Query<(&Camera, &GlobalTransform), With<GameCamera>>,
+) {
+    // let allow_painting = key_input.pressed(KeyCode::LControl);
+    let allow_painting = true;
+
+    // Change brush
+    for event in mouse_wheel.iter() {
+        if allow_painting {
+            brush.radius = (brush.radius + event.y.round() as i32).clamp(1, 128);
+        }
+    }
+
+    if !allow_painting {
+        return;
+    }
+
+    // https://bevy-cheatbook.github.io/cookbook/cursor2world.html#2d-games
+    // get the camera info and transform
+    // assuming there is exactly one main camera entity, so query::single() is OK
+    let (camera, camera_transform) = camera_query.single();
+
+    // get the window that the camera is displaying to (or the primary window)
+    let window = if let RenderTarget::Window(id) = camera.target {
+        windows.get(id).unwrap()
+    } else {
+        windows.get_primary().unwrap()
+    };
+
+    // check if the cursor is inside the window and get its position
+    let world_pos = if let Some(screen_pos) = window.cursor_position() {
+        // get the size of the window
+        let window_size = Vec2::new(window.width() as f32, window.height() as f32);
+
+        // convert screen position [0..resolution] to ndc [-1..1] (gpu coordinates)
+        let ndc = (screen_pos / window_size) * 2.0 - Vec2::ONE;
+
+        // matrix for undoing the projection and camera transform
+        let ndc_to_world = camera_transform.compute_matrix() * camera.projection_matrix().inverse();
+
+        // use it to convert ndc to world-space coordinates
+        let world_pos = ndc_to_world.project_point3(ndc.extend(-1.0));
+
+        // reduce it to a 2D value
+        world_pos.truncate()
+    } else {
+        return;
+    };
+
+    for (index, key) in vec![
+        KeyCode::Key1,
+        KeyCode::Key2,
+        KeyCode::Key3,
+        KeyCode::Key4,
+        KeyCode::Key5,
+        KeyCode::Key6,
+        KeyCode::Key7,
+        KeyCode::Key8,
+        KeyCode::Key9,
+    ]
+    .iter()
+    .enumerate()
+    {
+        if key_input.just_pressed(*key) {
+            brush.tile = index as u8 + 1;
+        }
+    }
+
+    let origin = Vector2I::from(world_pos);
+    let radius = brush.radius;
+    let id = match (
+        mouse_input.pressed(MouseButton::Left),
+        mouse_input.pressed(MouseButton::Right),
+    ) {
+        (true, false) => brush.tile,
+        (_, _) => 0,
+    };
+    let color = TexelBehaviour2D::from_id(&brush.tile)
+        .map_or(Color::rgba(0.0, 0.0, 0.0, 0.0), |tb| tb.color);
+
+    for y in origin.y - (radius - 1)..origin.y + radius {
+        for x in origin.x - (radius - 1)..origin.x + radius {
+            let dx = (x - origin.x).abs();
+            let dy = (y - origin.y).abs();
+            let draw = dx * dx + dy * dy <= (radius - 1) * (radius - 1);
+
+            if draw {
+                let pos: Vector2I = Vector2I { x, y };
+                debug_draw.line_colored(
+                    Vec3::from(pos) + Vec3::new(0.45, 0.45, 1.0),
+                    Vec3::from(pos) + Vec3::new(0.55, 0.55, 1.0),
+                    0.0,
+                    color,
+                );
+                if mouse_input.pressed(MouseButton::Left) || mouse_input.pressed(MouseButton::Right)
+                {
+                    terrain.set_texel(&pos, Texel2D { id, ..default() }, None)
+                }
+            }
+        }
+    }
+}
+
+/**
+    Visualize dirty rects
+*/
+fn dirty_rect_visualizer(terrain: Res<Terrain2D>, mut debug_draw: ResMut<DebugLines>) {
+    for (chunk_index, chunk) in terrain.chunk_iter() {
+        let rect = if let Some(rect) = chunk.dirty_rect {
+            rect
+        } else {
+            continue;
+        };
+
+        let offset = Vec3::from(chunk_index_to_global(chunk_index));
+        let min = offset + Vec3::from(rect.min);
+        let max = offset + Vec3::from(rect.max + Vector2I::ONE);
+        draw_box(&mut debug_draw, min, max, Color::RED, 0.0);
+    }
+}
+
+fn chunk_debugger(terrain: Res<Terrain2D>, mut debug_draw: ResMut<DebugLines>) {
+    for (chunk_index, chunk) in terrain.chunk_iter() {
+        println!("chunk contents: {chunk_index:?}");
+        let offset = Vec3::from(chunk_index_to_global(chunk_index));
+        let min = offset + Vec3::ZERO;
+        let max = offset + Vec3::from(Chunk2D::SIZE);
+        draw_box(
+            &mut debug_draw,
+            min,
+            max,
+            Color::rgba(0.5, 0.0, 0.5, 0.5),
+            0.0,
+        );
+
+        let mut tile_counter: HashMap<TexelID, (u32, u32)> = HashMap::new();
+        for y in 0..Chunk2D::SIZE_Y as i32 {
+            for x in 0..Chunk2D::SIZE_X as i32 {
+                let local = Vector2I::new(x, y);
+                let global = local_to_global(&local, chunk_index);
+                if let (Some(texel), _) = terrain.get_texel_behaviour(&global) {
+                    if !tile_counter.contains_key(&texel.id) {
+                        tile_counter.insert(texel.id, (0, 0));
+                    }
+                    let (old_count, old_density) = tile_counter[&texel.id].clone();
+                    tile_counter.insert(
+                        texel.id,
+                        (old_count + 1, old_density + texel.density as u32),
+                    );
+                }
+            }
+        }
+
+        let mut counts: Vec<(u8, String, u32, u32)> = vec![];
+
+        for (id, (count, total_density)) in tile_counter.iter() {
+            let name =
+                TexelBehaviour2D::from_id(id).map_or("unknown".to_string(), |b| b.name.to_string());
+            counts.push((*id, name, *count, *total_density));
+        }
+        counts.sort_unstable_by_key(|c| c.0);
+        for (id, name, count, total_density) in counts {
+            println!(
+                "\tmaterial: {name:<24}id: {id:<8}count: {count:<8}total_density: {total_density:<8}"
+            );
+        }
+    }
+}
+
+pub fn draw_box(debug_draw: &mut DebugLines, min: Vec3, max: Vec3, color: Color, duration: f32) {
+    let points = vec![
+        Vec3::new(min.x, min.y, min.z),
+        Vec3::new(max.x, min.y, min.z),
+        Vec3::new(max.x, max.y, min.z),
+        Vec3::new(min.x, max.y, min.z),
+    ];
+    for i in 0..points.len() {
+        debug_draw.line_colored(points[i], points[(i + 1) % points.len()], duration, color);
+    }
+}

src/game/kinematic.rs

@@ -18,31 +18,23 @@ impl Plugin for KinematicPlugin {
 
 #[derive(Bundle)]
 pub struct KinematicBundle {
-    pub kinematic: KinematicState,
+    pub state: KinematicState,
+    pub properties: KinematicProperties,
     pub rigidbody: RigidBody,
-    pub velocity: Velocity,
-    pub gravity_scale: GravityScale,
-    pub collider: Collider,
-    pub locked_axes: LockedAxes,
     pub events: ActiveEvents,
     pub collisions: ActiveCollisionTypes,
-    pub properties: KinematicProperties,
     pub transform: TransformBundle,
 }
 
 impl Default for KinematicBundle {
     fn default() -> Self {
         KinematicBundle {
-            kinematic: KinematicState::default(),
-            rigidbody: RigidBody::Dynamic,
-            gravity_scale: GravityScale(3.0),
-            locked_axes: LockedAxes::ROTATION_LOCKED,
+            state: KinematicState::default(),
+            properties: KinematicProperties::default(),
+            rigidbody: RigidBody::KinematicPositionBased,
             events: ActiveEvents::COLLISION_EVENTS,
             collisions: ActiveCollisionTypes::all(),
-            collider: Collider::default(),
-            properties: KinematicProperties::default(),
             transform: TransformBundle::default(),
-            velocity: Velocity::default(),
         }
     }
 }
@@ -50,27 +42,14 @@ impl Default for KinematicBundle {
 #[derive(Component, Reflect, Default)]
 #[reflect(Component)]
 pub struct KinematicState {
-    on_ground: bool,
-    did_jump: bool,
-    air_jump_counter: u8,
+    #[reflect(ignore)]
+    pub last_move: Option<MoveShapeOutput>,
+    pub did_jump: bool,
 }
 
 impl KinematicState {
-    #[inline]
-    pub fn on_ground(&self) -> bool {
-        self.on_ground
-    }
-    #[inline]
-    pub fn did_jump(&self) -> bool {
-        self.did_jump
-    }
-    #[inline]
-    pub fn air_jump_counter(&self) -> u8 {
-        self.air_jump_counter
-    }
-
     pub fn can_jump(&self) -> bool {
-        self.on_ground && !self.did_jump
+        self.last_move.as_ref().map_or(false, |last| last.grounded) && !self.did_jump
     }
 }
 
