点坐标旋转方法

以一已知点为基点旋转另一点的方法

        /**
         * 旋转点
         * @param x
         * @param y
         * @param x1
         * @param y1
         * @param a
         */
        _translatePoint:function(x,y,x1,y1,a){
            var l = Math.sqrt(Math.pow(2,(x1-x)) + Math.pow(2,(y1-y)));
            var cosb= (x1-x)/l;
            var sinb = (y1-y)/l;
            var sina = Math.sin(a);
            var cosa = Math.cos(a);
            var cosc = cosa*cosb-sina*sinb;
            var sinc = sina*cosb+cosa*sinb; 
            var $x = l*cosc;
            var $y = l*sinc;
            var point = [x+$x, y+$y];
            return point;
        },