Inverse discrete cosine transform
x = idct(y)
x = idct(y,n)
The inverse discrete cosine transform reconstructs a sequence from its discrete cosine transform (DCT) coefficients. The idct function is the inverse of the dct function.
and N = length(x), which is the same as length(y). The series is indexed from n = 1 and k = 1 instead of the usual n = 0 and k = 0 because MATLAB® vectors run from 1 to N instead of from 0 to N-1.
If y is a matrix, idct transforms its columns.
 Jain, A.K., Fundamentals of Digital Image Processing, Prentice-Hall, 1989.
 Pennebaker, W.B., and J.L. Mitchell, JPEG Still Image Data Compression Standard, Van Nostrand Reinhold, 1993, Chapter 4.