中文摘要：

文中通过大量的实验,在研究现有的基于Freeman方向链码的方法的基础上,对提高Freeman四方向链码压缩率的方法进行了深入的研究。从改变码值含义定义并对码值进行Huffman编码,进而对出现频率最高的码值进行计算编码等不同角度,进行大量的实验、比较与分析。提出了一个Freeman四方向链码新方法：计算编码不等长相对四方向Freeman链码———AVRF4。实验结果表明,其链码压缩率比Freeman八方向链码提高了26%,而比原始Freeman四方向链码提高了15%。

英文摘要：

To study the methods for improving the efficiency of 4direction Freeman chain code, the methods based on existing Freeman direction chain code is researched though a large number of experiments. A large number of experiments, comparison and analysis are carried from the different views, such as changing the definition of code elements and employing combining enco ding for code elements, and applying arithmetic encoding for the code elements with highest probability. At last, a new method based on 4direction Freeman chain code entitled arithmetic encoding variablelength relative 4direction Freeman chain code, namely AVRF4 is put forward. The experimental results show that the compressibility of AVRF4 increases 26 % more than 8di rection Freeman chain code and 15 % more than 4direction Freeman chain code.