2017-02-18 15:08:51 +00:00
|
|
|
|
.TH GALAXY 1
|
|
|
|
|
|
.SH NAME
|
|
|
|
|
|
galaxy, mkgalaxy \- galactic n-body simulator
|
|
|
|
|
|
.SH SYNOPSIS
|
|
|
|
|
|
.B games/galaxy
|
|
|
|
|
|
[
|
|
|
|
|
|
.I options
|
|
|
|
|
|
] [
|
|
|
|
|
|
.B -i
|
|
|
|
|
|
] [
|
|
|
|
|
|
.I file
|
|
|
|
|
|
]
|
|
|
|
|
|
.br
|
|
|
|
|
|
.B games/mkgalaxy
|
|
|
|
|
|
[
|
|
|
|
|
|
.I options
|
|
|
|
|
|
] [
|
|
|
|
|
|
.B -i
|
|
|
|
|
|
] [
|
|
|
|
|
|
.B -f
|
|
|
|
|
|
.I file
|
|
|
|
|
|
]
|
|
|
|
|
|
.I size
|
|
|
|
|
|
.SH DESCRIPTION
|
|
|
|
|
|
.I Galaxy
|
|
|
|
|
|
is an n-body simulator that uses a Barnes-Hut quad-tree
|
|
|
|
|
|
to calculate gravitational interactions.
|
|
|
|
|
|
Typical usage is to read a galaxy file (see
|
|
|
|
|
|
.IR galaxy (6))
|
|
|
|
|
|
from standard input
|
|
|
|
|
|
using the
|
|
|
|
|
|
.B -i
|
|
|
|
|
|
command-line option or from a
|
|
|
|
|
|
.I file
|
|
|
|
|
|
using the
|
|
|
|
|
|
.B -f
|
|
|
|
|
|
option. If no file is read then the simulator starts with an empty
|
|
|
|
|
|
universe.
|
|
|
|
|
|
.SS Mouse commands
|
|
|
|
|
|
.PP
|
|
|
|
|
|
New planetary bodies can be created with mouse button 1.
|
|
|
|
|
|
Holding button 1 will reposition the body.
|
|
|
|
|
|
Holding a button 1-2 chord changes the mass/size
|
|
|
|
|
|
of the body. Holding a button 1-3 chord
|
|
|
|
|
|
changes the initial velocity of the body. Releasing button 1
|
|
|
|
|
|
restarts the simulator with the new body in motion. When new
|
|
|
|
|
|
bodies are created, the simulator maintains the Galilean (inertial)
|
|
|
|
|
|
reference frame.
|
|
|
|
|
|
.PP
|
|
|
|
|
|
Mouse button 3 opens a menu with the following options:
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B move
|
|
|
|
|
|
Change the visible region of the simulation
|
|
|
|
|
|
by holding button 1 and dragging. Any other mouse
|
|
|
|
|
|
button restarts the simulation.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B zoom
|
|
|
|
|
|
Prompts for a floating point value to change the scale of the
|
|
|
|
|
|
simulation. E.g. a value of 2 will halve the scale (zoom in)
|
|
|
|
|
|
and a value of 0.5 will double the scale (zoom out).
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B speed
|
|
|
|
|
|
Prompts for a floating point value to change the speed of
|
|
|
|
|
|
the simulation. E.g. a value of 2 will double the speed
|
|
|
|
|
|
of the simulation and a value of 0.5 will
|
|
|
|
|
|
halve the speed. Accuracy is sacrificed for greater speed.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B gravity
|
|
|
|
|
|
Prompts for a floating point value to change the gravitational
|
|
|
|
|
|
constant. E.g. a value of 2 will double the force exerted by
|
|
|
|
|
|
gravity and a value of 0.5 will halve it.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B save
|
|
|
|
|
|
Prompts for a file name to save the current galaxy as a
|
|
|
|
|
|
.IR galaxy (6)
|
|
|
|
|
|
file.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B load
|
|
|
|
|
|
Prompts for a file name to load the galaxy from the
|
|
|
|
|
|
.IR galaxy (6)
|
|
|
|
|
|
file.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B exit
|
|
|
|
|
|
Exits the simulator.
|
|
|
|
|
|
.SS Keyboard commands
|
|
|
|
|
|
The following keys are recognized as commands:
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B a
|
|
|
|
|
|
Show accelerations as vectors.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B v
|
|
|
|
|
|
Show velocities as vectors.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B s
|
|
|
|
|
|
Show statistics such as the number of bodies being
|
|
|
|
|
|
simulated, the maximum depth of the quad-tree, and the
|
|
|
|
|
|
average number of calculations made per body.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B q
|
|
|
|
|
|
Exit the simulator.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B space
|
|
|
|
|
|
Pause and unpause the simulator.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B del
|
|
|
|
|
|
Exit the simulator.
|
|
|
|
|
|
.SS Command-line options
|
|
|
|
|
|
Certain aspects of the galaxy simulator are controlled by
|
|
|
|
|
|
the following options:
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -t " throttle"
|
|
|
|
|
|
Causes the process that calculates forces to relinquish
|
|
|
|
|
|
the processor for
|
|
|
|
|
|
.I throttle
|
|
|
|
|
|
milliseconds after each calculation.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -G " gravity"
|
|
|
|
|
|
Sets the gravitational constant to
|
|
|
|
|
|
.I gravity.
|
|
|
|
|
|
The default value is 1.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -ε " softening"
|
|
|
|
|
|
Sets the
|
|
|
|
|
|
.I softening
|
|
|
|
|
|
factor to prevent gravitational singularities during
|
|
|
|
|
|
collisions or near-collisions. The default value is 500.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -f " file"
|
|
|
|
|
|
Reads the galaxy file
|
|
|
|
|
|
.I file
|
|
|
|
|
|
(see
|
|
|
|
|
|
.IR galaxy (6)).
