Discrete Cosine Transform

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The Discrete Cosine Transform (DCT) is similar to the Discrete Fourier Transform: It transforms a signal from the time domain into the frequency domain. And just as the fourier transform uses sine and cosine waves to represent a signal, the DCT only uses cosine waves. The DCT I and II, is mostly used in image compression, while MDCT (DCT-IV) is used in audio encoding.

The DCT is an invertible, discrete orthogonal transformation.