# Point in Polygon

Actionscript:
1. // function found here:
2. // http://www.alienryderflex.com/polygon/
3. function pointInPolygon(x:Number, y:Number, polyX:Array, polyY:Array):Boolean{
4.     var j:int = polyX.length - 1;
5.     var oddNodes:Boolean = false;
6.     for (var i:int=0; i <polyX.length; i++) {
7.     if (polyY[i] <y && polyY[j]>= y ||  polyY[j] <y && polyY[i]>= y) {
8.
9.       if (polyX[i] + (y - polyY[i]) / (polyY[j] - polyY[i]) * (polyX[j] - polyX[i]) <x) {
10.         oddNodes = !oddNodes;
11.       }
12.     }
13.     j = i;
14.   }
15.   return  oddNodes;
16. }
17.
18. // draw an overly complex poly and store all  x y coords
19. var pX:Array= new Array();
20. var pY:Array = new Array();
21. graphics.beginFill(0xCC0000);
22. for (var i:int = 0; i<60; i++){
23.     pX[i] = Math.random()*stage.stageWidth;
24.     pY[i] =Math.random()*stage.stageHeight;
25.     if (i == 0){
26.         graphics.moveTo(pX[i], pY[i]);
27.     }else{
28.         graphics.lineTo(pX[i], pY[i]);
29.     }
30. }
32. function onLoop(evt:Event):void {
33.        alpha = 1;
34.        if (pointInPolygon(mouseX, mouseY, pX, pY)){
35.             alpha = .5;
36.        }
37. }

This snippet shows how to test to see if a point is inside a polygon. This may not really be useful as hitTestPoint() is likely faster - however this could be modified for other purposes such as polygon to polygon collision. This algorithm works nicely on all kinds of polygons ... convex, concave etc...

I found this algorithm in an article by Darel Rex Finley ... here.

I've used this function in Processing a few times...

1. 62316e
Posted November 11, 2010 at 2:50 am | Permalink

I love u!

Btw, is there a way to get intersection point?

2. Posted November 22, 2010 at 3:59 pm | Permalink

thanks. I think this post might help with intersections.

http://actionsnippet.com/?p=956