Shot curve using a simulated Magnus effect

src/equipment/balls/physics_ball/game_ball.gd

@@ -12,6 +12,7 @@ enum Type {
 }
 
 const TERRAIN_DAMPING_EPSILON := 1e-6
+const MAGNUS_EPSILON := 1e-3
 const IRON_DAMPING := 9999.0
 
 ## Angular damping while in air
@@ -21,6 +22,16 @@ const IRON_DAMPING := 9999.0
 ## Angular damping for iron balls
 @export var iron_damping := 9999.0
 
+#@export var fluid_density := 1.225
+#@export var lift_coefficient := 0.05
+#@export var radius := 0.05
+## Coefficient of angular velocity influence on linear velocity
+## This is approximately 1/2 * rho * C_L * pi * r^2
+## where `rho` is the fluid density of the medium, or 1.225 for air at sea level,
+## and `C_L` is the lift coefficient which for our purposes is 0.05,
+## and `r` is the radius of the ball, which is 5cm.
+@export var magnus_coefficient := 0.00024
+
 ## Causes the ball to act more like a brick
 @export var iron_ball := false:
 	set(value):
@@ -59,15 +70,22 @@ func _total_terrain_angular_damping() -> float:
 	)
 
 
+func _magnus_force() -> Vector3:
+	return magnus_coefficient * angular_velocity.cross(linear_velocity)
+
+
 func _integrate_forces(state: PhysicsDirectBodyState3D) -> void:
 	if not _awake:
 		# Triggered on first frame after waking
 		_awake = true
 		_position_on_last_wake = global_position
 		_last_contact_normal = Vector3.UP
+		# TODO something's fucky here... I think this gets called once after the ball sleeps
 
 	var damping := air_damping
 	if state.get_contact_count():
+		constant_force = Vector3.ZERO
+
 		# We want the contact normal which minimizes the angle to the up vector
 		var min_dot := -1.0
 		for i: int in range(state.get_contact_count()):
@@ -85,6 +103,13 @@ func _integrate_forces(state: PhysicsDirectBodyState3D) -> void:
 	angular_damp = damping
 
 
+func _physics_process(_delta: float) -> void:
+	# Simulate magnus effect
+	var magnus := _magnus_force()
+	if magnus.length_squared() > MAGNUS_EPSILON:
+		apply_central_force(magnus)
+
+
 func enter_zone(zone: BallZone) -> void:
 	_zones.push_back(zone)
 

src/equipment/balls/physics_ball/physics_ball.tscn

@@ -34,8 +34,13 @@ script/source = "extends Control
 const COLOR_X := Color(1, 0, 0)
 const COLOR_Y := Color(0, 1, 0)
 const COLOR_Z := Color(0, 0, 1)
+const COLOR_MAGNUS := Color.CYAN
+const MAGNUS_SCALE := 3
 const WIDTH := 4
 
+@export var draw_reoriented_basis := true
+@export var draw_magnus_effect := true
+
 @onready var physics_ball: GameBall = $\"..\"
 
 
@@ -45,9 +50,9 @@ func _process(_delta: float) -> void:
 
 
 func _draw() -> void:
-	if physics_ball._last_contact_normal != null:
-		var camera := get_viewport().get_camera_3d()
-		var start := camera.unproject_position(physics_ball.global_position)
+	var camera := get_viewport().get_camera_3d()
+	var start := camera.unproject_position(physics_ball.global_position)
+	if draw_reoriented_basis and physics_ball._last_contact_normal != null:
 		var basis := physics_ball.get_reoriented_basis()
 		var end_x := camera.unproject_position(physics_ball.global_position + basis.x)
 		var end_y := camera.unproject_position(physics_ball.global_position + basis.y)
@@ -55,6 +60,10 @@ func _draw() -> void:
 		draw_line(start, end_x, COLOR_X, WIDTH)
 		draw_line(start, end_y, COLOR_Y, WIDTH)
 		draw_line(start, end_z, COLOR_Z, WIDTH)
+
+	if draw_magnus_effect:
+		var end := camera.unproject_position(physics_ball.global_position + physics_ball._magnus_force() * MAGNUS_SCALE)
+		draw_line(start, end, COLOR_MAGNUS, WIDTH)
 "
 
 [node name="PhysicsBall" type="RigidBody3D"]
@@ -86,5 +95,6 @@ anchor_bottom = 1.0
 grow_horizontal = 2
 grow_vertical = 2
 script = SubResource("GDScript_p4v7o")
+draw_reoriented_basis = false
 
 [connection signal="sleeping_state_changed" from="." to="." method="_on_sleeping_state_changed"]

src/player/shot_setup/shot_setup.gd

@@ -35,6 +35,12 @@ const BIG_POWER_THRESHOLD := 0.7
 ## Amount of life lost when landing in water
 const WATER_DAMAGE := 10.0
 
+## Angle of influence that shot curve has, in radians
+const CURVE_INFLUENCE := PI / 8
+
+## Maximum absolute curve for the "nice shot" animation to play
+const NICE_THRESHOLD := 0.2
+
 ## In Driving Range mode, the ball can be retrieved in the shot phase.
 @export var driving_range := false
 
@@ -108,7 +114,7 @@ var shot_curve: float:
 	set(value):
 		hud.curve_bar.value = value
 	get:
-		return hud.power_bar.value
+		return hud.curve_bar.value
 
 var game_ball: GameBall:
 	get:
@@ -231,13 +237,22 @@ func take_shot() -> void:
 
 	print_debug("WHACK!\nPower: ", shot_power, "\nCurve: ", shot_curve)
 
+	# Momentary impulse applied to the ball
 	var impulse := get_shot_impulse(shot_power)
 	print_debug("Shot impulse: ", impulse, "; ", impulse.length(), " N*s")
 
+	var curve := shot_ref.global_basis.x.normalized() * shot_curve
+
+	# Position where the ball is hit (imparts spin)
+	var offset := curve * 0.001
+	print_debug("Shot offset: ", offset, "; ", offset.length(), " m")
+	# Rotate impulse
+	impulse = impulse.rotated(Vector3.UP, -shot_curve * CURVE_INFLUENCE)
+
 	if game_ball:
 		game_ball.iron_ball = club == Club.Type.IRON
 		game_ball.freeze = false
-		game_ball.apply_central_impulse(impulse)
+		game_ball.apply_impulse(impulse, offset)
 
 	# Use a ball if a limited type is selected
 	if player.get_balls(ball_type) > 0:
@@ -407,7 +422,11 @@ func _on_phase_change(new_phase: Phase) -> void:
 			hud.power_bar.hide()
 			hud.curve_bar.hide()
 
-			hud.play_nice_animation()
+			if perfect_aim:
+				shot_curve = 0.0
+
+			if abs(shot_curve) <= NICE_THRESHOLD:
+				hud.play_nice_animation()
 
 			if not driving_range:
 				shot_animation.play("shoot")