@@ -84,20 +63,20 @@ pub struct KinematicProperties {
     pub air_acceleration: f32,
     pub air_friction: f32,
     pub jump_height: f32,
-    pub air_jumps: u8,
+    pub gravity: Option<f32>,
 }
 
 impl Default for KinematicProperties {
     fn default() -> Self {
         Self {
-            ground_speed: 100.0,
+            ground_speed: 75.0,
             ground_acceleration: 20.0,
             ground_friction: 30.0,
-            air_speed: 100.0,
+            air_speed: 75.0,
             air_acceleration: 10.0,
             air_friction: 10.0,
-            jump_height: 150.0,
-            air_jumps: 1,
+            jump_height: 100.0,
+            gravity: Some(1.0),
         }
     }
 }
@@ -110,40 +89,38 @@ pub struct KinematicInput {
 }
 
 fn kinematic_movement(
-    time: Res<Time>,
     mut query: Query<(
         Entity,
-        &mut Velocity,
         &mut KinematicState,
+        &mut Transform,
         &KinematicProperties,
         &GlobalTransform,
         Option<&KinematicInput>,
-        Option<&GravityScale>,
-        Option<&Collider>,
+        Option<&CollisionGroups>,
     )>,
-    rapier_context: Res<RapierContext>,
+    shape_query: Query<&Collider, Without<Sensor>>,
+    child_query: Query<&Children>,
+    mut rapier_context: ResMut<RapierContext>,
 ) {
+    let dt = rapier_context.integration_parameters.dt;
     for (
         entity,
-        mut velocity,
         mut kinematic_state,
+        mut transform,
         props,
         global_transform,
         input,
-        gravity,
-        collider,
+        collision_groups,
     ) in query.iter_mut()
     {
         let default = &KinematicInput::default();
         let input = input.unwrap_or(default);
 
-        let has_gravity = if let Some(gravity) = gravity {
-            gravity.0.abs() > f32::EPSILON
-        } else {
-            false
-        };
-
-        let (speed, acceleration, friction) = if kinematic_state.on_ground {
+        let (speed, acceleration, friction) = if kinematic_state
+            .last_move
+            .as_ref()
+            .map_or(false, |last| last.grounded)
+        {
             (
                 props.ground_speed,
                 props.ground_acceleration,
@@ -154,10 +131,21 @@ fn kinematic_movement(
         };
 
         const GRAVITY_DIR: Vec2 = Vec2::NEG_Y;
+        const GRAVITY_COEFFICIENT: f32 = 2.0;
 
-        let current_velocity = velocity.linvel;
-        let target_velocity =
-            input.movement * speed + current_velocity.project_onto_normalized(GRAVITY_DIR);
+        let current_velocity = kinematic_state
+            .last_move
+            .as_ref()
+            .map_or(Vec2::ZERO, |last| {
+                if last.grounded {
+                    last.effective_translation
+                        .reject_from_normalized(GRAVITY_DIR)
+                } else {
+                    last.effective_translation
+                }
+            })
+            / dt;
+        let target_velocity = input.movement * speed;
 
         let angle_lerp = if current_velocity.length_squared() > 0.01 {
             let result = inverse_lerp(
@@ -178,42 +166,104 @@ fn kinematic_movement(
         let delta_interpolation = angle_lerp.clamp(0.0, 1.0);
         let velocity_change_speed = lerp(acceleration, friction, delta_interpolation) * speed;
 
-        velocity.linvel = move_towards_vec2(
-            current_velocity,
-            target_velocity,
-            velocity_change_speed * time.delta_seconds(),
-        );
+        let mut velocity = if let Some(gravity) = props.gravity {
+            // Also apply gravity
+            move_towards_vec2(
+                current_velocity,
+                target_velocity.reject_from_normalized(GRAVITY_DIR)
+                    + current_velocity.project_onto_normalized(GRAVITY_DIR),
+                velocity_change_speed * dt,
+            ) + GRAVITY_DIR * GRAVITY_COEFFICIENT * gravity
+        } else {
+            move_towards_vec2(
+                current_velocity,
+                target_velocity,
+                velocity_change_speed * dt,
+            )
+        };
 
         if input.want_jump && kinematic_state.can_jump() {
-            velocity.linvel = Vec2 {
+            velocity = Vec2 {
                 y: props.jump_height,
-                ..velocity.linvel
+                ..velocity
             };
             kinematic_state.did_jump = true;
         }
 
-        if has_gravity {
-            // Reset any possible jump snapping and stuff after the peak of jump
-            if velocity.linvel.y <= 0.0 {
-                kinematic_state.did_jump = false;
+        let shape = if let Ok(shape) = shape_query.get(entity) {
+            Some(shape)
+        } else if let Ok(children) = child_query.get(entity) {
+            children
+                .iter()
+                .find_map(|child| shape_query.get(*child).ok())
+        } else {
+            None
+        };
+
+        // move
+        kinematic_state.last_move = if let Some(shape) = shape {
+            let (_scale, rotation, translation) = global_transform.to_scale_rotation_translation();
+
+            let move_options = &MoveShapeOptions {
+                up: Vec2::Y,
+                autostep: Some(CharacterAutostep {
+                    min_width: CharacterLength::Absolute(0.5),
+                    max_height: CharacterLength::Absolute(2.1),
+                    include_dynamic_bodies: false,
+                }),
+                slide: true,
+                max_slope_climb_angle: (50.0_f32).to_radians(),
+                min_slope_slide_angle: (50.0_f32).to_radians(),
+                snap_to_ground: Some(CharacterLength::Absolute(5.0)),
+                // snap_to_ground: props.gravity.map_or(None, |_| {
+                //     if velocity.y <= 0.0 {
+                //         Some(CharacterLength::Absolute(5.0))
+                //     } else {
+                //         None
+                //     }
+                // }),
+                offset: CharacterLength::Absolute(0.01),
+                ..MoveShapeOptions::default()
+            };
+
+            let mut filter = QueryFilter::new();
+            let predicate = |coll_entity| coll_entity != entity;
+            filter.predicate = Some(&predicate);
+
+            if let Some(collision_groups) = collision_groups {
+                filter.groups(InteractionGroups::new(
+                    bevy_rapier2d::rapier::geometry::Group::from_bits_truncate(
+                        collision_groups.memberships.bits(),
+                    ),
+                    bevy_rapier2d::rapier::geometry::Group::from_bits_truncate(
+                        collision_groups.filters.bits(),
+                    ),
+                ));
             }
-            if let Some(collider) = collider {
-                let (_, rot, pos) = global_transform.to_scale_rotation_translation();
-                let angle = rot.to_euler(EulerRot::YXZ).2;
-                let mut shape = collider.clone();
-                shape.set_scale(Vec2::ONE * 0.9, 1);
-                if let Some((_coll_entity, _hit)) = rapier_context.cast_shape(
-                    Vec2::new(pos.x, pos.y),
-                    angle,
-                    Vec2::NEG_Y,
-                    &shape,
-                    2.0,
-                    QueryFilter::new().exclude_collider(entity),
-                ) {
-                    kinematic_state.on_ground = true;
-                } else {
-                    kinematic_state.on_ground = false;
-                }
+
+            let last_move: MoveShapeOutput = rapier_context.move_shape(
+                velocity * dt,
+                shape,
+                translation.truncate(),
+                rotation.to_euler(EulerRot::ZYX).0,
+                shape.raw.0.mass_properties(1.0).mass(),
+                move_options,
+                filter,
+                |_coll: CharacterCollision| (),
+            );
+
+            // Apply movement
+            transform.translation += last_move.effective_translation.extend(0.0);
+
+            Some(last_move)
+        } else {
+            None
+        };
+
+        if props.gravity.is_some() {
+            // Reset any possible jump snapping and stuff after the peak of jump
+            if velocity.y <= 0.0 {
+                kinematic_state.did_jump = false;
             }
         }
     }

src/game/player.rs

@@ -38,8 +38,9 @@ pub fn player_system(
 
     let movement = Vec2 {
         x: input_to_axis(input.pressed(KeyCode::A), input.pressed(KeyCode::D)),
-        // y: input_to_axis(input.pressed(KeyCode::S), input.pressed(KeyCode::W)),
-        y: 0.0,
+        // x: -1.0,
+        y: input_to_axis(input.pressed(KeyCode::S), input.pressed(KeyCode::W)),
+        // y: 0.0,
     };
 
     kinematic_input.movement = movement;
@@ -59,10 +60,13 @@ fn input_to_axis(negative: bool, positive: bool) -> f32 {
 
 pub fn player_spawn(mut commands: Commands) {
     let kinematic = KinematicBundle {
-        collider: Collider::round_cuboid(4.0, 8.0, 1.0),
         transform: TransformBundle::from_transform(Transform::from_translation(Vec3::new(
             256.0, 128.0, 0.0,
         ))),
+        properties: KinematicProperties {
+            gravity: None,
+            ..default()
+        },
         ..default()
     };
 
