Restrict generated clam spots to visible sand

tests/digging_prototype_validation.gd

@@ -11,6 +11,15 @@ const Gatherables: GatherableCatalog = preload(
 	"res://gathering/catalog/gatherable_catalog.tres"
 )
 
+class PrimaryTerrainProvider:
+	extends Node3D
+
+	var primary_mesh: MeshInstance3D
+
+
+	func get_primary_terrain_meshes() -> Array[MeshInstance3D]:
+		return [primary_mesh] if primary_mesh != null else []
+
 
 func _initialize() -> void:
 	call_deferred("_run")
@@ -20,6 +29,7 @@ func _run() -> void:
 	_validate_catalog_content()
 	_validate_flat_shovel()
 	await _validate_beach_authoring()
+	_validate_primary_terrain_provider()
 	print("Digging prototype validation: PASS")
 	quit()
 
@@ -81,6 +91,52 @@ func _validate_beach_authoring() -> void:
 	region.queue_free()
 
 
+func _validate_primary_terrain_provider() -> void:
+	var fixture := Node3D.new()
+	fixture.name = "PrimaryTerrainFixture"
+	root.add_child(fixture)
+	var provider := PrimaryTerrainProvider.new()
+	provider.name = "Provider"
+	fixture.add_child(provider)
+	provider.primary_mesh = _flat_sand_triangle(0.0)
+	provider.add_child(provider.primary_mesh)
+	var buried_base := _flat_sand_triangle(10.0)
+	buried_base.name = "BuriedSandBase"
+	provider.add_child(buried_base)
+	var area := DiggableArea3D.new()
+	area.name = "DiggableArea"
+	area.area_id = &"provider_test"
+	area.terrain_source = NodePath("../Provider")
+	area.surface_materials = [&"sand"]
+	area.generation_bounds = Rect2(-20.0, -20.0, 40.0, 40.0)
+	fixture.add_child(area)
+	var triangles := area.get_surface_triangles()
+	assert(triangles.size() == 1)
+	var center := (triangles[0][0] + triangles[0][1] + triangles[0][2]) / 3.0
+	assert(center.x < 2.0)
+	fixture.free()
+
+
+func _flat_sand_triangle(x_offset: float) -> MeshInstance3D:
+	var arrays: Array = []
+	arrays.resize(Mesh.ARRAY_MAX)
+	arrays[Mesh.ARRAY_VERTEX] = PackedVector3Array([
+		Vector3(x_offset, 0.0, 0.0),
+		Vector3(x_offset, 0.0, 1.0),
+		Vector3(x_offset + 1.0, 0.0, 0.0),
+	])
+	arrays[Mesh.ARRAY_INDEX] = PackedInt32Array([0, 1, 2])
+	var mesh := ArrayMesh.new()
+	mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, arrays)
+	var material := StandardMaterial3D.new()
+	material.resource_name = "sand"
+	mesh.surface_set_material(0, material)
+	var mesh_instance := MeshInstance3D.new()
+	mesh_instance.name = "PrimarySand"
+	mesh_instance.mesh = mesh
+	return mesh_instance
+
+
 func _collect_meshes(
 	root_node: Node,
 	result: Array[MeshInstance3D],

world/digging/diggable_area_3d.gd

@@ -18,7 +18,7 @@ func get_surface_triangles() -> Array[PackedVector3Array]:
 	var terrain_root: Node = get_node_or_null(terrain_source)
 	if terrain_root == null:
 		return triangles
-	for mesh_instance: MeshInstance3D in _collect_mesh_instances(terrain_root):
+	for mesh_instance: MeshInstance3D in _terrain_mesh_instances(terrain_root):
 		var mesh: Mesh = mesh_instance.mesh
 		if mesh == null:
 			continue
@@ -92,6 +92,23 @@ func _append_triangle(
 	result.append(PackedVector3Array([a, b, c]))
 
 
+func _terrain_mesh_instances(terrain_root: Node) -> Array[MeshInstance3D]:
+	# Generated terrain can contain authored base layers beneath raised visual
+	# overlays. Those meshes are useful for closing terrain seams, but they are
+	# not necessarily the visible surface and must not produce buried dig spots.
+	# A terrain provider may therefore expose its authoritative primary meshes.
+	if terrain_root.has_method(&"get_primary_terrain_meshes"):
+		var provided: Variant = terrain_root.call(&"get_primary_terrain_meshes")
+		var meshes: Array[MeshInstance3D] = []
+		if provided is Array:
+			for value: Variant in provided:
+				var mesh_instance := value as MeshInstance3D
+				if mesh_instance != null:
+					meshes.append(mesh_instance)
+		return meshes
+	return _collect_mesh_instances(terrain_root)
+
+
 func _collect_mesh_instances(root: Node) -> Array[MeshInstance3D]:
 	var meshes: Array[MeshInstance3D] = []
 	if root is MeshInstance3D: