鉴于H.261是多媒体通信领域中图象编码的重要算法，因此实现H.261算法的四画面合成具有重要意义，因传统四画面合成是在模拟域中完成，故存在成本较高和质量较差的问题，为了降低其实现成本及提高合成质量，首先提出了一种在压缩域中实现四画面合成的算法；然后在详细地论述H.320和H.323两种视频会议框架下的四画面合成方法（包括合成原理）的基础上，给出了该合成算法以及程序流程；最后给 该算法的实验结果，并和模拟域的实现方法进行了比较，实验结果表明，不仅合成质量有了提高，而且实现成本也有料大的降低。

H 261 is an important algorithm for video composition in multimedia communication field. Therfore, it is very significant to implement four-image composition of H 261 algorithm. The traditional four-image is in analog domain. So there is a problem that the cost of implementation is high and its quality isn't very satisfying. In order to lower the cost of implementation and enhance its quality, this paper provides a kind of algorithm of four-image composition which is completed in compressed domain. This paper discusses in detail four-image composition methods in the two kinds of videoconference systems of both H 320 recommendation and H 323 recommendation. First, the composition principle is given. Then based on this principle, the composition algorithm and program flow chart are provided. Finally, its experiment's result is given and compared with that of implementation in analog domain. According to the result we can find not only its quality is enhanced but also its cost is lowered largely.