@@ -72,18 +76,21 @@ pub fn player_spawn(mut commands: Commands) {
         .insert(SpriteBundle {
             sprite: Sprite {
                 color: Color::rgb(0.75, 0.25, 0.25),
-                custom_size: Some(Vec2 { x: 8.0, y: 16.0 }),
+                custom_size: Some(Vec2 { x: 6.0, y: 12.0 }),
                 ..default()
             },
             ..default()
         })
+        .insert(Collider::cuboid(3.0, 6.0))
         .insert(PlayerBundle {
             kinematic,
             ..default()
         })
         .insert(KinematicInput::default())
+        .insert(Ccd::enabled())
+        .insert(Sleeping::disabled())
         .insert(CameraFollow {
             priority: 1,
-            movement: FollowMovement::Smooth(18.0),
+            movement: FollowMovement::Instant,
         });
 }

src/terrain2d.rs

@@ -3,196 +3,349 @@ use std::collections::{
     HashMap,
 };
 
-use bevy::{input::mouse::MouseWheel, prelude::*, render::camera::RenderTarget};
-use bevy_prototype_debug_lines::DebugLines;
+use bevy::ecs::prelude::SystemStage;
+use bevy::prelude::*;
 use bevy_rapier2d::prelude::*;
 
 mod chunk2d;
 mod terrain_gen2d;
 mod texel2d;
+mod texel_behaviour2d;
 
 pub use chunk2d::*;
 pub use terrain_gen2d::*;
 pub use texel2d::*;
+pub use texel_behaviour2d::*;
 
-use crate::{
-    game::camera::GameCamera,
-    util::{math::*, Vector2I},
-};
+use crate::util::{frame_counter::FrameCounter, math::*, Vector2I};
 
 pub struct Terrain2DPlugin;
 
 impl Plugin for Terrain2DPlugin {
     fn build(&self, app: &mut App) {
+        // Add terrain stages
+        app.add_stage_before(
+            CoreStage::Update,
+            TerrainStages::Simulation,
+            SystemStage::parallel(),
+        );
+        // After update, but before rapier
+        app.add_stage_before(
+            PhysicsStages::SyncBackend,
+            TerrainStages::EventHandler,
+            SystemStage::parallel(),
+        )
+        .add_stage_after(
+            TerrainStages::EventHandler,
+            TerrainStages::ChunkSync,
+            SystemStage::parallel(),
+        );
+
         app.register_type::<TerrainChunk2D>()
-            .insert_resource(Terrain2D::new())
-            .insert_resource(TerrainBrush2D::default())
+            .insert_resource(Terrain2D::new(
+                Some(Terrain2D::WORLD_HEIGHT),
+                Some(0),
+                Some(0),
+                Some(Terrain2D::WORLD_WIDTH),
+            ))
             .add_event::<TerrainEvent2D>()
-            .add_system(debug_painter)
+            .add_system_to_stage(TerrainStages::Simulation, terrain_simulation)
+            .add_system_to_stage(TerrainStages::EventHandler, emit_terrain_events)
             .add_system_to_stage(
-                CoreStage::PostUpdate,
-                dirty_rect_visualizer.before(emit_terrain_events),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
+                TerrainStages::EventHandler,
+                // TODO: Figure out why .after() creates a lagspike for the first frame
                 chunk_spawner.before(emit_terrain_events),
             )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                chunk_sprite_sync.after(chunk_spawner),
-            )
-            .add_system_to_stage(
-                CoreStage::PostUpdate,
-                chunk_collision_sync.after(chunk_spawner),
-            )
-            .add_system_to_stage(CoreStage::PostUpdate, emit_terrain_events);
+            .add_system_to_stage(TerrainStages::ChunkSync, chunk_sprite_sync)
+            .add_system_to_stage(CoreStage::PostUpdate, chunk_collision_sync);
     }
 }
 
-#[derive(Resource)]
-struct TerrainBrush2D {
-    pub radius: i32,
-    pub tile: TexelID,
+#[derive(StageLabel)]
+pub enum TerrainStages {
+    /// Terrain simulation stage. Should run before update.
+    Simulation,
+    /// The stage that Handles collected events and creates new chunk entities as needed. Should run after update.
+    EventHandler,
+    /// Chunk sync systems (e.g. collsion and sprite) run in this stage.
+    ChunkSync,
 }
 
-impl Default for TerrainBrush2D {
-    fn default() -> Self {
-        TerrainBrush2D { radius: 7, tile: 3 }
-    }
-}
-
-// REM: Dirty and hopefully temporary
-fn debug_painter(
+fn terrain_simulation(
     mut terrain: ResMut<Terrain2D>,
-    mut debug_draw: ResMut<DebugLines>,
-    mut brush: ResMut<TerrainBrush2D>,
-    windows: Res<Windows>,
-    mouse_input: Res<Input<MouseButton>>,
-    key_input: Res<Input<KeyCode>>,
-    mut mouse_wheel: EventReader<MouseWheel>,
-    camera_query: Query<(&Camera, &GlobalTransform), With<GameCamera>>,
+    frame_counter: Res<FrameCounter>,
+    mut debug_draw: ResMut<bevy_prototype_debug_lines::DebugLines>,
 ) {
-    let allow_painting = key_input.pressed(KeyCode::LControl);
+    let simulation_frame = (frame_counter.frame % u8::MAX as u64) as u8 + 1;
 
-    // Change brush
-    for event in mouse_wheel.iter() {
-        if allow_painting {
-            brush.radius = (brush.radius + event.y.round() as i32).clamp(1, 128);
-        }
-    }
+    let indices = terrain
+        .chunk_iter()
+        .map(|(chunk_index, _)| *chunk_index)
+        .collect::<Vec<Chunk2DIndex>>()
+        .clone();
 
-    if !allow_painting {
-        return;
-    }
+    for chunk_index in indices.iter() {
+        // Mark few chunks dirty in interval. Should help activate stale chunks
+        if let Some(chunk) = terrain.index_to_chunk_mut(&chunk_index) {
+            let interval = 1;
+            if frame_counter.frame % interval == 0 {
+                let i = ((frame_counter.frame / interval) % 100) as i32;
+                if (chunk_index.y % 10) * 10 + (chunk_index.x % 10) == i {
+                    chunk.mark_all_dirty();
+                }
+            }
+        };
 
-    // https://bevy-cheatbook.github.io/cookbook/cursor2world.html#2d-games
-    // get the camera info and transform
-    // assuming there is exactly one main camera entity, so query::single() is OK
-    let (camera, camera_transform) = camera_query.single();
+        if let Some(rect) = &terrain
+            .index_to_chunk(&chunk_index)
+            .map_or(None, |chunk| chunk.dirty_rect.clone())
+        {
+            if let Some(chunk) = terrain.index_to_chunk_mut(&chunk_index) {
+                chunk.mark_clean();
+            } else {
+                continue;
+            };
 
-    // get the window that the camera is displaying to (or the primary window)
-    let window = if let RenderTarget::Window(id) = camera.target {
-        windows.get(id).unwrap()
-    } else {
-        windows.get_primary().unwrap()
-    };
+            // Texel simulation
+            let mut y_range: Vec<_> = (rect.min.y..rect.max.y + 1).collect();
+            let mut x_range: Vec<_> = (rect.min.x..rect.max.x + 1).collect();
+            if frame_counter.frame % 2 == 0 {
+                y_range.reverse();
+            }
+            if frame_counter.frame / 2 % 2 == 0 {
+                x_range.reverse();
+            }
 
-    // check if the cursor is inside the window and get its position
-    let world_pos = if let Some(screen_pos) = window.cursor_position() {
-        // get the size of the window
-        let window_size = Vec2::new(window.width() as f32, window.height() as f32);
+            for y in y_range.iter() {
+                for x in x_range.iter() {
+                    let local = Vector2I::new(*x, *y);
+                    let global = local_to_global(&local, &chunk_index);
 
-        // convert screen position [0..resolution] to ndc [-1..1] (gpu coordinates)
-        let ndc = (screen_pos / window_size) * 2.0 - Vec2::ONE;
+                    if terrain
+                        .get_latest_simulation(&global)
+                        .map_or(true, |frame| frame == simulation_frame)
+                    {
+                        continue;
+                    };
 
-        // matrix for undoing the projection and camera transform
-        let ndc_to_world = camera_transform.compute_matrix() * camera.projection_matrix().inverse();
+                    simulate_texel(global, &mut terrain, &frame_counter);
+                }
+            }
 
