Spirogram

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. ]