Spirogram



          Moving in circles can produce some

          interesting results as Simon Palmer

                     demonstrates.



Almost everybody will have played with a Spirograph at some

time. It is a set of toothed wheels, like gears which pro-

duce intricate patterns. One of the gears is pinned to a

piece of paper, while the other is pushed around it by a

pen which pokes through onto the paper. The patterns which

are produced depend on the sizes of the gears, and the

position of the pen hole in the moving gear. These effects

are reproduced on a Spectrum computer by this program.

   The format for a call to the machine code program is

RANDOMIZE USR 65274: REM a,b,f,m,r where a and b are the

coordinates of the center of the pattern on the screen,

f and m are integers which are the radii of the fixed and

moving gears, and r is the distance of the pen hole from

the centre of the moving gear.



[ The rest of the article was a description of the internal

workings of the program, which makes extensive use of the

RST 28 calculator. It is interesting, but better read in

the original magazine layout than in a text file.

  There were also four sample printouts. Of course I can't

reproduce these here, but luckily the parameters required

for their reproduction were printed alongside. They were:



 (128,88) f37 m18 r18

 (128,88) f37 m-18 r-18



 (128,88) f38 m18 r18

 (128,88) f38 m-18 r-18



 (128,88) f39 m18 r18

 (128,88) f39 m-18 r-18



 (128,88) f51 m15 r15

 (128,88) f51 m-21 r-21



(The first two sets used one after another without clearing

the screen in between produce the first sample, and simi-

larly for the others. The effect of using a negative moving

gear is like using one of the "hollow" rings as the fixed

gear.)



One tip which wasn't mentioned in the article is that,

because this is a computer program and not a physical set

of gears, you can put the pen outside the moving gear!

Try the following set: (128,88) f30 m23 r27. ]