2-D discrete cosine transform
If you apply the DCT to real data, the result is also real. The DCT tends to concentrate information, making it useful for image compression applications.
To invert the DCT transformation, use idct2
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[1] Jain, Anil K., Fundamentals of Digital Image Processing, Englewood Cliffs, NJ, Prentice Hall, 1989, pp. 150–153.
[2] Pennebaker, William B., and Joan L. Mitchell, JPEG: Still Image Data Compression Standard, Van Nostrand Reinhold, 1993.