According to three-dimensional geometrical properties, some theorems and corollaries of face identification from line
drawing with hidden lines are proposed in this paper. This paper improves the depth-first search algorithm by adding
constraints. By this means, the cycles that are unreasonable are deleted directly and the efficiency of face identification is
increased. The examples show that the face identification algorithm proposed in this paper not only can identify faces
exactly from line drawing but increases the efficiency of the original algorithm.