Vectorization refers to conversion from raster to vector data, which is often called raster vector conversion. Vectorization is not very easy as compared with rasterization, because vector format needs topological structure, for example, direction of line or chain, boundaris and nodes of polygons, order of series of chains that form a polygon, left and right polygons ID of a chain and so on.

A simple algorithm of vectorization is explained in Figure 3.8, in which the original image in raster format is converted to vector data through thinning and chain coding (see 2.6). This algorithm is useful to convert raster image to vector data with the coordinates, but it is not sufficient because the agorithm will not build topological structure.

Raster vector conversion with automatic building of topology is possible if 2 x 2 window is continuously moved along the boundary from a node. The boundary and node can be identitified by the method as described in 2.6 and Figure 2.8.

Figure 3.9 shows schematically the raster vector conversion by which left and right polygons are identified.

In order to automate raster vector conversion as much as possible, a clean image without noises or unnecessary marks should be scanned in the beginning.