此代码是在保持信噪比的情况下，进一步提高了压缩比。灰度级的减少，势必会降低信噪比，但因为是处以比较小的整数，并没有引起熵值的剧烈变化，编码后信噪比基本不变。-This code is to maintain the signal to noise ratio of the case, to further improve the compression ratio. Gray-scale reduction, will inevitably reduce the signal to noise