From 906e161b678507cd1ac43c0bb5acbf2b437f63f9 Mon Sep 17 00:00:00 2001
From: Rob Kelly <contact@robkel.ly>
Date: Mon, 9 Dec 2024 14:40:23 -0700
Subject: [PATCH] Projectile arc computes local gravity at each step

---
 src/ui/3d/projectile_arc/projectile_arc.gd | 65 +++++++++++++++++++++-
 1 file changed, 63 insertions(+), 2 deletions(-)

diff --git a/src/ui/3d/projectile_arc/projectile_arc.gd b/src/ui/3d/projectile_arc/projectile_arc.gd
index 6ec7898..18618ab 100644
--- a/src/ui/3d/projectile_arc/projectile_arc.gd
+++ b/src/ui/3d/projectile_arc/projectile_arc.gd
@@ -22,7 +22,7 @@ const PUTT_ATTRITION := 0.8325  # green?
 ## If enabled, project a linear putt instead of an arcing shot
 @export var putt_projection := false
 
-@export_category("Collision")
+@export_category("Collision & Physics")
 ## Enables collision checking. Projection will end at the point where a collision is detected.
 ## Uses continuous collision detection.
 @export var check_collision := true
@@ -32,6 +32,10 @@ const PUTT_ATTRITION := 0.8325  # green?
 ## This should probably include the ball!
 @export var excluded_bodies: Array[CollisionObject3D] = []
 
+## Enables checking local gravity at each point along the trajectory.
+## If disabled, global gravity will be used instead.
+@export var check_gravity := true
+
 var _tick_counter := 0
 
 var _debug_points: Array[Vector3] = []
@@ -73,8 +77,13 @@ func _process(_delta: float) -> void:
 		# TODO: smooth curve with bezier handles
 		path.curve.add_point(pos - global_position)
 
+		# Get local gravity if enabled
+		var local_gravity := gravity * gravity_vec
+		if check_gravity:
+			local_gravity = _get_gravity(pos)
+
 		# Integrate projectile path
-		vel += gravity * gravity_vec * time_step
+		vel += local_gravity * time_step
 		var next_pos := pos + vel * time_step
 
 		# Collision
@@ -110,6 +119,58 @@ func _process(_delta: float) -> void:
 	(%DebugDraw as CanvasItem).queue_redraw()
 
 
+func _get_gravity(point: Vector3) -> Vector3:
+	# Start with global gravity
+	var local_gravity := gravity * gravity_vec
+
+	# TODO this is awful, surely there has to be a better way than this!!!
+	# Get areas at point
+	var point_params := PhysicsPointQueryParameters3D.new()
+	point_params.collide_with_areas = true
+	point_params.collide_with_bodies = false
+	point_params.collision_mask = collision_mask
+	point_params.position = point
+	var collisions := get_world_3d().direct_space_state.intersect_point(point_params)
+	var gravity_areas: Array[Area3D] = []
+	gravity_areas.assign(
+		collisions.map(func(d: Dictionary) -> Area3D: return d["collider"] as Area3D)
+	)
+	gravity_areas.sort_custom(func(a: Area3D, b: Area3D) -> bool: return a.priority < b.priority)
+
+	# Iteratively integrate gravity
+	for area: Area3D in gravity_areas:
+		var point_local := point - area.global_position
+		var area_gravity: Vector3
+		if area.gravity_point:
+			# TODO: `point` may need to be local
+			var v := area.transform * area.gravity_direction - point_local
+			if area.gravity_point_unit_distance > 0:
+				var v_sq := v.length_squared()
+				if v_sq > 0:
+					area_gravity = (
+						v.normalized()
+						* area.gravity
+						* pow(area.gravity_point_unit_distance, 2)
+						/ v_sq
+					)
+				else:
+					area_gravity = Vector3.ZERO
+			else:
+				area_gravity = v.normalized() * area.gravity
+		else:
+			area_gravity = area.gravity * area.gravity_direction
+
+		match area.gravity_space_override:
+			Area3D.SPACE_OVERRIDE_COMBINE, Area3D.SPACE_OVERRIDE_COMBINE_REPLACE:
+				local_gravity += area_gravity
+			Area3D.SPACE_OVERRIDE_REPLACE_COMBINE, Area3D.SPACE_OVERRIDE_REPLACE:
+				local_gravity = area_gravity
+			Area3D.SPACE_OVERRIDE_COMBINE_REPLACE, Area3D.SPACE_OVERRIDE_REPLACE:
+				break
+
+	return local_gravity
+
+
 func _on_visibility_changed() -> void:
 	# Force update as soon as visible
 	_tick_counter = 0