Discrete Cosine Transform. As a basic operation of MPEG video compression it is widely used as the first stage of compression of digital video pictures. DCT operates on blocks (hence DCT blocks) of the picture (usually 8 x 8 pixels) resolving their content into frequencies, giving an amplitude for each frequency component. In itself DCT may not reduce the amount of data but it prepares it for following processes that will. Besides MPEG, JPEG, VC9, WM9 and DV compression all depend on DCT. The use of blocks can lead to blocks being visible on screen where data rates that are too low are used.