TRACING MADE EASIER
ONE OF the most outstanding achievements in the graphics field is the RD tracer, a device which allows you to draw across a piece of paper with a cross hair - like a rifle sight - and have the computer copy the same thing onto the screen. The three-part arm needs to be fixed to the board on which you are working by the sticky strip provided - or Blu-tack. A template is provided to allow you to line up this and the graph-type map over which the arm works.
The map is marked in pixels and PRINT positions, so that letters and numbers can be added to the drawing. The interface black box, which contains an edge connector and PCB continuing strip, plugs into the expansion port of the ZX-81 or Spectrum. As it fits both computers it has only a 23-way edge connector and must therefore be used in the same way as the printer - placed after any Spectrum equipment.
On the ZX-81 it is memory-mapped into location 15391; on the 48K Spectrum it is input/output mapped (port 31), but if required on the 16K Spectrum it can also be memory-mapped via an adaptor. LEDs are provided on the module to indicate when it is being used and also if it is being used on the input/output map.
The tracer tape has ZX-81 and Spectrum programs on the same side. That is satisfactory for the Spectrum but ZX-81 owners should locate the ZX-81 program before attempting to LOAD the program.
The tracer has four programs for the Spectrum and two for the ZX-81. For the Spectrum the tracer can be used to create lines, circles or just to follow the trace of the arm in all the colours available. The drawing can them have areas "painted" in or shaded in the same or a different colour.
One thing to remember is that each character square can have only two colours. All the commands are available by one key press and the entire program is written in Basic so that it can be customised easily.
The other use of the tracer is to input data in the form of co-ordinates for graphs: they can be stored as strings and the tracing program made to execute the co-ordinates one after another, creating an automatic drawing routine.
The pictures can be saved on to tape from within the program and can also be loaded back for modification. The cost of the tracer is £55.50 with 10-percent discount for cash. It is available from RD Labs, 5 Kennedy Road, Dane End, Ware, Herts SG12 0LU. Tel: 0920 84380.
The drawing can be scaled to give a 3D effect, with the program taking care of the perspective. With user-definable graphics plus text and the tracer producing any other shapes, all kinds of pictures can be created.