中文说明：离散余弦变换（DC T），表示一个图像作为变化幅度和频率的正弦曲线的和。所述DCT2函数计算两维离散余弦变换的图像的（DC T）。直流T有，对于一个典型的图像，大多数关于图像的视觉显著信息被集中在DC T。由于这个原因仅仅几个系数的特性，直流T被经常用于图像压缩的应用程序。例如，直流T是在被称为JPEG国际标准有损图像压缩算法的心脏。（这个名字来自于开发的标准工作组：联合图像专家组）。在实践中，DC T被通常使用相同的基本的效率的技术如在FFT算法实现
English Description：
The discrete cosine transform (DC T) represents an image as a sum of sinusoids of varying magnitudes and frequencies. The dct2 function computes the two-dimensional discrete cosine transform (DC T) of an image. The DC T has the property that, for a typical image, most of the visually significant information about the image is concentrated in just a few coefficients of the DC T. For this reason, the DC T is often used in image compression applications. For example, the DC T is at the heart of the international standard lossy image compression algorithm known as JPEG. (The name comes from the working group that developed the standard: the Joint Photographic Experts Group). In practice, the DC T is normally implemented using the same basic efficiency techniques as in FFT algorithms