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基于JPEG的图像压缩与解码技术

毕业论文

基于JPEG的图像压缩与解码技术
 
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。
在图象传送过程中,我们经常采用JPEG格式对静态图象进行编码。JPEG基本系统是1种有损编码,无法完全恢复出原图象,信息有1定的丢失,称为有损压缩。尽管我们希望能够无损压缩,但是通常有损压缩的压缩比(即原图象占的字节数与压缩后图象占的字节数之比,压缩比越大,说明压缩效率越高)比无损压缩的高。JPEG编码先把图象色彩RBG变成亮度Y和色度Cr、Cb,它利用人的视觉对色度不敏感的特点,减少1部分色度数据,以达到压缩。
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)。
论文介绍了用VC++实现JPEG编解码技术,该程序的编码部分能把1张BMP格式的24位真彩色图象进行JEPG编码,压缩成以2进制形式保存的文件;通过相应的解码程序又可以把图象解压缩出来。
关键字:离散余弦变换;熵编码;Huffman编码;bmp;jpeg
 
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

注释:不含源代码

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