In this paper

we present an optimized Freeman chain coding method for bi level line structured images and discuss its application in the storage and automatic plotting of engineering drawings. Compared with the traditional Freeman chain coding that uses single ring or multi ring image quantizers

the proposed method is based on already quantized dot matrix digital images

and is a lossless coding method. We select the code length according to the statistics of the length of straight line segment in the image and thus minimize the average number of bits needed to encode the segment. Theoretical rate analysis and several experimental results show that the proposed method achieves lower bit rate than the existing one and can be easily applied to the storage and plotting of engineering drawings.