Comments on: Line-Line Intersection in AS3

By: Michael

Michael — Sat, 17 Dec 2011 21:18:58 +0000

Just wanted to add that i have tried Keith Hairs code and it seems to work much better:
http://keith-hair.net/blog/2008/08/04/find-intersection-point-of-two-lines-in-as3/

By: Michael

Michael — Sat, 17 Dec 2011 21:07:41 +0000

Sorry, your code doesn’t work.

// this will give null
var p1= new Point(x=343.4729858398438, y=351.9473937988281);
var p2= new Point(x=628.6473937988281, y=351.9473937988281);
var p3= new Point(x=486.06018981933596, y=494.5345977783203);
var p4= new Point(x=486.06018981933596, y=209.36018981933594);
trace (intersection(p1,p2,p3,p4));

// but this will draw two intersecting lines
graphics.lineStyle(1);
graphics.moveTo(p1.x,p1.y);
graphics.lineTo(p2.x,p2.y);
graphics.moveTo(p3.x,p3.y);
graphics.lineTo(p4.x,p4.y);

By: Lucas

Lucas — Tue, 11 Oct 2011 05:55:49 +0000

if you use int instead of number works well =)

By: Lucas

Lucas — Tue, 11 Oct 2011 05:50:00 +0000

trace(new GLine(475.5, 468.05, 475.5, 468.05 + 57.099999999999966).intersectsLinePoints(pA, pB)); // this return null

By: Lucas

Lucas — Mon, 10 Oct 2011 23:05:05 +0000

I create framework with this code, and i have problem to specific coors:

var r:GRectangle = new GRectangle(475.5, 468.05, 475.5 + 52.39999999999998, 468.05 + 57.099999999999966);
var pA:GPoint = new GPoint(183, 504);
var pB:GPoint = new GPoint(883, 504);
trace(r.intersectsLinePoints(pA, pB)); // return FALSE

By: Robbe

Robbe — Sat, 05 Mar 2011 10:19:57 +0000

Thanks! This will really come in handy.
(and also thanks john for pointing out the performance thingy, because I was about to go and test it myself :o)

By: john

john — Fri, 10 Dec 2010 20:13:25 +0000

over 1 1/2 years… THANKS! this is a really fast function, It out performs Keith Hairs version which i was currently using.

By: Adam

Adam — Mon, 13 Jul 2009 20:11:38 +0000

Before using this for something like collision detection in a game, you will want to expand your analysis for the cases where your determinant is zero. A zero determinant (d==0) tells you there is no UNIQUE point of intersection between the lines passing through your points. The lines are either parallel or collinear. But… your function is not a line-line intersection function, it’s really a line segment intersection function, so there are more cases to breakdown when d==0:

You could have a valid intersection if the line segments are collinear and share a point. Consider the simple case where all 4 points are on the same line, and the segments share an endpoint. Your function will return null, but it really should return the intersection point (the common endpoint). Ditto for the completely degenerate case where p1==p2==p3==p4.

In the case where your segments are collinear and overlap by more than an endpoint, the most correct thing to do would be to return the segment that represents the intersection. This will require a more general return type (a segment type, i.e. two points).