-        // use it to convert ndc to world-space coordinates
-        let world_pos = ndc_to_world.project_point3(ndc.extend(-1.0));
+            // Gas dispersion
+            let alternate_dispersion = frame_counter.frame % 2 == 0;
+            let alternate = if alternate_dispersion { 1 } else { 0 };
+            let y_range =
+                ((rect.min.y - alternate)..rect.max.y + 1 + alternate).collect::<Vec<_>>();
+            let x_range =
+                ((rect.min.x - alternate)..rect.max.x + 1 + alternate).collect::<Vec<_>>();
+            const DISPERSION_WIDTH: usize = 2;
+            const DISPERSION_HEIGHT: usize = 2;
+            for y_arr in y_range.chunks(DISPERSION_HEIGHT) {
+                for x_arr in x_range.chunks(DISPERSION_WIDTH) {
+                    let mut global_positions = vec![];
+                    for y in y_arr.iter() {
+                        for x in x_arr.iter() {
+                            let local = Vector2I::new(*x, *y);
+                            let global = local_to_global(&local, &chunk_index);
+                            global_positions.push(global);
+                        }
+                    }
 
-        // reduce it to a 2D value
-        world_pos.truncate()
-    } else {
-        return;
-    };
-
-    if key_input.just_pressed(KeyCode::Key1) {
-        brush.tile = 1;
-    }
-    if key_input.just_pressed(KeyCode::Key2) {
-        brush.tile = 2;
-    }
-    if key_input.just_pressed(KeyCode::Key3) {
-        brush.tile = 3;
-    }
-
-    let colors = vec![
-        Color::rgba(1.0, 0.25, 0.25, 1.0),
-        Color::rgba(0.25, 1.0, 0.25, 1.0),
-        Color::rgba(0.25, 0.25, 1.0, 1.0),
-    ];
-
-    let origin = Vector2I::from(world_pos);
-    let radius = brush.radius;
-    let color = colors[brush.tile as usize % colors.len()];
-    let id = match (
-        mouse_input.pressed(MouseButton::Left),
-        mouse_input.pressed(MouseButton::Right),
-    ) {
-        (true, false) => brush.tile,
-        (_, _) => 0,
-    };
-
-    for y in origin.y - (radius - 1)..origin.y + radius {
-        for x in origin.x - (radius - 1)..origin.x + radius {
-            let dx = (x - origin.x).abs();
-            let dy = (y - origin.y).abs();
-
-            if dx * dx + dy * dy <= (radius - 1) * (radius - 1) {
-                let pos: Vector2I = Vector2I { x, y };
-                debug_draw.line_colored(
-                    Vec3::from(pos) + Vec3::new(0.45, 0.45, 0.0),
-                    Vec3::from(pos) + Vec3::new(0.55, 0.55, 0.0),
-                    0.0,
-                    color,
-                );
-                if mouse_input.pressed(MouseButton::Left) || mouse_input.pressed(MouseButton::Right)
-                {
-                    terrain.set_texel(&pos, id)
+                    // Distribute gas
+                    disperse_gas(
+                        global_positions,
+                        &mut terrain,
+                        &frame_counter,
+                        &mut debug_draw,
+                    )
                 }
             }
         }
     }
 }
 
-/**
-    Visualize dirty rects
-*/
-fn dirty_rect_visualizer(terrain: Res<Terrain2D>, mut debug_draw: ResMut<DebugLines>) {
-    for (chunk_index, chunk) in terrain.chunk_iter() {
-        let rect = if let Some(rect) = chunk.dirty_rect {
-            rect
+// TODO: Don't update if the result of dispersion is similar to before
+fn disperse_gas(
+    global_positions: Vec<Vector2I>,
+    terrain: &mut Terrain2D,
+    frame_counter: &FrameCounter,
+    debug_draw: &mut bevy_prototype_debug_lines::DebugLines,
+) {
+    use u32 as Capacity;
+    use u8 as Min;
+    use u8 as Max;
+    let mut total_densities: HashMap<TexelID, (Capacity, Min, Max)> = HashMap::new();
+    let mut valid_globals = vec![];
+    for global in global_positions.iter() {
+        let (texel, behaviour) = terrain.get_texel_behaviour(global);
+        if behaviour.clone().map_or(true, |b| b.form == TexelForm::Gas) {
+            valid_globals.push(*global);
+        }
+        match (texel, behaviour) {
+            (Some(texel), Some(behaviour)) => {
+                if behaviour.form == TexelForm::Gas {
+                    if let Some((old_density, old_min, old_max)) = total_densities.get(&texel.id) {
+                        total_densities.insert(
+                            texel.id,
+                            (
+                                texel.density as u32 + *old_density,
+                                texel.density.min(*old_min),
+                                texel.density.max(*old_max),
+                            ),
+                        );
+                    } else {
+                        total_densities.insert(
+                            texel.id,
+                            (texel.density as u32, texel.density, texel.density),
+                        );
+                    }
+                }
+            }
+            (_, _) => (),
+        }
+    }
+
+    let mut total_densities: Vec<(TexelID, Capacity, Min, Max)> = total_densities
+        .iter()
+        .map(|(t, (d, min, max))| (*t, *d, *min, *max))
+        .collect();
+
+    if total_densities.len() == 0 {
+        return;
+    }
+
+    total_densities.sort_unstable_by_key(|(_, density, _, _)| *density);
+    total_densities.reverse();
+
+    const TILE_CAPACITY: u32 = u8::MAX as u32;
+    let free_slots = valid_globals.len() as u32
+        - total_densities
+            .iter()
+            .map(|(_, v, _, _)| (*v / (TILE_CAPACITY + 1)) + 1)
+            .sum::<u32>();
+
+    // Stop if the gas is already close to a stable state
+    const STABLE_TRESHOLD: u8 = 3;
+    if total_densities.iter().all(|(_, _, min, max)| {
+        if u8::abs_diff(*min, *max) > STABLE_TRESHOLD {
+            return false;
+        }
+        free_slots > 0 && *max <= STABLE_TRESHOLD
+    }) {
+        // // DEBUG: draw box for stabilized area
+        // let mut min = valid_globals.first().unwrap().clone();
+        // let mut max = valid_globals.first().unwrap().clone();
+        // for global in valid_globals.iter() {
+        //     min = Vector2I::min(&min, global);
+        //     max = Vector2I::max(&max, global);
+        // }
+        // max = max + Vector2I::ONE;
+        // crate::game::debug::terrain::draw_box(
+        //     debug_draw,
+        //     Vec3::from(min),
+        //     Vec3::from(max),
+        //     Color::CYAN,
+        //     0.0,
+        // );
+        return;
+    }
+
+    // Allocate slots
+    let mut slots: Vec<(TexelID, u32)> = vec![];
+    for (id, density, _, _) in total_densities.iter() {
+        let min_slots = (density / (TILE_CAPACITY + 1)) + 1;
+        slots.push((*id, min_slots));
+    }
+    for i in 0..free_slots as usize {
+        let len = slots.len();
+        slots[i % len].1 += 1;
+    }
+
+    // Disperse into given slots
+    let mut texels: Vec<Texel2D> = vec![];
+    for (id, total_density, _, _) in total_densities.iter() {
+        let slots = slots.iter().find(|s| s.0 == *id).unwrap().1;
+        let mut density_left = *total_density;
+        for i in 0..slots {
+            let density = if i < (slots - 1) {
+                (total_density / slots).min(density_left)
+            } else {
+                density_left
+            }
+            .min(u8::MAX as u32);
+            if density > 0 {
+                texels.push(Texel2D {
+                    id: *id,
+                    density: density as u8,
+                });
+                density_left -= density;
+            }
+        }
+    }
+
+    // Apply changes
+    if texels.len() > valid_globals.len() {
+        panic!("disperse_gas() - valid_globals is shorter than texels");
+    }
+
+    fastrand::shuffle(&mut valid_globals);
+    for i in 0..valid_globals.len() {
+        let global = valid_globals[i];
+        if i < texels.len() {
+            let texel = texels[i];
+            terrain.set_texel(&global, texel, None);
         } else {
-            continue;
-        };
+            terrain.set_texel(&global, Texel2D::default(), None)
+        }
+    }
+}
 
-        let color = Color::RED;
+fn simulate_texel(global: Vector2I, terrain: &mut Terrain2D, frame_counter: &FrameCounter) {
+    let (_, behaviour) = match terrain.get_texel_behaviour(&global) {
+        (Some(texel), Some(behaviour)) => (texel, behaviour),
+        (_, _) => return,
+    };
 
-        let points = vec![
-            Vec3::new(rect.min.x as f32, rect.min.y as f32, 0.0),
-            Vec3::new((rect.max.x + 1) as f32, rect.min.y as f32, 0.0),
-            Vec3::new((rect.max.x + 1) as f32, (rect.max.y + 1) as f32, 0.0),
-            Vec3::new(rect.min.x as f32, (rect.max.y + 1) as f32, 0.0),
-        ];
-        for i in 0..points.len() {
-            let offset = Vec3::from(chunk_index_to_global(chunk_index));
-            debug_draw.line_colored(
-                offset + points[i],
-                offset + points[(i + 1) % points.len()],
-                0.0,
-                color,
-            );
+    let simulation_frame = (frame_counter.frame % u8::MAX as u64) as u8 + 1;
+
+    // Gravity
+    if let Some(gravity) = behaviour.gravity {
+        let grav_offset = Vector2I::from(gravity);
+        let grav_pos = global + grav_offset;
+
+        if behaviour.form != TexelForm::Gas || gravity.abs() > fastrand::u8(0..u8::MAX) {
+            // Try falling
+            {
+                let (_, other_behaviour) = terrain.get_texel_behaviour(&grav_pos);
+                if TexelBehaviour2D::can_displace(&behaviour, &other_behaviour) {
+                    terrain.swap_texels(&global, &grav_pos, Some(simulation_frame));
+                    return;
+                }
+                if terrain.can_transfer_density(&global, &grav_pos) {
+                    terrain.transfer_density(&global, &grav_pos, gravity, Some(simulation_frame))
+                }
+            }
+
+            // Try "sliding"
+            let mut dirs = vec![Vector2I::RIGHT, Vector2I::LEFT];
+            if ((frame_counter.frame / 73) % 2) as i32 == global.y % 2 {
+                dirs.reverse();
+            }
+            for dir in dirs.iter() {
+                let slide_pos = match behaviour.form {
+                    TexelForm::Solid => grav_pos + *dir,
+                    TexelForm::Liquid | TexelForm::Gas => global + *dir,
+                };
+                let (_, other_behaviour) = terrain.get_texel_behaviour(&slide_pos);
+                if TexelBehaviour2D::can_displace(&behaviour, &other_behaviour) {
+                    terrain.swap_texels(&global, &slide_pos, Some(simulation_frame));
+                    return;
+                }
+                if terrain.can_transfer_density(&global, &grav_pos) {
+                    terrain.transfer_density(&global, &grav_pos, gravity, Some(simulation_frame))
+                }
+            }
         }
     }
 }
@@ -207,7 +360,6 @@ fn emit_terrain_events(
     for (chunk_index, chunk) in terrain.chunk_iter_mut() {
         if let Some(rect) = &chunk.dirty_rect {
             terrain_events.send(TerrainEvent2D::TexelsUpdated(*chunk_index, *rect));
-            chunk.mark_clean();
         }
     }
 }
@@ -222,13 +374,29 @@ pub enum TerrainEvent2D {
 pub struct Terrain2D {
     chunk_map: HashMap<Chunk2DIndex, Chunk2D>,
     events: Vec<TerrainEvent2D>,
+    pub top_boundary: Option<i32>,
+    pub bottom_boundary: Option<i32>,
+    pub left_boundary: Option<i32>,
+    pub right_boundary: Option<i32>,
 }
 
 impl Terrain2D {
-    pub fn new() -> Terrain2D {
-        Terrain2D {
+    pub const WORLD_WIDTH: i32 = 512;
+    pub const WORLD_HEIGHT: i32 = Self::WORLD_WIDTH * 2;
+
+    pub fn new(
+        top_boundary: Option<i32>,
+        bottom_boundary: Option<i32>,
+        left_boundary: Option<i32>,
+        right_boundary: Option<i32>,
+    ) -> Self {
+        Self {
             chunk_map: HashMap::new(),
             events: Vec::new(),
+            top_boundary,
+            bottom_boundary,
+            left_boundary,
+            right_boundary,
         }
     }
 
@@ -280,16 +448,161 @@ impl Terrain2D {
         }
     }
 
-    pub fn set_texel(&mut self, global: &Vector2I, id: TexelID) {
+    pub fn mark_dirty(&mut self, global: &Vector2I) {
         let index = global_to_chunk_index(global);
-        match self.index_to_chunk_mut(&index) {
-            Some(chunk) => chunk.set_texel(&global_to_local(global), id),
-            None => {
-                let mut chunk = Chunk2D::new();
-                chunk.set_texel(&global_to_local(global), id);
-                self.add_chunk(index, chunk);
+        if let Some(chunk) = self.index_to_chunk_mut(&index) {
+            chunk.mark_dirty(&global_to_local(global));
+        }
+    }
+
+    pub fn is_within_boundaries(&self, global: &Vector2I) -> bool {
+        if let Some(top) = self.top_boundary {
+            if global.y >= top {
+                return false;
             }
         }
+        if let Some(bottom) = self.bottom_boundary {
+            if global.y < bottom {
+                return false;
+            }
+        }
+        if let Some(left) = self.left_boundary {
+            if global.x < left {
+                return false;
+            }
+        }
+        if let Some(right) = self.right_boundary {
+            if global.x >= right {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    pub fn get_texel(&self, global: &Vector2I) -> Option<Texel2D> {
+        self.global_to_chunk(global)
+            .map_or(None, |chunk| chunk.get_texel(&global_to_local(global)))
+    }
+
+    pub fn get_latest_simulation(&self, global: &Vector2I) -> Option<u8> {
+        self.global_to_chunk(global).map_or(None, |chunk| {
+            chunk.get_latest_simulation(&global_to_local(global))
+        })
+    }
+
+    pub fn get_texel_behaviour(
+        &self,
+        global: &Vector2I,
+    ) -> (Option<Texel2D>, Option<TexelBehaviour2D>) {
+        let texel = self.get_texel(global);
+        (
+            texel,
+            if self.is_within_boundaries(global) {
+                texel.map_or(None, |t| TexelBehaviour2D::from_id(&t.id))
+            } else {
+                Some(TexelBehaviour2D::OUT_OF_BOUNDS)
+            },
+        )
+    }
+
+    pub fn set_texel(
+        &mut self,
+        global: &Vector2I,
+        new_texel: Texel2D,
+        simulation_frame: Option<u8>,
+    ) {
+        if !self.is_within_boundaries(global) {
+            return;
+        }
+        let index = global_to_chunk_index(global);
+        let changed = match self.index_to_chunk_mut(&index) {
+            Some(chunk) => chunk.set_texel(&global_to_local(global), new_texel, simulation_frame),
+            None => {
+                let mut chunk = Chunk2D::new();
+                let changed =
+                    chunk.set_texel(&global_to_local(global), new_texel, simulation_frame);
+                self.add_chunk(index, chunk);
+                changed
+            }
+        };
+        if changed {
+            self.mark_dirty(&(*global + Vector2I::UP));
+            self.mark_dirty(&(*global + Vector2I::RIGHT));
+            self.mark_dirty(&(*global + Vector2I::DOWN));
+            self.mark_dirty(&(*global + Vector2I::LEFT));
+        }
+    }
+
+    pub fn swap_texels(
+        &mut self,
+        from_global: &Vector2I,
+        to_global: &Vector2I,
+        simulation_frame: Option<u8>,
+    ) {
+        let from = self.get_texel(from_global).unwrap_or_default();
+        let to = self.get_texel(to_global).unwrap_or_default();
+        self.set_texel(to_global, from, simulation_frame);
+        // REM: The displaced texel is also marked as simulated
+        self.set_texel(from_global, to, simulation_frame);
+    }
+
+    fn can_transfer_density(&self, from_global: &Vector2I, to_global: &Vector2I) -> bool {
+        let from = self.get_texel(from_global).unwrap_or_default();
+        let to = self.get_texel(to_global).unwrap_or_default();
+        if from.id != to.id {
+            return false;
+        }
+
+        let behaviour = if let Some(behaviour) = from.behaviour() {
+            behaviour
+        } else {
+            return false;
+        };
+
+        behaviour.form == TexelForm::Gas
+    }
+
+    fn transfer_density(
+        &mut self,
+        from_global: &Vector2I,
+        to_global: &Vector2I,
+        gravity: TexelGravity,
+        simulation_frame: Option<u8>,
+    ) {
+        let from = self.get_texel(from_global).unwrap_or_default();
+        let to = self.get_texel(to_global).unwrap_or_default();
+        let max_transfer = gravity.abs();
+
+        // DEBUG: Test this out, another property?
+        const MAX_TARGET_DENSITY: u8 = 25;
+        let transfer = (u8::MAX - to.density)
+            .min(max_transfer)
+            .min(from.density)
+            .min(MAX_TARGET_DENSITY.max(to.density) - to.density);
+        if transfer == 0 {
+            return;
+        }
+
+        if from.density - transfer == 0 {
+            self.set_texel(&from_global, Texel2D::default(), simulation_frame);
+        } else {
+            self.set_texel(
+                &from_global,
+                Texel2D {
+                    density: from.density - transfer,
+                    ..from
+                },
+                simulation_frame,
+            );
+        }
+        self.set_texel(
+            &to_global,
+            Texel2D {
+                density: to.density + transfer,
+                ..to
+            },
+            simulation_frame,
+        );
     }
 }
 
