[Week2.3]JPEG采用8×8分块处理


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JPEG分块对图像进行处理

  JPEG压缩图像的第一步,是将图像分解成一个个8×8的小图像,之后再分别对这些小图像进行变换量化编码。
02_03 - Video -[00_04_23][20140407-001045-0]

为什么JPEG要使用8×8的分块?

1. 分块小,比如2×2,图像还原质量差;
2. 分块大,比如将整幅图作为一块,消耗计算资源多;
平衡质量和资源,JPEG默认使用8×8的分块。当然,我觉得一定是有更深层原因的....
02_04 - Video -[00_21_06][20140408-233518-0]

用YCrCb表示颜色

  JPEG是“色盲”的,它不能直接作用于彩色图像。要处理彩色图像,显而易见的解决办法是分别对RGB三个通道进行处理,后果是,图像中三种颜色的联系被活活的拆散了。因此,JPEG采用YCrCb来表示颜色。
  想当年,黑白电视占主流,彩色电视刚面世。为了使黑白电视也能看到彩色电视信号装载的节目,聪明的人类想到了一种办法:用Y通道表示图像的亮度,CrCb表示色差,这样黑白电视只接收Y通道的信号,就能看到黑白图像;彩色电视则通过YCbCr,来获得彩色图像。也因为人对图像的亮度比较敏感,可以用更多数据来传输Y通道,用更少数据来传送CbCr通道,从而提升图像压缩率。
  RGB与YCbCr可以通过线性运算互相转换。
从RGB到YCbCr:




从YCbCr到RGB: