intersectSegmentCircleDisplace does not work when the segment is partially inside the circle

Issue details

The Intersector.intersectSegmentCircleDisplace() function is wrong when one of the endpoint of the segment is inside the circle. The current algorithm calcultates the projection of the circle center on the infinite line (start, end), but returns no intersection if the projection point is not within the segment endpoints (as in the diagram above)

Also, the function is supposed to “returns by how much and in what direction the line has to move away from the circle to not intersect.” but it returns by how much and in what direction the line has to move to cross the circle center.

Here’s my correction:

	public static float intersectSegmentCircleDisplace(Vector2 start, Vector2 end, Vector2 point, float radius, Vector2 displacement) {
		float u = (point.x - start.x) * (end.x - start.x) + (point.y - start.y) * (end.y - start.y);
		float d = start.dst(end);
		u /= d * d;
		if (u < 0)
			tmp2.set(start);
		else if (u > 1)
			tmp2.set(end);
		else {
			tmp.set(end.x, end.y).sub(start.x, start.y);
			tmp2.set(start.x, start.y).add(tmp.scl(u));
		}
		d = tmp2.dst(point.x, point.y);
		if (d < radius) {
			displacement.set(tmp2).sub(point).nor();
			return radius - d;
		} else
			return Float.POSITIVE_INFINITY;
	}

Reproduction steps/code

System.out.println(Intersector.intersectSegmentCircleDisplace(new Vector2(0,0), new Vector2(0,1), new Vector2(0,2),1.5f, new Vector2())); returns Infinity instead of 0.5

Version of LibGDX and/or relevant dependencies

1.9.6

Please select the affected platforms

• Android
• iOS (robovm)
• iOS (MOE)
• HTML/GWT
• Windows
• Linux
• MacOS

[obigu]

Wow! I guess nobody has ever used this one, it’s pretty bad. The implementation of intersectSegmentCircleDisplace is wrong but I believe it should be aligned with the intersectSegmentCircle implementation which is correct and more efficient:

1. Operators should also check for equality (<= and >=)
2. Avoid square roots such as Vector2.dst for efficiency

On a more general note I see several things that could be improved:

1. I miss basic important utility mehods such as the orthoghonal projection of a point in a line or the intersection between line and circle (not a segment).
2. Use Circle as argument instead of center and radius. The reason why some of these methods don’t accept it is because Circle class was created afterwards (they were there on first commit! 😃.
3. Reuse code when possible
4. Add some tests

I’ll work on a PR see where it goes that includes a fix for this.

[obigu]

@anigart Thank you, it’s been fixed