DCT

Discrete Cosine Transform, based on Fourier-related transforms, uses real numbers to provide algorithmic compression for audio, video, and images. Roughly twice the length of a DFT (Discrete Fourier Transform The cosine in DCT implies that "some variants the input and/or output data are shifted by half a sample" and two most common variants of DCT are Type-II (DCT), Type-III (inverse or IDCT).