Abstract: A display system having a display memory, a palette and a digital-to-analogue converter stage is able to support both an indirect color mode and a direct color mode. In the indirect color mode. pixel data from the display memory is used to access chrominance and/or luminance data from the palette for conversion in the converter stage into analogue values for controlling a display device. In the direct color mode. pixel data in the display memory directly specifies the chrominance and/or luminance data for conversion in the converter stage into analogue values for controlling the display device. In the direct color mode, however, the converter stage is also responsive to data from the palette for producing the analogue values for controlling the display device. The invention permits the provision of direct as well as indirect color modes in a flexible and efficient manner, with the provision of a plurality of formats for the direct color modes being achieved by loading appropriate information into the palette.

Abstract: A curve generator for a display system with an arc generator has for generating an arc (10) of a circle (14) from data defining the locations of two end points P.sub.1 and P.sub.3 and an intermediate point P.sub.2 on the arc. The arc generator has an initializer (40) for calculating the angle subtended between a first vector (21), from a first of the end points, P.sub.1 to the intermediate point P.sub.2, and a second vector (32), from the second of the end points P.sub.3 to the intermediate point P.sub.2, and an arc plotter (44) for defining a succession of further vectors n1 from the first end point P.sub.1 and for calculating, for each further vector, its point of intersection P.sub.n with a counterpart vector 3n, from the second end point P.sub.3, with which it subtends the same angle, whereby a succession of further points P.sub.n are plotted on the circular arc.

Abstract: The specification describes a method of storing alphanumeric characters in a graphics display terminal comprising a raster-scan display device and a refresh buffer including a plurality of bit planes each having a respective bit storage location corresponding to each addressable pel position on the screen of the display device. In the method, a first bit plane stores high resolution luminance data defining alphanumeric characters each as a selection of "on" bits within a respective n.times.m array where n is the width of the character box in the scan line direction, and at least one further bit plane stores low resolution color data for the characters. The attribute plane comprises a respective n-bit set of storage locations which corresponds to each n-bit wide by one pel deep portion of a character box in the luminance plane and defines at least the color and/or intensity of the foreground and background of the character for the width of the character box in respect of a single scan line.

Abstract: In order to compensate for image distortion introduced into a digitally-controlled raster-scan CRT by the finite video amplifier rise and fall times, the digital video drive waveform is subject to selective pulse stretching to extend where possible the duration of pels which represent critical features of the image. This is achieved by decoding means for examining each pel at least in relation to its two immediate neighbors on either side in order to detect predetermined relationships between the values of the pels, and retiming means for selectively advancing or delaying the transitions between consecutive pels of different value in accordance with the relationships so detected.