Patent GB-1434783-A (GB)
Title: Apparatus and methods for computer graphics
IPC: G06F3/0484
Filed: 1973-03-09
Abstract: 1434783 Cathode - ray tube displays NATIONAL RESEARCH DEVELOPMENT CORP 4 March 1974 [9 March 1973] 11433/73 Heading H4T In an arrangement for carrying out spatial transforms a display is produced on a cathoderay tube (CRT) 41 (Fig. 1) in response to serially received data from a store 10, an operator, using a stylus in co-operation with a graphical input tablet 13 provided with an "overlay" display of X and Y co-ordinates, selects a point the X, Y co-ordinates of which are supplied in digital form via A/D converters 14, 15 to a combiner 16 (some of the least significant bits being omitted) to form a number which is compared in comparator 17 with the number in a counter 12 (clocked on via OR gate 53 on the termination of each display operation of the generators 42, 55 or 56), the output of the comparator then operating via modulator 18 to intensify (or flash or otherwise emphasise) the portion of the display having a predetermined relationship to the point selected on tablet 13. Thus, as shown in Figs. 2a, 2b, when the CRT display comprises a family of parabolae 21, ... 28 each defined by X = AT<SP>2</SP>, Y = 2AT, the overlay on the tablet indicates the value of A and T and selection of, e.g. the point 30 causes, e.g. the point 31 on the display to be emphasized, movement of the stylus along the line 32 causing the trace (shown in heavy black) to be emphasized. Figs. 3a, 3b show a spatial transform relating to data on different mathematical planes. As shown, the CRT displays a Nyquist plot on the S plane in the form of segments between frequency values f 3 , f 4 , f 5  for various values of system performance a 1 , a 2 , a 3  ... a 8  (e.g. gain) and the tablet "overlay" shows the boundaries between segments and the system performance. Thus if the stylus selects point 35 the segment 36 between frequencies f 3  and f 4  for system performance a 3 , is emphasized. In another example Figs. 4a, 4b, the CRT display is of the physical position of circuit components and selection of an element on the "overlay", which comprises a circuit diagram causes the actual component to be emphasized. Instead of identifying each instruction read from store 10 by the output of OR gate 53 the counter 12 and address generator 54 may be clocked by a pulse generator providing pulses at intervals which are separated by the greatest time required for any vector, function or character to be displayed (Fig. 5, not shown). This arrangement is also useful when the tube 41 is a storage tube and each data item is read only once (i.e. no refresh is necessary). In this case the recycling loop around the store 10 is omitted. In a further arrangement counter 12 is omitted and each data item in store 10 ineludes a code identifying the item such code being held in a register the output of which forms one input (formally that from counter 12) to comparator 17 (Fig. 6, not shown). The tablet 13 (Fig. 1) may be replaced by a cathoderay tube (Fig. 7, not shown) having the "overlay" as a static display produced by a display generator, point being selected by a "light-pen". When the tube is of the storage type so that refresh of the "overlay" is not required, the display generator may provide signals for displaying a non-stored cursor the X-Y positions of which are adjusted by manual controls. Fig. 8, shows an embodiment in which the co-ordinates of a point on display CRT 63 selected by a light-pen 71 are compared in comparator 77 with the co-ordinates of a succession of displays on a second display CRT 70 the comparator acting to emphasize a portion of the second display when its two inputs are equal. This arrangement is termed a "store position to store position transformation". Where a "store position to spatial co-ordinate transformation" is required the number held in register 76 (Fig. 8) may be divided by a logic circuit into two portions controlling, respectively, the X and Y inputs of an XY plotter (Fig. 9, not shown). Fig. 10 shows an alternative arrangement for producing "store position to store position transformation" in which stores 84, 85 clocked by 83 produce a succession of displays on respective CRT's 81, 82. A light-pen 86 selects a portion of the display on CRT 81 and causes the CRT 82 to emphasize the portion of display corresponding to the data read out from store 85 at the same time as the selected data is read out from store 84.