<title>Grayscale/resolution trade-off for text: model predictions and psychophysical results for letter confusion and letter discrimination</title>

In a series of papers presented in 1994 at SPIE1' SID2 and IDRC3, we examined the grayscale/resolution trade-off for natural images displayed on devices with discrete pixellation, such as AMLCDs. In the present paper we extend this study by examining the grayscale/resolution trade-off for text images on discrete-pixel displays. Haiftoning in printing is an example of the grayscale/resolution trade-off. In printing, spatial resolution is sacrificed to produce grayscale. Another example of this trade-off is the inherent low-pass spatial filter of a CRT, caused by the point-spread function of the electron beam in the phosphor layer. On a CRT, sharp image edges are blurred by this inherent low-pass filtering, and the block noise created by spatial quantization is greatly reduced. A third example of this trade-off is text anti-aliasing, where grayscale is used to improve letter shape, size and location when rendered at a low spatial resolution. There are additional implications for display system design from the grayscale/resolution trade-off. For example, reduced grayscale can reduce system costs by requiring less complexity in the framestore, allowing the use of lower cost drivers, potentially increasing data transfer rates in the image subsystem, and simplifying the manufacturing processes that are used to construct the active matrix for AMLCD (active-matrix liquid-crystal display) or AMTFEL (active-matrix thin-film electroluminescent) devices. Therefore, the study of these trade-offs is important for display designers and manufacturing and systems engineers who wish to create the highest performance, lowest cost device possible. Our strategy for investigating this trade-off is to generate a set of simple test images, manipulate grayscale and resolution, predict discrimination performance using the ViDEOSSarnoff Human Vision Model (see Larimer et al., 1994), conduct an empirical study of thscrimination using psychophysical procedures, and verify the computational results using the psychophysical results.