GB1244659A - Apparatus for decoding graphic-display information - Google Patents

Abstract

1,244,659. Cathode-ray tube displays. NATIONAL RESEARCH DEVELOPMENT CORP. 17 July, 1969 [29 July, 1968], No. 36172/68. Heading H4T. In an arrangement for displaying curves (including straight lines) by orthogonal deflections of the beam of a cathode-ray tube (or the pen of an incremental plotter) the curves being specified by first and second items of information specifying respectively, in which of a plurality of sectors the curve or portion thereof is to be displayed and in which of a plurality of directions elemental straight lines making up the curve are to lie, decoding apparatus is provided comprising sector-selection means which in response to the first item of information provides output signals specifying elemental lines in the selected sector, the directions of such lines being as specified by the second item. Thus by employing a plurality of sectors (Fig. 3) in each of which the direction of the elemental lines may be parallel to either of the defining bounding lines, the accuracy with which curves are produced is increased in comparison with systems utilizing only two directions, e.g. parallel to the horizontal or at 45 degrees thereto (Fig. 2 not shown). In the operation of the present invention the complete code for one curve (which may be a portion of a larger curve) comprises: an initial bit-"beam-on"; ten bits specifying the X co-ordinate and ten bits specifying the Y coordinate of the start position of the curve; eight bits, N, specifying the number of elemental lines which make up the curve; three bits W, X, Y, specifying the sector (in this ease, octant); a number of bits m specifying the direction of each of the elemental lines and one bit c specifying whether or not the next curve starts, at the end of the curve just specified. Information coded in this manner for all the curves is deposited in a file 10 (Fig. 1) and when a display is required such information is sent, e.g. by a telephone line, to a recycling buffer-store 11 which may be a delay line from which it is supplied to the decoding apparatus (logic Hardware) 12. The flow chart of Fig. 5 indicates the requirements for the logic hardware (Fig. 6, not shown) which operates by testing (30, 31, 32) the first m bit against the w, x and y bits to provide the required change in the X, Y co-ordinates necessary to trace the first elemental line in the required direction. The number N is then decreased by 1, (37) and if the remainder is zero the beam is switched off. If not zero, the next m bit is tested against the w, x and y bits and this cycle continues until N becomes equal to zero. After each cycle the c bit is tested and if this is "1" the next curve continues from the co-ordinate portion of the end of the previous curve. If not zero the system waits for the next beam "ON" bit. Fig. 4 shows a curved character "S" produced by the system.