A vanishing point or a vanishing line indicates the direction information of a set of parallel lines or a set of parallel planes.The vanishing points of all lines in the same plane are colinear at the vanishing line of the plane.The cross ratio of four points in the vanishing line is a projective invariant.It is related inherently with the intersection angles among the lines that the vanishing points correspond.So

this paper presents an approach based on the projective invariants of vanishing points to calculate the intersection angles of planar lines.Without any calibration or 3D reconstruction

the approach can be used to describe the relationship between planar lines

such as parallelism

perpendicularity or intersection

and so on.Our method is applied to some real images for interpreting the relationship of lines

and the experimental results verify its efficacy.