中文说明：利用matlab的DCT2函数，对图像做变换并压缩。并用8*8对图像分块，对每一块计算均方值作为阈值。
English Description：
The DCT2 function of MATLAB is used to transform and compress the image. 8 * 8 pairs of images are divided into blocks, and the mean square value of each block is calculated as the threshold.