intersect_polyline_with_polygon, but getting the cut polygons

[opyate]

intersect_polyline_with_polygon returns the intersected polylines, but I was wondering if a similar function exists which would return the intersected polygons. I couldn't see any such function, so perhaps it's more of a question of how this can be achieved using the existing API.

E.g. left side is a polygon intersected by the red polyline, and the result is the 3 polygons on the right.

PS the line doesn't have thickness, hence there aren't really gaps between the polygons on the right, but it's just illustrative.

[Xrayez]

Yeah that's an interesting problem, there was Q&A question I answered some time ago:

There's no such method exposed because the underlying library for polygon clipping in Godot (Clipper) doesn't support such operation.

You can in theory simulate this by using Geometry.offset_polyline_2d with a very small delta, which will grow your polyline into a thin polygon, and then do the clipping between polygons. Or, you can generate two rectangles along with a polyline (if it's a straight line), and do Geometry.intersect_polygons_2d two times against your subject polygon on the both side of the polyline.

Example

extends Node2D

func _draw():
	var circle = GoostGeometry2D.circle(50)

	var polyline = PoolVector2Array([
		Vector2(-45, -54.5),
		Vector2(6, -19.5),
		Vector2(-8, 20.5),
		Vector2(66, 29.5)
	])
	var crack_width = 1.0
	var deflate_result = GoostGeometry2D.deflate_polyline(polyline, crack_width)

	assert(not deflate_result.empty())
	var deflated_polyline = deflate_result[0]

	var polygons = GoostGeometry2D.clip_polygons(circle, deflated_polyline)
	for poly in polygons:
		draw_colored_polygon(poly, Random.color)

It looks to me that this kind of task is more or less common (you're not the only one asking this question, at least), so it might make sense to provide API for this, like split_polygon(polygon, polyline), as the solution is not immediately clear and that's something which would make current API more complete on functional level, perhaps.

But it could be considered hack-ish depending on the exact use case. Deflating polyline into a polygon could possibly result in unwanted artifacts like cutting the edges slightly of the original polygon if crack_width is large. However, making the width small enough is likely acceptable for most use cases.

There's also another potential solution to this problem: you can generate a partial rectangular-ish polygon with desired curve (in such a way that would cover parts that you want to cut), and do intersect operation between original and generated polygons, and then clipping the original polygon with the result of previous operation! This way, you'll have no gap, but you have to make sure to properly cover your subject polygon in order to cut all parts.

See also discussion upstream at the Clipper repository: "Splitting polygons with split lines?".

[opyate]

Thanks for the reply. I can't wait to try these techniques. EDIT: the technique from the posted code example works nicely, thanks. I didn't have time to experiment with the other potential solution, but the first one works.