Unable to set margin on CollisionPolygon resulting in noticeable gaps when using Bullet engine

[RobTheFiveNine]

Godot version:
v3.2.3.stable.official

OS/device including version:
Ubuntu 18.04

Issue description:
When a RigidBody with a CollisionShape collides with a StaticBody that contains a CollisionPolygon, a noticeable gap can be seen which seems to be the default margin that is applied. Unlike other collision objects, it's not possible to set the margin of the CollisionPolygon though.

A potential work around was mentioned in This Similar Issue, but I was unable to get it to work.

An example of the gap can be seen in this screenshot:

Minimal reproduction project:
https://github.com/RobTheFiveNine/godot-collision-polygon-margin-example

[hoontee]

Author of the workaround here. Are you sure this code doesn't work?:

func _ready() -> void:
	fix_collision_polygons($"/root/Game/World") # Your nodes to fix here

func fix_collision_polygons(parent: Node) -> void:
	for child in parent.get_children():
		if parent is PhysicsBody and child is CollisionPolygon and not child.disabled:
			var body := (parent as CollisionObject).get_rid()
			for i in PhysicsServer.body_get_shape_count(body):
				var shape := PhysicsServer.body_get_shape(body, i)
				if PhysicsServer.shape_get_type(shape) == PhysicsServer.SHAPE_CONVEX_POLYGON:
					var new_convex_polygon_shape := ConvexPolygonShape.new()
					new_convex_polygon_shape.margin = 0
					new_convex_polygon_shape.set_points(PhysicsServer.shape_get_data(shape))
					var new_collision_shape := CollisionShape.new()
					new_collision_shape.shape = new_convex_polygon_shape
					new_collision_shape.transform = child.transform
					parent.add_child(new_collision_shape)
			child.free()
		else:
			fix_collision_polygons(child)

[RobTheFiveNine]

Author of the workaround here. Are you sure this code doesn't work?:

func _ready() -> void:
	fix_collision_polygons($"/root/Game/World") # Your nodes to fix here

func fix_collision_polygons(parent: Node) -> void:
	for child in parent.get_children():
		if parent is PhysicsBody and child is CollisionPolygon and not child.disabled:
			var body := (parent as CollisionObject).get_rid()
			for i in PhysicsServer.body_get_shape_count(body):
				var shape := PhysicsServer.body_get_shape(body, i)
				if PhysicsServer.shape_get_type(shape) == PhysicsServer.SHAPE_CONVEX_POLYGON:
					var new_convex_polygon_shape := ConvexPolygonShape.new()
					new_convex_polygon_shape.margin = 0
					new_convex_polygon_shape.set_points(PhysicsServer.shape_get_data(shape))
					var new_collision_shape := CollisionShape.new()
					new_collision_shape.shape = new_convex_polygon_shape
					new_collision_shape.transform = child.transform
					parent.add_child(new_collision_shape)
			child.free()
		else:
			fix_collision_polygons(child)

I must have made a typo somewhere when trying it last time, because I can't break it this time! Above example and the one from the original post seems to work

[hoontee]

Glad we could get it to work. I do a lot of CSG / collision workarounds like this for my projects so let me know if you need any more help.

[RobTheFiveNine]

Ahhh OK, this issue can probably be closed then and just added as a note to #27427 then. I didn't realise CollisionPolygons get turned into convex shapes under the hood.

Thanks for the fix too! I ended up scaling everything up in the project I ran into it with, which has actually helped fix some lighting issues too, but I'll definitely keep this solution bookmarked for future use 😃

[hoontee]

I'm sorry, I take it back entirely - adding a margin property and setting it to zero will still have an effect if #27427 isn't fixed first...