@@ -311,10 +624,21 @@ pub fn texel_index_to_local(i: usize) -> Vector2I {
     }
 }
 
-pub fn global_to_local(position: &Vector2I) -> Vector2I {
+pub fn texel_index_to_global(i: usize, chunk_index: &Chunk2DIndex) -> Vector2I {
     Vector2I {
-        x: wrapping_remainder(position.x, Chunk2D::SIZE.x),
-        y: wrapping_remainder(position.y, Chunk2D::SIZE.y),
+        x: i as i32 % Chunk2D::SIZE.x,
+        y: i as i32 / Chunk2D::SIZE.y,
+    } + chunk_index_to_global(chunk_index)
+}
+
+pub fn local_to_global(local: &Vector2I, chunk_index: &Chunk2DIndex) -> Vector2I {
+    chunk_index_to_global(chunk_index) + *local
+}
+
+pub fn global_to_local(global: &Vector2I) -> Vector2I {
+    Vector2I {
+        x: wrapping_remainder(global.x, Chunk2D::SIZE.x),
+        y: wrapping_remainder(global.y, Chunk2D::SIZE.y),
     }
 }
 

src/terrain2d/chunk2d.rs

@@ -1,31 +1,15 @@
-use std::collections::{HashMap, VecDeque};
+use std::collections::VecDeque;
 
-use super::{
-    local_to_texel_index, texel_index_to_local, Terrain2D, TerrainEvent2D, Texel2D, TexelID,
-    NEIGHBOUR_INDEX_MAP,
-};
-use crate::util::{Segment2I, Vector2I};
-use bevy::{
-    prelude::*,
-    render::{render_resource::Extent3d, texture::ImageSampler},
-};
-use bevy_rapier2d::prelude::*;
+use super::*;
+use crate::util::{CollisionLayers, Segment2I, Vector2I};
+use bevy::render::{render_resource::Extent3d, texture::ImageSampler};
 use lazy_static::lazy_static;
 
 type Island = VecDeque<Segment2I>;
+pub type Chunk2DIndex = Vector2I;
+pub type NeighbourMask = u8;
 
 lazy_static! {
-    pub static ref COLOR_MAP: HashMap<TexelID, [u8; 4]> = {
-        let mut map = HashMap::new();
-        map.insert(0, [0x00, 0x00, 0x00, 0x00]);
-        // map.insert(0, [0x03, 0x03, 0x03, 0xff]);
-        // map.insert(1, [0x47, 0x8e, 0x48, 0xff]);
-        map.insert(1, [0x9e, 0x7f, 0x63, 0xff]);
-        map.insert(2, [0x38, 0x32, 0x2d, 0xff]);
-        map.insert(3, [0x1e, 0x1e, 0x1e, 0xff]);
-        map
-    };
-
     /// Marching Square case dictionary.
     ///
     /// Key is a bitmask of neighbouring tiles (up, right, down, left - least significant bit first).
@@ -60,6 +44,14 @@ lazy_static! {
         /* down  */ Segment2I { from: Vector2I::RIGHT, to: Vector2I::ZERO },
         /* left  */ Segment2I { from: Vector2I::ZERO, to: Vector2I::UP },
     ];
+
+    static ref NEIGHBOUR_INDEX_MAP: HashMap<Vector2I, u8> = {
+        let mut map = HashMap::new();
+        for i in 0..Chunk2D::NEIGHBOUR_OFFSET_VECTORS.len() {
+            map.insert(Chunk2D::NEIGHBOUR_OFFSET_VECTORS[i], i as u8);
+        }
+        map
+    };
 }
 
 #[derive(Reflect, Component, Default)]
@@ -90,17 +82,28 @@ pub struct ChunkColliderBundle {
     pub transform: TransformBundle,
 }
 
-pub type Chunk2DIndex = Vector2I;
-
 #[derive(Clone, Copy)]
 pub struct ChunkRect {
     pub min: Vector2I,
     pub max: Vector2I,
 }
 
+impl ChunkRect {
+    pub fn include_point(&self, point: Vector2I) -> Self {
+        ChunkRect {
+            min: Vector2I::min(&self.min, &point),
+            max: Vector2I::max(&self.max, &point),
+        }
+    }
+}
+
 pub struct Chunk2D {
-    pub texels: [Texel2D; (Self::SIZE_X * Self::SIZE_Y) as usize],
-    // TODO: handle multiple dirty rects
+    pub texels: [Texel2D; Self::SIZE_X * Self::SIZE_Y],
+    /// bitmask of empty/non-empty neighbours, see NEIGHBOUR_OFFSET_VECTORS for the order
+    pub neighbour_mask: [NeighbourMask; Self::SIZE_X * Self::SIZE_Y],
+    /// Used in simulation step so that texels won't be updated twice. Value of 0 is always updated.
+    pub simulation_frames: [u8; Self::SIZE_X * Self::SIZE_Y],
+    // TODO: handle multiple dirty rects?
     pub dirty_rect: Option<ChunkRect>,
 }
 
@@ -111,65 +114,22 @@ impl Chunk2D {
         x: Self::SIZE_X as i32,
         y: Self::SIZE_Y as i32,
     };
+    pub const NEIGHBOUR_OFFSET_VECTORS: [Vector2I; 4] = [
+        Vector2I { x: 0, y: 1 },
+        Vector2I { x: 1, y: 0 },
+        Vector2I { x: 0, y: -1 },
+        Vector2I { x: -1, y: 0 },
+    ];
 
     pub fn new() -> Chunk2D {
         Chunk2D {
-            texels: Self::new_texel_array(),
+            texels: [Texel2D::default(); Self::SIZE_X * Self::SIZE_Y],
+            neighbour_mask: [0; Self::SIZE_X * Self::SIZE_Y],
+            simulation_frames: [0; Self::SIZE_X * Self::SIZE_Y],
             dirty_rect: None,
         }
     }
 
-    pub fn new_full() -> Chunk2D {
-        let mut chunk = Chunk2D {
-            texels: Self::new_texel_array(),
-            dirty_rect: None,
-        };
-        for y in 0..Self::SIZE_Y {
-            for x in 0..Self::SIZE_X {
-                chunk.set_texel(&Vector2I::new(x as i32, y as i32), 1);
-            }
-        }
-        chunk
-    }
-
-    pub fn new_half() -> Chunk2D {
-        let mut chunk = Chunk2D {
-            texels: Self::new_texel_array(),
-            dirty_rect: None,
-        };
-        for y in 0..Self::SIZE_Y {
-            for x in 0..Self::SIZE_X {
-                if x <= Self::SIZE_Y - y {
-                    chunk.set_texel(&Vector2I::new(x as i32, y as i32), 1);
-                }
-            }
-        }
-        chunk
-    }
-
-    pub fn new_circle() -> Chunk2D {
-        let mut chunk = Chunk2D {
-            texels: Self::new_texel_array(),
-            dirty_rect: None,
-        };
-        let origin = Self::SIZE / 2;
-        let radius = Self::SIZE_X as i32 / 2;
-        for y in 0..Self::SIZE_Y {
-            for x in 0..Self::SIZE_X {
-                let dx = (x as i32 - origin.x).abs();
-                let dy = (y as i32 - origin.y).abs();
-                if dx * dx + dy * dy <= (radius - 1) * (radius - 1) {
-                    chunk.set_texel(&Vector2I::new(x as i32, y as i32), 1);
-                }
-            }
-        }
-        chunk
-    }
-
-    pub fn new_texel_array() -> [Texel2D; Self::SIZE_X * Self::SIZE_Y] {
-        [Texel2D::default(); Self::SIZE_X * Self::SIZE_Y]
-    }
-
     pub fn xy_vec() -> Vec<Vector2I> {
         let mut result = Vec::with_capacity(Self::SIZE_X * Self::SIZE_Y);
         for y in 0..Self::SIZE_Y {
@@ -186,18 +146,13 @@ impl Chunk2D {
     pub fn mark_all_dirty(&mut self) {
         self.dirty_rect = Some(ChunkRect {
             min: Vector2I::ZERO,
-            max: Self::SIZE,
+            max: Self::SIZE - Vector2I::ONE,
         });
     }
 
     pub fn mark_dirty(&mut self, position: &Vector2I) {
         match &self.dirty_rect {
-            Some(rect) => {
-                self.dirty_rect = Some(ChunkRect {
-                    min: Vector2I::min(&rect.min, position),
-                    max: Vector2I::max(&rect.max, position),
-                })
-            }
+            Some(rect) => self.dirty_rect = Some(rect.include_point(*position)),
             None => {
                 self.dirty_rect = Some(ChunkRect {
                     min: *position,
@@ -215,58 +170,69 @@ impl Chunk2D {
         local_to_texel_index(position).map(|i| self.texels[i])
     }
 
