Discrete Cosine Transform, much like the Discrete Fourier Transform (i.e. what most people think of as the Fourier Transform or even FFT), when all your data are real.

The Discrete Cosine Transform (DCT) is a method of reformating data which helps separate an image into spectral sub-bands of differing importance. The DCT is similar to the Discrete Fourier Transform (DFT) in that it transforms a signal or image from the spatial domain to the frequency domain. However, the DCT does a better job of concentrating energy into lower order coefficients the DFT for image data.

This is one part in several steps to generating a JPEG or MPEG compressed file.

This method is used in compressing video files, too. It stores the data with frequency instead of spatial domains. Mpeg uses DCT to encode the spatial information into the frequency domain. The way you de-code it is to use the iDCT (Inverse Descrete Cosine Transform).

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