DCT image fidelity metric and its application to a text-based scheme for image display
D. Amnon Silverstein, Stanley A. Klein · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1993
The discrete cosine transform (DCT) can be used to transform two images into a space where it is easy to obtain an estimate of their perceptual distance. We used this method to find the closest fit of the ASCII symbols (which includes the English alphabet, numbers, punctuation, and common symbols) to rectangular segments of a gray-scale image. Each segment was converted into a DCT coefficient matrix which was compared to the coefficient matrix of each ASCII symbol. The image segment was replaced with the symbol that had the least weighted Euclidean distance. Thus, a page of text was generated that resembled the original image. The text image format has the advantage that it can be displayed on a non-graphic terminal or printer. It can also be sent via electronic mail without requiring further processing by the receiver. The processing scheme can also be used to preview stored images when transmission bandwidth is limited or a graphic output device is unavailable.