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//右回りの座標
var _pts1:Vector.<Point>	= Vector.<Point>( [
	new Point( -50, -50 ),
	new Point(  50, -50 ),
	new Point(  50,  50 ),
	new Point( -50,  50 )
] );
 
//左周りの座標
_pts2:Vector.<Point>	= Vector.<Point>( [
	new Point( -50, -50 ),
	new Point( -50,  50 ),
	new Point(  50,  50 ),
	new Point(  50, -50 )
] );
 
//符号付面積を返す
function getArea( pts:Vector.<Point> ):Number{
	var S:Number	= 0;
	for( var i:uint=0; i<pts.length; i++ ){
		var a:Point = pts[i];
		var b:Point = ( i<pts.length-1 ) ? pts[ i+1 ] : pts[0];
		S += a.x * b.y - a.y * b.x;
	}
	return S / 2;
}
 
//右回りの座標より取得
getArea( _pts1 );
 
//左回りの座標より取得
getArea( _pts2 );