Data Compression: The Complete Reference

Glossary 861Dictionary-Based Compression. Compression methods (Chapter 3) that save pieces ofthe data in a “dictionary” data structure (normally a tree). If a string of new data isidentical to a piece already saved in the dictionary, a pointer to that piece is output tothe compressed stream. (See also LZ Methods.)Differential Image Compression. A lossless image compression method where eachpixel p is compared to a reference pixel, which is one of its immediate neighbors, and isthen encoded in two parts: a prefix, which is the number of most significant bits of pthat are identical to those of the reference pixel, and a suffix, which is (almost all) theremaining least significant bits of p. (See also DPCM.)Digital Video. Digital video is a form of video in which the original image is generated,in the camera, in the form of pixels. (See also High-Definition Television.)Digram. A pair of consecutive symbols.Discrete Cosine Transform. A variant of the discrete Fourier transform (DFT) thatproduces just real numbers. The DCT (Sections 4.6 and 4.8.2) transforms a set ofnumbers by combining n numbers to become an n-dimensional point and rotating it inn-dimensions such that the first coordinate becomes dominant. The DCT and its inverse,the IDCT, are used in JPEG (Section 4.8) to compress an image with acceptable loss, byisolating the high-frequency components of an image, so that they can later be quantized.(See also Fourier Transform, Transform.)Discrete-Tone Image. A discrete-tone image may be bi-level, grayscale, or color. Suchimages are (with few exceptions) artificial, having been obtained by scanning a document,or grabbing a computer screen. The pixel colors of such an image do not varycontinuously or smoothly, but have a small set of values, such that adjacent pixels maydiffer much in intensity or color. Figure 4.61 is an example of such an image. (See alsoBlock Decomposition, Continuous-Tone Image.)Discrete Wavelet Transform. The discrete version of the continuous wavelet transform.A wavelet is represented by means of several filter coefficients, and the transform is carriedout by matrix multiplication (or a simpler version thereof) instead of by calculatingan integral. (See also Continuous Wavelet Transform, Multiresolution Decomposition.)Dithering. A technique for printing or displaying a grayscale image on a bi-level outputdevice, such as a monochromatic screen or a black and white printer. The tradeoff isloss of image detail. (See also Halftoning.)DjVu. Certain images combine the properties of all three image types (bi-level, discretetone,and continuous-tone). An important example of such an image is a scanned documentcontaining text, line drawings, and regions with continuous-tone pictures, such aspaintings or photographs. DjVu (pronounced “déjà vu”), is designed for high compressionand fast decompression of such documents.It starts by decomposing the document into three components: mask, foreground, andbackground. The background component contains the pixels that constitute the picturesand the paper background. The mask contains the text and the lines in bi-level form(i.e., one bit per pixel). The foreground contains the color of the mask pixels. The