Although line drawings consist of only line segments on a plane

they convey much information about the three-dimensional object structures. For a computer interpreting line drawings

some intelligent mechanism is required to extract three-dimensional information from the two-dimensional line drawings. The labels of the line drawing as an efficient aidded method is take part in recovering the 3-D scene the line drawing. Many impossible configuration can be exclude by consistent labeling.In this paper

according to current the theory of labeling

a new labeling method for the line drawing of an object with tangent curve surface is proposed. The labeling rules are established. The natural line drawings can be labeled correctly by using of our labeling method. There are 38 kinds of possible junctions for a hidden-part-draw natural line drawing that is the projection of a three-dimensional object with tangent curve surface

in which there are 6 possible Y junctions

18 possible W junctions

14 possible V junctions. The labeling method is efficient to the line drawing of a complex object consisting of polyhedrons and objects with curved surfaces.