code: function ( opX1, opY1, npX1, npY1, ocX, ocY, ncX, ncY ) {
          //중심점 보정
          var cGapX = ncX - ocX,
              cGapY = ncY - ocY,
              ccX = ncX - cGapX,//보정된 ncX
              ccY = ncY - cGapY,//보정된 ncY
              cX1 = npX1 - cGapX,//보정된 npX1
              cY1 = npY1 - cGapY;//보정된 npY1
  
          //중심점 기준의 좌표로 보정
          var ox = opX1 - ocX,
              oy = opY1 - ocY,
              nx = npX1 - ncX,
              ny = npY1 - ncY,
              cw = this._isClockwise( ox, oy, nx, ny ),
              angle = this._vectorToAngle( opX1, opY1, cX1, cY1, ccX, ccY );
  
          return cw? angle : -angle;
      }
docs: 움직인 각도 반환