Provided by: xscreensaver-data-extra_5.36-1ubuntu1_amd64 
NAME
epicycle - draws a point moving around a circle which moves around a cicle which...
SYNOPSIS
epicycle [-display host:display.screen] [-root] [-window] [-mono] [-install] [-noinstall]
[-visual viz] [-colors N] [-foreground name] [-color-shift N] [-delay microseconds]
[-holdtime seconds] [-linewidth N] [-min_circles N] [-max_circles N] [-min_speed number]
[-max_speed number] [-harmonics N] [-timestep number] [-divisor_poisson probability]
[-size_factor_min number] [-size_factor_max number] [-fps]
DESCRIPTION
The epicycle program draws the path traced out by a point on the edge of a circle. That
circle rotates around a point on the rim of another circle, and so on, several times. The
random curves produced can be simple or complex, convex or concave, but they are always
closed curves (they never go in indefinitely).
You can configure both the way the curves are drawn and the way in which the random
sequence of circles is generated, either with command-line options or X resources.
OPTIONS
-display host:display.screen
Specifies which X display we should use (see the section DISPLAY NAMES in X(1) for
more information about this option).
-root Draw on the root window.
-window Draw on a newly-created window. This is the default.
-mono If on a color display, pretend we're on a monochrome display. If we're on a mono
display, we have no choice.
-install
Install a private colormap for the window.
-noinstall
Don't install a private colormap for the window.
-visual viz
Specify which visual to use. Legal values are the name of a visual class, or the
id number (decimal or hex) of a specific visual. Possible choices include
default, best, mono, monochrome, gray, grey, color, staticgray, staticcolor,
truecolor, grayscale, greyscale, pseudocolor, directcolor, number
If a decimal or hexadecimal number is used, XGetVisualInfo(3X) is consulted to
obtain the required visual.
-colors N
How many colors should be used (if possible). The colors are chosen randomly.
-foreground name
With -mono, this option selects the foreground colour.
-delay microseconds
Specifies the delay between drawing successive line segments of the path. If you
do not specify -sync, some X servers may batch up several drawing operations
together, producing a less smooth effect. This is more likely to happen in
monochrome mode (on monochrome servers or when -mono is specified).
-holdtime seconds
When the figure is complete, epicycle pauses this number of seconds.
-linewidth N
Width in pixels of the body's track. Specifying values greater than one may
cause slower drawing. The fastest value is usually zero, meaning one pixel.
-min_circles N
Smallest number of epicycles in the figure.
-max_circles N
Largest number of epicycles in the figure.
-min_speed number
Smallest possible value for the base speed of revolution of the epicycles. The
actual speeds of the epicycles vary from this down to min_speed / harmonics.
-max_speed number
Smallest possible value for the base speed of revolution of the epicycles.
-harmonics N
Number of possible harmonics; the larger this value is, the greater the possible
variety of possible speeds of epicycle.
-timestep number
Decreasing this value will reduce the distance the body moves for each line
segment, possibly producing a smoother figure. Increasing it may produce faster
results.
-divisor_poisson probability
Each epicycle rotates at a rate which is a factor of the base speed. The speed of
each epicycle is the base speed divided by some integer between 1 and the value of
the -harmonics option. This integer is decided by starting at 1 and tossing a
biased coin. For each consecutive head, the value is incremented by one. The
integer will not be incremented above the value of the -harmonics option. The
argument of this option decides the bias of the coin; it is the probability that
that coin will produce a head at any given toss.
-size_factor_min number
Epicycles are always at least this factor smaller than their parents.
-size_factor_max number
Epicycles are never more than this factor smaller than their parents.
-fps Display the current frame rate and CPU load.
RESOURCES
Option Resource Default Value
------ -------- -------------
-colors .colors 100
-delay .delay 1000
-holdtime .holdtime 2
-linewidth .lineWidth 4
-min_circles .minCircles 2
-max_circles .maxCircles 10
-min_speed .minSpeed 0.003
-max_speed .maxSpeed 0.005
-harmonics .harmonics 8
-timestep .timestep 1.0
-divisor_poisson .divisorPoisson 0.4
-size_factor_min .sizeFactorMin 1.05
-size_factor_max .sizeFactorMax 2.05
.timestepCoarseFactor 1.0
Before the drawing of the figure is begun, a preliminary calculation of the path is done
in order to scale the radii of the epicycles so as to fit the figure on the screen or
window. For the sake of speed, This calculation is done with a larger timestep than the
actual drawing. The time-step used is the value of the -timestep option multiplied by the
timestepCoarseFactor resource. The default value of 1 will almost always work fast enough
and so this resource is not available as a command-line option.
USER INTERFACE
The program runs mostly without user interaction. When running on the root window, no
input is accepted. When running in its own window, the program will exit if mouse button
3 is pressed. If any other mouse button is pressed, the current figure will be abandoned
and another will be started.
HISTORY
The geometry of epicycles was perfected by Hipparchus of Rhodes at some time around 125
B.C., 185 years after the birth of Aristarchus of Samos, the inventor of the heliocentric
universe model. Hipparchus applied epicycles to the Sun and the Moon. Ptolemy of
Alexandria went on to apply them to what was then the known universe, at around 150 A.D.
Copernicus went on to apply them to the heliocentric model at the beginning of the
sixteenth century. Johannes Kepler discovered that the planets actually move in
elliptical orbits in about 1602. The inverse-square law of gravity was suggested by
Boulliau in 1645. Isaac Newton's Principia Mathematica was published in 1687, and proved
that Kepler's laws derived from Newtonian gravitation.
BUGS
The colour selection is re-done for every figure. This may generate too much network
traffic for this program to work well over slow or long links.
COPYRIGHT
Copyright © 1998, James Youngman. Permission to use, copy, modify, distribute, and sell
this software and its documentation for any purpose is hereby granted without fee,
provided that the above copyright notice appear in all copies and that both that copyright
notice and this permission notice appear in supporting documentation. No representations
are made about the suitability of this software for any purpose. It is provided "as is"
without express or implied warranty.
AUTHOR
James Youngman <jay@gnu.org>, April 1998.