Abstract: For the sake of efficient storage and transmission, more and more images are compressed, thus compressed domain image enhancement techniques are attracting more and more attention. This paper illustrates the background of Discrete Cosine Transform（DCT） domain color image enhancement techniques. Among these algorithms, they often suffer from blocking artifact around the boundaries of image blocks. The blocking artifact reason of recently published Color Image Enhancement by Scaling（CES） algorithm is analyzed, and then two improved methods are proposed to suppress the blocking artifact. Simulation results show that the improved methods can not only perform well in suppressing blocking artifact, but also can decrease the computation complexity.

Key words: image enhancement, DCT（Discrete Cosine Transform）, blocking artifact, LPF（Low-Pass Filtering）

摘要： 为了减少图像的存储量，提高图像的传输效率，大量的图像被压缩，因此基于压缩域图像的增强技术越来越引起人们的重视。介绍了DCT域彩色图像增强技术的各种算法，其问题是会在图像块的交界处产生块状效应。着重分析了CES算法产生块状效应的原因，提出了两种改进方法来抑制块状效应。仿真结果表明，改进的算法不仅能够很好地抑制块状效应，而且降低了计算的复杂度。

关键词: 图像增强, 离散余弦变换, 块状效应, 低通滤波