# Rename file to "spiro2.gp"
# usage: gnuplot < spiro2.gp ; xv spiro2.png
#
###spiro2.gp	gnuplot script to plot trochoids
###22aug07	from http://linuxgazette.net/133/luana.html
#
set terminal png 
 set output "spiro2.png"
 set size ratio -1
 set nokey
 set noxtics
 set noytics
 set noborder
 set parametric
#
 x(t)=(R-r)*cos(t) + p*cos((R-r)*t/r)
 y(t)=(R-r)*sin(t) - p*sin((R-r)*t/r)
#
### circle: p=0;  ellipse: R=2r and p≠r, with axes (r+p) and |r-p|;
### R=2r=2p gives a line of length 2a; 
###  p=r; produces hypocycloids
### negative p and/or r results in some extraordinary specimens.
#
 R=100.0; r=2.0; p=80.0
 set samples 2001
#
 plot [t=0:2*pi] x(t), y(t)