利用离散余弦变换函数（dct2和idct2）模拟JPEG压缩过程。 对“lena.bmp”做压缩，观察压缩效果。（JPEG压缩过程： 首先把原始的图像划分为8×8的子图像，然后利用dct2分别

对各个子图像进行离散余弦变换，对变换系数仅保留左上角15个值， 然后利用这15个系数进行离散余弦反变换重新得到各个子图像，再 将子图像拼接成完整图像） -Using discrete cosine transform function (dct2 and idct2) analog JPEG compression process. On " lena.bmp"　do compression, the compression effect was observed. (JPEG compression process: First, the original image is divided into 88 sub-image, and then use dct2　of each sub-image, respectively, discrete cosine transform, the upper left corner of the transform coefficients to retain only 15 values, then use the 15 coefficients discrete cosine transform regain each sub-image, and then sub-images together into a complete picture)