对DCT系数进行计算，使用一下

代码说明：

考虑到DCT变换中的系数要重复计算，可使用查找表来提高运行的效率，只要开始DCT变换之前计算一次，DCT变换中就可以只查找而无需计算系数。-to the DCT coefficients to be calculated, the use of look-up tables to improve operational efficiency, as long as the DCT started once before, on DCT can only search without calculation of the coefficient.