@property<EOL><INDENT>def isEmpty(self):<DEDENT>
if self.isRectangular:<EOL><INDENT>return False<EOL><DEDENT>eps = <NUM_LIT><EOL>ul = Point()<EOL>ur = (self.ur - self.ul).abs_unit<EOL>lr = (self.lr - self.ul).abs_unit<EOL>ll = (self.ll - self.ul).abs_unit<EOL>if max(ur.y, lr.y, ll.y) - min(ur.y, lr.y, ll.y) < eps:<EOL><INDENT>return True<EOL><DEDENT>return False<EOL>
Check if quad is empty retangle. If rectangular, we are done (not empty).
        But all 4 points may still be on one line. We check this out here.
        In that case all 3 lines connecting corners to ul will have same angle with
        x-axis.