Keith Hair

Here is an ActionScript 3 function that will return intersection information between a segment and circle.
It's useful to me to know if a segment is partially intersecting with a circle or going completely through.

 
/*---------------------------------------------------------------------------
Returns an Object with the following properties:
	enter			-Intersection Point entering the circle.
	exit			-Intersection Point exiting the circle.
	inside			-Boolean indicating if the points of the line are inside the circle.
	tangent		-Boolean indicating if line intersect at one point of the circle.
	intersects		-Boolean indicating if there is an intersection of the points and the circle.
 
If both "enter" and "exit" are null, or "intersects" == false, it indicates there is no intersection.
 
This is a customization of the intersectCircleLine Javascript function found here:
 
http://www.kevlindev.com/gui/index.htm
 
----------------------------------------------------------------------------*/
function lineIntersectCircle(A : Point, B : Point, C : Point, r : Number = 1):Object {
	var result : Object = new Object ();
	result.inside = false;
	result.tangent = false;
	result.intersects = false;
	result.enter=null;
	result.exit=null;
	var a : Number = (B.x - A.x) * (B.x - A.x) + (B.y - A.y) * (B.y - A.y);
	var b : Number = 2 * ((B.x - A.x) * (A.x - C.x) +(B.y - A.y) * (A.y - C.y));
	var cc : Number = C.x * C.x + C.y * C.y + A.x * A.x + A.y * A.y - 2 * (C.x * A.x + C.y * A.y) - r * r;
	var deter : Number = b * b - 4 * a * cc;
	if (deter <= 0 ) {
		result.inside = false;
	} else {
		var e : Number = Math.sqrt (deter);
		var u1 : Number = ( - b + e ) / (2 * a );
		var u2 : Number = ( - b - e ) / (2 * a );
		if ((u1 < 0 || u1 > 1) && (u2 < 0 || u2 > 1)) {
			if ((u1 < 0 && u2 < 0) || (u1 > 1 && u2 > 1)) {
				result.inside = false;
			} else {
				result.inside = true;
			}
		} else {
			if (0 <= u2 && u2 <= 1) {
				result.enter=Point.interpolate (A, B, 1 - u2);
			}
			if (0 <= u1 && u1 <= 1) {
				result.exit=Point.interpolate (A, B, 1 - u1);
			}
			result.intersects = true;
			if (result.exit != null && result.enter != null && result.exit.equals (result.enter)) {
				result.tangent = true;
			}
		}
	}
	return result;
}
 

This entry was posted on Tuesday, August 5th, 2008 at 3:45 am and is filed under ActionScript 3, Flash 9.You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.