+    pub fn get_latest_simulation(&self, position: &Vector2I) -> Option<u8> {
+        local_to_texel_index(position).map(|i| self.simulation_frames[i])
+    }
+
     pub fn get_texel_mut(&mut self, position: &Vector2I) -> Option<&mut Texel2D> {
         local_to_texel_index(position).map(|i| &mut self.texels[i])
     }
 
-    pub fn set_texel(&mut self, position: &Vector2I, id: TexelID) {
+    pub fn set_texel(
+        &mut self,
+        position: &Vector2I,
+        new_texel: Texel2D,
+        simulation_frame: Option<u8>,
+    ) -> bool {
         let i = local_to_texel_index(position).expect("Texel index out of range");
-        if self.texels[i].id != id {
-            self.mark_dirty(position);
+        if self.texels[i] == new_texel {
+            return false;
+        }
+        self.mark_dirty(position);
+        let update_neighbours = self.texels[i].has_collision() != new_texel.has_collision();
+        self.texels[i] = new_texel;
+        // Update simulation frame
+        if let Some(simulation_frame) = simulation_frame {
+            self.simulation_frames[i] = simulation_frame;
         }
-        let update_neighbours = self.texels[i].is_empty()
-            != (Texel2D {
-                id,
-                ..self.texels[i]
-            })
-            .is_empty();
-        self.texels[i].id = id;
         // Update neighbour mask
         if update_neighbours {
-            for offset in Texel2D::NEIGHBOUR_OFFSET_VECTORS {
+            for offset in Self::NEIGHBOUR_OFFSET_VECTORS {
                 // Flip neighbour's bit
-                match self.get_texel_mut(&(*position + offset)) {
-                    Some(mut neighbour) => {
-                        neighbour.neighbour_mask ^= 1 << NEIGHBOUR_INDEX_MAP[&-offset];
+                match local_to_texel_index(&(*position + offset)) {
+                    Some(index) => {
+                        self.neighbour_mask[index] ^= 1 << NEIGHBOUR_INDEX_MAP[&-offset];
                     }
                     None => (),
                 }
             }
         }
+        true
     }
 
     pub fn create_texture_data(&self) -> Vec<u8> {
         let mut image_data = Vec::with_capacity(Chunk2D::SIZE_X * Chunk2D::SIZE_Y * 4);
-        let fallback: [u8; 4] = [0x00, 0x00, 0x00, 0x00];
         for y in (0..Chunk2D::SIZE_Y).rev() {
             for x in 0..Chunk2D::SIZE_X {
-                image_data.append(
-                    &mut COLOR_MAP
-                        .get(
-                            &self
-                                .get_texel(&Vector2I::new(x as i32, y as i32))
-                                .unwrap()
-                                .id,
-                        )
-                        .unwrap_or(&fallback)
-                        .to_vec()
-                        .clone(),
-                );
+                let texel = &self.get_texel(&Vector2I::new(x as i32, y as i32)).unwrap();
+                let behaviour = texel.behaviour();
+                let mut color =
+                    behaviour.map_or(Color::rgba_u8(0, 0, 0, 0), |behaviour| behaviour.color);
+                color.set_a(color.a() * ((texel.density as f32) / 256.0));
+                let color_data = color.as_rgba_u32();
+                let mut color_data: Vec<u8> = vec![
+                    ((color_data >> 0) & 0xff) as u8,
+                    ((color_data >> 8) & 0xff) as u8,
+                    ((color_data >> 16) & 0xff) as u8,
+                    ((color_data >> 24) & 0xff) as u8,
+                ];
+                image_data.append(&mut color_data);
             }
         }
         image_data
     }
 
+    // TODO: Don't create collision for falling texels, it's pretty annoying that a stream of small grains blocks movement
     pub fn create_collision_data(&self) -> Vec<Vec<Vec2>> {
         let mut islands: Vec<Island> = Vec::new();
         for i in 0..self.texels.len() {
@@ -284,8 +250,9 @@ impl Chunk2D {
                 | if local.x == 0 { 1 << 3 } else { 0 };
 
             let mut sides: Vec<Segment2I>;
-            if self.texels[i].is_empty() {
-                sides = MST_CASE_MAP[self.texels[i].neighbour_mask as usize]
+            let has_collision = TexelBehaviour2D::has_collision(&self.texels[i].id);
+            if !has_collision {
+                sides = MST_CASE_MAP[self.neighbour_mask[i] as usize]
                     .iter()
                     .clone()
                     .map(|side| Segment2I {
@@ -293,7 +260,7 @@ impl Chunk2D {
                         to: side.to + local,
                     })
                     .collect();
-            } else if !self.texels[i].is_empty() && edge_mask != 0 {
+            } else if has_collision && edge_mask != 0 {
                 sides = Vec::with_capacity(Chunk2D::SIZE_X * 2 + Chunk2D::SIZE_Y * 2);
                 for i in 0..MST_EDGE_CASE_MAP.len() {
                     if edge_mask & (1 << i) != 0 {
@@ -434,7 +401,7 @@ pub fn chunk_spawner(
                                 ..default()
                             },
                             texture,
-                            transform: Transform::from_translation(Vec3::new(pos.x, pos.y, 0.0)),
+                            transform: Transform::from_translation(Vec3::new(pos.x, pos.y, 1.0)),
                             ..default()
                         },
                         ..default()
@@ -471,9 +438,7 @@ pub fn chunk_spawner(
     }
 }
 
-/**
-    Update the chunk sprite as needed
-*/
+/// Update the chunk sprite as needed
 pub fn chunk_sprite_sync(
     mut terrain_events: EventReader<TerrainEvent2D>,
     mut images: ResMut<Assets<Image>>,
@@ -530,9 +495,7 @@ pub fn chunk_sprite_sync(
     }
 }
 
-/**
-    Create and update colliders for chunk as needed
-*/
+/// Create and update colliders for chunk as needed
 pub fn chunk_collision_sync(
     mut terrain_events: EventReader<TerrainEvent2D>,
     mut commands: Commands,
@@ -574,7 +537,9 @@ pub fn chunk_collision_sync(
         }
     }
 
-    // Kinda messy, partly due do how entity creatin is queued
+    // let layer_membership = CollisionLayers::WORLD;
+
+    // REM: Kinda messy, partly due do how entity creation is timed
     for (entity, chunk_component) in updated_chunks.iter() {
         let chunk = terrain.index_to_chunk(&chunk_component.index).unwrap();
         let new_islands = chunk.create_collision_data();
@@ -594,6 +559,7 @@ pub fn chunk_collision_sync(
                         builder
                             .spawn(Collider::polyline(island.clone(), None))
                             .insert(TransformBundle::default())
+                            .insert(CollisionGroups::new(CollisionLayers::WORLD, Group::ALL))
                             .insert(Name::new(format!("Island #{}", index)));
                     });
                 }
@@ -605,6 +571,7 @@ pub fn chunk_collision_sync(
                     builder
                         .spawn(Collider::polyline(island.clone(), None))
                         .insert(TransformBundle::default())
+                        .insert(CollisionGroups::new(CollisionLayers::WORLD, Group::ALL))
                         .insert(Name::new(format!("Island #{}", index)));
                 });
             }

src/terrain2d/terrain_gen2d.rs

@@ -1,6 +1,7 @@
 use noise::{NoiseFn, PerlinSurflet};
 
-use super::{chunk_index_to_global, Chunk2D, Chunk2DIndex};
+use super::*;
+use crate::util::{inverse_lerp, lerp};
 
 pub struct TerrainGen2D {
     pub seed: u32,
@@ -30,16 +31,16 @@ impl TerrainGen2D {
 
             let mut id = 0;
             if value > 0.35 {
-                id = 1;
+                id = 11;
             }
             if value > 0.42 {
-                id = 2;
+                id = 12;
             }
             if value > 0.9 {
-                id = 3;
+                id = 13;
             }
 
-            chunk.set_texel(&local, id);
+            chunk.set_texel(&local, Texel2D { id, ..default() }, None);
         }
         chunk
     }

src/terrain2d/texel2d.rs

@@ -1,38 +1,32 @@
-use lazy_static::lazy_static;
-use std::collections::HashMap;
-
 pub use u8 as TexelID;
-pub use u8 as NeighbourMask;
 
-use crate::util::Vector2I;
+use super::TexelBehaviour2D;
 
-#[derive(Clone, Copy, Default)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct Texel2D {
+    /// Identifier for a set of properties
     pub id: TexelID,
-    /// bitmask of empty/non-empty neighbours, see NEIGHBOUR_OFFSET_VECTORS for the order
-    pub neighbour_mask: NeighbourMask,
+    /// Used by gas materials
+    pub density: u8,
 }
 
