DCT. The other, which is more prevalent in proprietary codecs, is vector quantization (VQ). DISCRETE COSINE TRANSFORM Any student of image compression will encounter DCT; it forms the basis for three very important video communications compression standards. One is the Joint Photographic Experts Group (JPEG) standard, which addresses still-image compression. Another is the Moving Picture Experts Group (MJPEG) standard that compresses TV-quality or near-TV-quality moving pictures (typically for playback applications). Furthermore, the ITU-T's H.26x Recommendations, the primary standards-based videoconferencing codecs, rely upon DCT. The DCT is a technique that converts pixel intensities into their frequency-based equivalents. To do this, it applies numeric transformation to the data contained in a pixel block. After the DCT is complete, a block of video data is described in terms of frequencies and amplitudes rather than a series of colored pixels. Frequencies describe how rapidly colors shift. For instance, high frequencies denote very rapidly shifting colors such as edges where one color stops and another starts. Amplitudes describe the magnitude of a color change associated with each shift. To understand DCT, one must possess some knowledge of sines and cosines. In trigonometry, the sine is the ratio between the side opposite an acute angle in a right triangle and the hypotenuse (the side of a right-angled triangle opposite the right angle). Examine Figure 6.3 for a straightforward picture. A sine wave is a graphical representation of the sine ratio. Remember that the polarity of an AC power source is constantly changing from positive to negative. The waveform produced from alternating current can be plotted. The result is a sine wave that undulates with the shape of the wave as it continues from zero polarity to its peak positive value, then back to zero and beyond, to its peak negative value and then back to zero. Webster describes a cosine as "the sine of the complement," the ratio between the side adjacent 