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摘   要:压缩的理论基础是信息论。从信息论的角度来看,压缩就是去掉信息中的冗余,即保留不确定的信息,去掉确定的信息(可推知的),也就是用1种更接近信息本质的描述来代替原有冗余的描述。这样就可以减轻网络带宽日益紧张的问题。图像压缩1般通过改变图像的表示方式来达到,因此压缩和编码是分不开的。
JPEG采取多种编码方式,包含有行程编码(Run Length Coding)和哈夫曼(Huffman)编码,有很高的压缩比。在编码前,先对数据进行分块,离散余弦变换(DCT)及量化,保留能量大的低频信号,丢弃高频信号以达到压缩。解码时,进行熵解码,反量化,反离散余弦变换(IDCT)。
The technology of Graphics compression or decompression that is based on JPEG
Abstract:The theory of compression bases on the theory of information. From the theory of information, compress means that to get rid of the redundant information. In other words, it means that to reserve the uncertain information but get rid of the certain information which can be inferred.,And this can relieve the problem that the network band—width  is  busier  day by day. In general, compress picture can be always done by change the express way of the pictures. Therefore, the compress and encode can not be separated.
While conveying pictures we often encode still pictures into the style of JPEG.JPEG is of lossy compression which can not recover all the image data,some data are losed. Although we refer to non-lossy compression, however, the compression ratio of lossy compression( the times that data bytes before compression to that after compression )is bigger than that of non-lossy compression.JPEG encoding translate image color from RBG to luminance Y and chroma Cr、Cb. Because people’ eyes are less sensitive to chroma comparing with luminance , some of chroma data are abandoned to reduce data size.
JPEG envolves some kinds of ways of encoding including Run Length Coding and Huffman Coding, so it has high compression ratio. Before Coding ,we divide data into blocks , DCT and quantify each block . The low-frequency signals that have more energy are maintained and others are rejected . While decoding , data stream are readed , decoded , inverse-quantified and then IDCT .
This paper introduces the encoding and decoding of JPEG and the specific realization of program on VisualC++  platform.The encoding part of this program can encode a picture whose format is 24-bits-turecolor BMPg and save relevant data as binary system.The decoding program can decode the compression data and reconstruct the origin image.
Keywords: DCT; Entropy encode; Huffman encode; bmp; jpeg


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