# DCT - Computer Definition

A technique used in signal and image processing and particularly in lossy compression techniques. DCT separates an image into discrete blocks of pixels of differing importance with respect to the overall image. DCT expresses a function or signal in terms of a sum of sinusoidal waveforms that vary in amplitude and frequency, essentially transforming the image from the spatial domain into the frequency domain. In the process, the average luminance of each block is evaluated using the DC coefficient. Transform compression is based on the premise that the low-frequency components of a signal are more important than the high-frequency components.Therefore, a substantial reduction in the number of bits used to represent a high-frequency component will degrade the quality of the image only slightly. JPEG and MPEG both specify DCT, as do the ITU-T H.261 and H.263 videoconferencing standards. See also amplitude, DC, frequency, H.261, H.263, ITU-T, JPEG, lossy compression, MPEG, pixel, signal, and sine wave.

(**D**iscrete **C**osine **T**ransform) An algorithm that is widely used for data compression. Similar to Fast Fourier Transform, DCT converts data (pixels, waveforms, etc.) into sets of frequencies. The first frequencies in the set are the most meaningful; the latter, the least. To compress data, the least meaningful frequencies are stripped away based on allowable resolution loss. DCT is used to compress JPEG, MPEG, DV and H.263 frames.