- DCT is a Fourier-related transform similar to DFT, but using only real number.

- DCT is similar to the Fast Fourier Transform (FFT), but can approximate lines well with fewer coefficients (see following figure, the effect after IDCT and IDFT).
- The DCT concentrates most of the power on the lower frequencies.

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DFT:

DCT:

DCTs use only cosine functions, while DFTs use both cosine and sine functions.

The two-dimensional DCT is:

some of above are from http://en.wikipedia.org/wiki/Discrete_cosine_transform

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