Scanning display apparatus

Abstract

A head-up display involving display of pitch-bar symbols in the line of sight of an aircraft pilot is generated using raster scanning of a cathode-ray-tube screen and bright-up in accordance with stored information relating to the wings-level or other datum attitude of the aircraft. Variation of the pitch-bar mapping in the display area to follow change of attitude of the aircraft is made through transformation in accordance with pitch and bank angles applied to the point-by-point progression of the raster scan, and the visual staircase effect of symbols inclined to the line-scan is reduced by controlled variation in brightness from point to point. Each point (x.sub.d, y.sub.d) in the progression of the scan through the display area is mapped back incrementally into the corresponding point (x.sub.p, y.sub.p) of the stored datum-attitude map, through a transformation in rotation dependent on the bank angle .phi.. Computation of x.sub.p involves summation of discrete signals representative of x.sub.d cos.phi. and y.sub.d sin.phi., and of y.sub.p involves summation of discrete signals representative of -x.sub.d sin.phi. and y.sub.d cos.phi.. The eight most-significant bits of the eleven-bit computed values of x.sub.p and y.sub.p are used to read out four bits stored in respect of the point (x.sub.p, y.sub.p) to define the brightness of that point and three immediately-adjoining points in the datum-attitude mapping. Brightness-weightings applied to the four digits respectively are derived from read-only memories in accordance with the three least-significant bits of the computed values of x.sub.p and y.sub.p, and the four bits so weighted are added together to provide the bright-up video input to the cathode-ray tube for the point (x.sub.y, y.sub.d).