Improve tree gathering and rain occlusion

world/environment/precipitation_occlusion.gd

@@ -0,0 +1,56 @@
+class_name PrecipitationOcclusion
+extends RefCounted
+
+## A secondary visual layer used only when building the local rain height field.
+## Canopy meshes remain on layer 1 for ordinary cameras.
+const RENDER_LAYER_NUMBER: int = 20
+const RENDER_LAYER_MASK: int = 1 << (RENDER_LAYER_NUMBER - 1)
+const CANOPY_MATERIAL_NAMES: Array[String] = [
+	"leaf",
+	"leaf_light",
+	"leaf_mid",
+	"leaf_dark",
+	"pine",
+	"tree",
+]
+
+
+static func mark_canopy_meshes(root_node: Node) -> int:
+	if root_node == null:
+		return 0
+	var marked_count: int = 0
+	var mesh_instance := root_node as MeshInstance3D
+	if mesh_instance != null and _has_canopy_material(mesh_instance):
+		mesh_instance.layers |= RENDER_LAYER_MASK
+		marked_count += 1
+	for child: Node in root_node.get_children():
+		marked_count += mark_canopy_meshes(child)
+	return marked_count
+
+
+## Generated tree definitions are already explicit terrain metadata, so every
+## mesh in their visual scene can safely participate. This also supports older
+## combined tree meshes whose imported material name does not identify leaves.
+static func mark_tree_meshes(root_node: Node) -> int:
+	if root_node == null:
+		return 0
+	var marked_count: int = 0
+	var mesh_instance := root_node as MeshInstance3D
+	if mesh_instance != null:
+		mesh_instance.layers |= RENDER_LAYER_MASK
+		marked_count += 1
+	for child: Node in root_node.get_children():
+		marked_count += mark_tree_meshes(child)
+	return marked_count
+
+
+static func _has_canopy_material(mesh_instance: MeshInstance3D) -> bool:
+	if mesh_instance == null or mesh_instance.mesh == null:
+		return false
+	for surface_index: int in mesh_instance.mesh.get_surface_count():
+		var material := mesh_instance.mesh.surface_get_material(surface_index)
+		if material == null:
+			continue
+		if material.resource_name.to_lower() in CANOPY_MATERIAL_NAMES:
+			return true
+	return false