The scanning input and recognition of engineering drawings is a key step in CAD

and is to reuse lots of engineering drawings. In study on recognition of scanned image of engineering drawings

the recognition for circular arcs is an important and difficult problem. Recent algorithms of recognizing arcs are mainly about approximation with lines. This paper presents an algorithm for recognizing arcs and circles using Primitive Regions Adjacent Graph

which can directly extract arcs. The binary image is encoded with black horizontal runlength. A stripe region consists of correlative runlengths with the same width and topology. The stripe regions then can be segmented as some primitive regions (line and arc). The graph is used to describe geometrical property and topological constraint. The primitive region supplies shape information (line

arc

arrow etc.) improving integrality of recognition. After extraction of the arc region from regions

the seed for an arc is obtained. By traversals for the graph

the seed arc grows by constrains for the same circle. Some applications to recognize arcs and circles are finally provided

which show that the algorithm is effective and robust

can solve well intersection and tangency of between an arc and a line or an arc.