Point in Polygon

// function found here:
// http://www.alienryderflex.com/polygon/
function pointInPolygon(x:Number, y:Number, polyX:Array, polyY:Array):Boolean{
	var j:int = polyX.length - 1;
	var oddNodes:Boolean = false;
	for (var i:int=0; i < polyX.length; i++) {
    if (polyY[i] < y && polyY[j] >= y ||  polyY[j] < y && polyY[i] >= y) {

      if (polyX[i] + (y - polyY[i]) / (polyY[j] - polyY[i]) * (polyX[j] - polyX[i]) < x) {
        oddNodes = !oddNodes; 
      }
    }
    j = i; 
  }
  return  oddNodes;
}

// draw an overly complex poly and store all  x y coords
var pX:Array= new Array();
var pY:Array = new Array();
graphics.beginFill(0xCC0000);
for (var i:int = 0; i<60; i++){
	pX[i] = Math.random()*stage.stageWidth;
	pY[i] =Math.random()*stage.stageHeight;
	if (i == 0){
		graphics.moveTo(pX[i], pY[i]);
	}else{
		graphics.lineTo(pX[i], pY[i]);
	}
}
addEventListener(Event.ENTER_FRAME, onLoop);
function onLoop(evt:Event):void {
	   alpha = 1;
       if (pointInPolygon(mouseX, mouseY, pX, pY)){
		    alpha = .5;
	   }
}

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…