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1、Chapter9 ImageCompressionStandards2021/8/1219.1TheJPEGStandard*9.2TheJPEG2000Standard*9.3TheJPEG-LSStandard*9.4Bi-levelImageCompressionStandards2021/8/1229.1TheJPEGStandardJPEGisanimagecompressionstandardthatwasdevelopedbythe"JointPhotographicExpertsGroup".JPEGwasf
2、ormallyacceptedasaninternationalstandardin1992.JPEGisalossyimagecompressionmethod.ItemploysatransformcodingmethodusingtheDCT(DiscreteCosineTransform).2021/8/123ObservationsforJPEGImageCompressionTheeffectivenessoftheDCTtransformcodingmethodinJPEGrelieson3majorobser
3、vations:2021/8/124muchoftheinformationinanimageisrepeated,hence"spatialredundancy".thespatialredundancycanbereducedbylargelyreducingthehighspatialfrequencycontents.chromasubsampling(4:2:0)isusedinJPEG.2021/8/1252021/8/1269.1.1MainStepsinJPEGImageCompressionTransfor
4、mRGBtoYIQorYUVandsubsamplecolor.DCTonimageblocks.Quantization.Zig-zagorderingandrun-lengthencoding.Entropycoding.2021/8/1272021/8/1282021/8/129DCTonimageblocksEachimageisdividedinto88blocks.The2DDCTisappliedtoeachblockimagef(i,j),withoutputbeingtheDCTcoefficientsF(
5、u,v)foreachblock.Usingblocks,however,hastheeffectofisolatingeachblockfromitsneighboringcontext.ThisiswhyJPEGimageslookchoppy("blocky")whenahighcompressionratioisspecifiedbytheuser.2021/8/12102021/8/1211Quantization2021/8/12122021/8/12132021/8/12142021/8/12152021/8/
6、12162021/8/12172021/8/1218DPCMonDCcoefficientsTheDCcoefficientsarecodedseparatelyfromtheACones.DifferentialPulseCodeModulation(DPCM)isthecodingmethod.2021/8/1219EntropyCoding2021/8/12202021/8/1221附图源图像亮度数据块JPEG编码实例2021/8/1222附图量化结果2021/8/1223附表Zigzag排列及行程编码与哈夫曼编码（V
7、LC）结果2021/8/1224附表Zigzag排列及行程编码与哈夫曼编码（VLC）结果2021/8/1225例题2——亮度子快编码设亮度子快按Z字排序的系数如下：K0123456789~303132~63系数125-20200010-101）DC系数12，落入（-15~-8，8~15）2）2021/8/1226例子：512,57,45,0,0,0,0,23,0,-30,-16,0,0,1,0,0,0,0,0,0,0,..,0经过RLC压缩后就是(0,57);(0,45);(4,23);(1,-30);(0,-16);(
8、2,1);EOB由于后面huffman编码的要求,每组数字前一个表示0的数量的必须是4bit,就是说,只能是0~15,所以我们实际这样编码: (0,57);(15,0)(2,3);(4,2);(15,0)(15,0)(1,895),(0,0)注意：(15,0)表示了16个连续的0.2021