-lazy_static! {
-    pub static ref NEIGHBOUR_INDEX_MAP: HashMap<Vector2I, u8> = {
-        let mut map = HashMap::new();
-        for i in 0..Texel2D::NEIGHBOUR_OFFSET_VECTORS.len() {
-            map.insert(Texel2D::NEIGHBOUR_OFFSET_VECTORS[i], i as u8);
+impl Default for Texel2D {
+    fn default() -> Self {
+        Self {
+            id: TexelID::default(),
+            density: u8::MAX,
         }
-        map
-    };
+    }
 }
 
 impl Texel2D {
     pub const EMPTY: TexelID = 0;
-    pub const NEIGHBOUR_OFFSET_VECTORS: [Vector2I; 4] = [
-        Vector2I { x: 0, y: 1 },
-        Vector2I { x: 1, y: 0 },
-        Vector2I { x: 0, y: -1 },
-        Vector2I { x: -1, y: 0 },
-    ];
 
-    pub fn is_empty(&self) -> bool {
-        self.id == 0
+    pub fn has_collision(&self) -> bool {
+        TexelBehaviour2D::has_collision(&self.id)
+    }
+
+    pub fn behaviour(&self) -> Option<TexelBehaviour2D> {
+        TexelBehaviour2D::from_id(&self.id)
     }
 }

src/terrain2d/texel_behaviour2d.rs

@@ -0,0 +1,255 @@
+use crate::util::Vector2I;
+
+use super::TexelID;
+use bevy::prelude::*;
+use lazy_static::lazy_static;
+use std::{borrow::Cow, collections::HashMap};
+
+lazy_static! {
+    static ref ID_MAP: HashMap<TexelID, TexelBehaviour2D> = {
+        let mut result = HashMap::new();
+
+        result.insert(
+            1,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("loose sand"),
+                color: Color::rgb(0.61, 0.49, 0.38),
+                gravity: Some(TexelGravity::Down(200)),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result.insert(
+            2,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("loose stone"),
+                color: Color::rgb(0.21, 0.19, 0.17),
+                gravity: Some(TexelGravity::Down(200)),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result.insert(
+            3,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("loose sturdy stone"),
+                color: Color::rgb(0.11, 0.11, 0.11),
+                gravity: Some(TexelGravity::Down(200)),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result.insert(
+            4,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("water"),
+                color: Color::rgba(0.0, 0.0, 1.0, 0.5),
+                form: TexelForm::Liquid,
+                gravity: Some(TexelGravity::Down(50)),
+                ..default()
+            },
+        );
+
+        result.insert(
+            5,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("oil"),
+                color: Color::rgba(0.5, 0.5, 0.25, 0.5),
+                form: TexelForm::Liquid,
+                gravity: Some(TexelGravity::Down(20)),
+                ..default()
+            },
+        );
+
+        result.insert(
+            6,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("light gas"),
+                color: Color::rgba(0.0, 1.0, 0.0, 0.5),
+                form: TexelForm::Gas,
+                gravity: Some(TexelGravity::Up(160)),
+                ..default()
+            },
+        );
+
+        result.insert(
+            7,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("heavy gas"),
+                color: Color::rgba(1.0, 0.5, 0.5, 0.5),
+                form: TexelForm::Gas,
+                gravity: Some(TexelGravity::Down(60)),
+                ..default()
+            },
+        );
+
+        result.insert(
+            8,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("oxygen"),
+                color: Color::rgba(0.5, 0.5, 0.5, 0.5),
+                form: TexelForm::Gas,
+                ..default()
+            },
+        );
+
+        result.insert(
+            11,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("sand"),
+                color: Color::rgb(0.61, 0.49, 0.38),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result.insert(
+            12,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("stone"),
+                color: Color::rgb(0.21, 0.19, 0.17),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result.insert(
+            13,
+            TexelBehaviour2D {
+                name: Cow::Borrowed("sturdy stone"),
+                color: Color::rgb(0.11, 0.11, 0.11),
+                has_collision: true,
+                ..default()
+            },
+        );
+
+        result
+    };
+    static ref FORM_DISPLACEMENT_PRIORITY: HashMap<TexelForm, u8> = {
+        let mut result = HashMap::new();
+        result.insert(TexelForm::Gas, 0);
+        result.insert(TexelForm::Liquid, 1);
+        result.insert(TexelForm::Solid, 2);
+        result
+    };
+}
+
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
+pub enum TexelForm {
+    #[default]
+    // Solid materials, when affected by gravity, create pyramid-like piles
+    Solid,
+    // Liquid materials, when affected by gravity, act like solids but also try to stabilise the surface level by traveling flat surfaces
+    Liquid,
+    // Gas materials act like liquids, but also have density/pressure that causes them to disperse
+    Gas,
+}
+
+impl TexelForm {
+    fn priority(&self) -> u8 {
+        FORM_DISPLACEMENT_PRIORITY
+            .get(self)
+            .cloned()
+            .unwrap_or_default()
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum TexelGravity {
+    Down(u8),
+    Up(u8),
+}
+
+impl From<TexelGravity> for Vector2I {
+    fn from(gravity: TexelGravity) -> Self {
+        match gravity {
+            TexelGravity::Down(_) => Vector2I::DOWN,
+            TexelGravity::Up(_) => Vector2I::UP,
+        }
+    }
+}
+
+impl TexelGravity {
+    pub fn abs(&self) -> u8 {
+        match self {
+            TexelGravity::Down(grav) => *grav,
+            TexelGravity::Up(grav) => *grav,
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct TexelBehaviour2D {
+    pub name: Cow<'static, str>,
+    pub color: Color,
+    pub form: TexelForm,
+    pub has_collision: bool,
+    pub gravity: Option<TexelGravity>,
+    pub toughness: Option<f32>,
+}
+
+impl Default for TexelBehaviour2D {
+    fn default() -> Self {
+        TexelBehaviour2D {
+            name: Cow::Borrowed("Unnamed material"),
+            color: Color::PINK,
+            form: TexelForm::Solid,
+            has_collision: false,
+            gravity: None,
+            toughness: None,
+        }
+    }
+}
+
+impl TexelBehaviour2D {
+    pub const OUT_OF_BOUNDS: Self = TexelBehaviour2D {
+        name: Cow::Borrowed(":)"),
+        color: Color::BLACK,
+        has_collision: true,
+        form: TexelForm::Solid,
+        gravity: None,
+        toughness: None,
+    };
+
+    pub fn from_id(id: &TexelID) -> Option<Self> {
+        ID_MAP.get(id).cloned()
+    }
+
+    pub fn is_empty(id: &TexelID) -> bool {
+        ID_MAP.get(id).is_none()
+    }
+
+    pub fn has_collision(id: &TexelID) -> bool {
+        ID_MAP.get(id).map_or(false, |b| b.has_collision)
+    }
+
+    /// Can this type of material displace another?
+    pub fn can_displace(from: &TexelBehaviour2D, to: &Option<TexelBehaviour2D>) -> bool {
+        let to = if let Some(to) = to { to } else { return true };
+
+        match (from.form, to.form) {
+            (from_form, to_form) => {
+                if from_form.priority() != to_form.priority() {
+                    return from_form.priority() > to_form.priority();
+                }
+                if let (Some(from_grav), Some(to_grav)) = (from.gravity, to.gravity) {
+                    match (from_grav, to_grav) {
+                        (TexelGravity::Down(from_grav), TexelGravity::Down(to_grav)) => {
+                            from_grav > to_grav
+                        }
+                        (TexelGravity::Up(from_grav), TexelGravity::Up(to_grav)) => {
+                            from_grav > to_grav
+                        }
+                        (_, _) => true,
+                    }
+                } else {
+                    // Solids can also be displaced, but only if the other material has gravity
+                    to_form != TexelForm::Solid
+                }
+            }
+        }
+    }
+}

src/util.rs

@@ -1,10 +1,13 @@
 use bevy::prelude::*;
 
+mod collision_layers;
+pub mod frame_counter;
 pub mod math;
 mod segment2_i32;
 mod vector2;
 mod vector2_i32;
 
+pub use collision_layers::*;
 pub use segment2_i32::*;
 pub use vector2::*;
 pub use vector2_i32::*;

src/util/collision_layers.rs

@@ -0,0 +1,9 @@
+use bevy_rapier2d::prelude::*;
+
+pub struct CollisionLayers;
+
+impl CollisionLayers {
+    pub const WORLD: Group = Group::GROUP_1;
+    pub const PLAYER: Group = Group::GROUP_2;
+    pub const ENEMY: Group = Group::GROUP_3;
+}

src/util/frame_counter.rs

@@ -0,0 +1,19 @@
+use bevy::prelude::*;
+
+pub struct FrameCounterPlugin;
+
+impl Plugin for FrameCounterPlugin {
+    fn build(&self, app: &mut App) {
+        app.insert_resource(FrameCounter { frame: 0 })
+            .add_system_to_stage(CoreStage::First, frame_increment);
+    }
+}
+
+#[derive(Resource)]
+pub struct FrameCounter {
+    pub frame: u64,
+}
+
+fn frame_increment(mut frame_counter: ResMut<FrameCounter>) {
+    frame_counter.frame += 1;
+}