to a given acute angle in a right triangle and the hypotenuse, as depicted in Figure 6.4. It is the reciprocal of a sine. How does discrete cosine transform work? When the information in a picture is processed, using a technique called spatial- filtering, it is transformed into a series of frequencies that mimic the eye's perception Video Communications: The Whole Picture of light and color. The high-frequency portions of the picture are the areas of greatest change; for instance, complex patterns in clothing and black letters on a white page. The low frequency elements are the parts with relatively little change across an area such as blank walls and other objects of uniform color and shape. The DCT uses the reciprocal of those frequencies to describe a pixel block. The DCT process begins by dividing a video frame into eight-by-eight pixel blocks that each contains a set of waveforms of different horizontal and vertical frequencies. These waveforms, when scaled by numbers called coefficients then added together, represent any of the 64 sample values (a coefficient is nothing more than a multiplier). For instance, in the formula a(y + z), "a" is the coefficient. Y could be the sine and Z the cosine. A coefficient is the least common denominator; that is how DCT uses it. Cosine of angle ACB = AC/BC Figure 6-4. Cosine (Michel Bayard). After dividing a picture into blocks of pixels, the DCT scanning procedure starts in the upper left-hand corner of a block and finds the average luminance for the entire block. This value is the DC coefficient, and it is the first number produced by the transform. The DCT scanning process continues and as it does, 63 more values are produced. The second and third are expressions of the lowest frequency in the horizontal and vertical directions. As scanning proceeds, it concentrates the low- frequency components, which describe large areas of uniform color, in the upper left corner of the array. The high-frequency components, describing detail shifts in the image (edges and color chang