Vectorization of line drawing image based on junction analysis

Abstract

Converting a scanned or shot line drawing image into a vector graph can facilitate further edit and reuse, making it a hot research topic in computer animation and image processing. Besides avoiding noise influence, its main challenge is to preserve the topological structures of the original line drawings, such as line junctions, in the procedure of obtaining a smooth vector graph from a rough line drawing. In this paper, we propose a vectorization method of line drawings based on junction analysis, which retains the original structure unlike done by existing methods. We first combine central line tracking and contour tracking, which allows us to detect the encounter of line junctions when tracing a single path. Then, a junction analysis approach based on intensity polar mapping is proposed to compute the number and orientations of junction branches. Finally, we make use of bending degrees of contour paths to compute the smoothness between adjacent branches, which allows us to obtain the topological structures corresponding to the respective ones in the input image. We also introduce a correction mechanism for line tracking based on a quadratic surface fitting, which avoids accumulating errors of traditional line tracking and improves the robustness for vectorizing rough line drawings. We demonstrate the validity of our method through comparisons with existing methods, and a large amount of experiments on both professional and amateurish line drawing images.