Rotate 'N' Roll Dave Millen Welwyn Garden City, Hertfordshire. Rotate 'N' Roll is an intriguing program for any Spectrum written in Basic, which enables the operator to produce three-dimensional wire frame images of prisms or poly- hedrons. The number of sides is optional as is the size of the drawing. The shapes can be shown as a series of single frame movements or can be superimposed to create patterns. Apart from the fascination of the patterns produced, the program can be a valuable aid to secondary school teachers of technical graphics to help pupils to visualise objects in movement. The rotate facility turns clockwise around a vertical axis and the roll turns the object clockwise around a horizontal axis. The size increase gives the impression of movement towards the viewer. A combination of superimposed images using all three facilities simulates a solid moving in space, showing the loci of the sides for specified movements. The program can be paused at any stage of drawing by holding down M. To continue or abort the drawing, follow the instructions on the screen. Watching the Demo mode will give the operator an idea of what can be achieved. In Run mode the operator is required to program in his/her instructions. Here is a brief expla- nation of each input in turn. Each input must be terminated with Enter. Mistakes can be rectified by pressing Caps Shift and Delete. Polyhedron - a 3D shape with a flat base, number of sides optional, and lines from each corner joining in a point. Example: pyramid. Prism - a 3D shape with a flat base and top, number of sides optional. Lines from each corner join the base and top. Example: cube. Enter number of sides - This is self-explanatory. Enter Size (0-140) - Each unit represents one pixel on the vertical axis at 0 rotation and roll. 140 units is the maximum size, otherwise the program will stop drawing with the message "figures too big" when the roll approaches 45. Enter initial rotation - To establish the starting posi- tion of the object in degrees. 0 degrees rotation gives a side view with a flat face towards the viewer. 45 deg. rotation of a cube would give a corner towards the viewer. Enter initial roll - Again establishing the starting position of the object in degrees. 0 degrees roll gives a side view of the object with the face towards the viewer determined by the viewer. The top of the object rolls to- wards the viewer. 90 deg. roll will give a top view of the object. Repeat (Y/N) - If the viewer wishes just a drawing of the object already entered then N should be entered. If a sequence of drawings is required then enter Y. How many - Number of drawings required in the sequence. Enter size inc/dec - To increase or decrease the size of the object during a sequence. To avoid exceeding the maxi- mum size the initial size should be subtracted from 140 and the result divided by the number of repeats previously entered. When decreasing in size, enter a minus number in response to the prompt. If the size reaches 0 before the end of the sequence it will continue but start increasing in size. Enter rotation increase - The number of degrees by which the object must rotate each time, in a sequence. 360 deg. gives a complete rotation and would give no change. A minus number causes the object to rotate anticlockwise. Enter roll increase - The number of degrees by which the object must roll towards the viewer each time, in a se- quence. 360 deg. gives a complete roll. A minus number causes the object to roll away from the viewer. Superimpose? (Y/N) - If drawings are required as separate images one after another then enter N. To overlay each successive drawing onto the previous drawings, in sequence, enter Y. Enter paper colour - Use keys 0 to 7 to enter your de- sired paper colour. Enter ink colour - Use keys 0 to 7 to enter your desired ink colour. The program will now proceed to follow the instructions entered. Details of each drawing are printed on the left of the screen display. Rotate 'n' roll is very easy to use and provides endless fascination for young and old alike.