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B -i
|
|
|
|
|
|
Reads a galaxy file from standard input.
|
|
|
|
|
|
.SS Mkgalaxy
|
|
|
|
|
|
.I Mkgalaxy
|
|
|
|
|
|
is a utility to create galaxies for simulation.
|
|
|
|
|
|
Galaxies can be assembled incrementally by reading an
|
|
|
|
|
|
existing galaxy file from standard input with the
|
|
|
|
|
|
.B -i
|
|
|
|
|
|
command-line option or from a
|
|
|
|
|
|
.I file
|
|
|
|
|
|
with the
|
|
|
|
|
|
.B -f
|
|
|
|
|
|
option. Mkgalaxy then writes to standard output a
|
|
|
|
|
|
.IR galaxy (6)
|
|
|
|
|
|
file with a galaxy of the given
|
|
|
|
|
|
.I size
|
|
|
|
|
|
together with the previously read galaxy.
|
|
|
|
|
|
Galaxies generated by mkgalaxy have characteristics
|
|
|
|
|
|
determined by the following options:
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -d " distance"
|
|
|
|
|
|
.I Distance
|
|
|
|
|
|
determines the spacing between bodies.
|
|
|
|
|
|
The default value is 100.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -s " size"
|
|
|
|
|
|
Bodies have the given
|
|
|
|
|
|
.IR size .
|
|
|
|
|
|
The default value is 25.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -v " velocity"
|
|
|
|
|
|
Bodies have the given
|
|
|
|
|
|
.I velocity
|
|
|
|
|
|
in a random direction.
|
|
|
|
|
|
The default value is 0.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -av " angular velocity"
|
|
|
|
|
|
Bodies have the given
|
|
|
|
|
|
.I "angular velocity"
|
|
|
|
|
|
relative to the center of mass of the new galaxy being generated.
|
|
|
|
|
|
The default value is 0.
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -gv " x,y"
|
|
|
|
|
|
The entire galaxy being generated is given the directional velocity determined
|
|
|
|
|
|
by the vector
|
|
|
|
|
|
.RI ( x,y ).
|
|
|
|
|
|
The default value is (0, 0).
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.BI -o " x,y"
|
|
|
|
|
|
The entire galaxy being generated is offset by the vector
|
|
|
|
|
|
.RI ( x,y ).
|
|
|
|
|
|
The default value is (0, 0).
|
|
|
|
|
|
.TP
|
|
|
|
|
|
.B -sq
|
|
|
|
|
|
The galaxy being generated is a square. Without this option, the galaxy
|
|
|
|
|
|
will be circular.
|
|
|
|
|
|
.PP
|
|
|
|
|
|
The arguments to the
|
|
|
|
|
|
.BR -d ,
|
|
|
|
|
|
.BR -s ,
|
|
|
|
|
|
.BR -v ,
|
|
|
|
|
|
and
|
|
|
|
|
|
.B -av
|
|
|
|
|
|
arguments have the form
|
|
|
|
|
|
.B s
|
|
|
|
|
|
or
|
|
|
|
|
|
.B s±r
|
|
|
|
|
|
where s and r are double-precision floating point numbers.
|
|
|
|
|
|
.B S
|
|
|
|
|
|
is the base value and
|
|
|
|
|
|
.B r
|
|
|
|
|
|
if given determines a range in which the value will vary randomly
|
|
|
|
|
|
from the base.
|
|
|
|
|
|
.SH EXAMPLES
|
|
|
|
|
|
Two rotating circles destroy each other:
|
|
|
|
|
|
.IP
|
|
|
|
|
|
.EX
|
|
|
|
|
|
games/mkgalaxy -av 100 -d 60±50 -v 10 2000 |
|
|
|
|
|
|
games/mkgalaxy -i -av -70 -d 80±50 -v 10 -o 6000,2000 -gv -80,40 3000 |
|
|
|
|
|
|
games/galaxy -i
|
2017-02-19 19:05:43 +00:00
|
|
|
|
.EE
|
2017-02-18 15:08:51 +00:00
|
|
|
|
.PP
|
|
|
|
|
|
Cool patterns made by a square galaxy:
|
|
|
|
|
|
.IP
|
|
|
|
|
|
.EX
|
|
|
|
|
|
games/mkgalaxy -sq -av 20 5000 | games/galaxy -i
|
2017-02-19 19:05:43 +00:00
|
|
|
|
.EE
|
2017-02-18 15:08:51 +00:00
|
|
|
|
.SH SOURCE
|
|
|
|
|
|
.B /sys/src/games/galaxy
|
|
|
|
|
|
.SH SEE ALSO
|
|
|
|
|
|
J. Barnes & P. Hut (December 1986). "A hierarchical O(N log N) force-calculation algorithm".
|
|
|
|
|
|
.IR Nature .
|
|
|
|
|
|
324 (4): 446–449.
|
|
|
|
|
|
.PP
|
|
|
|
|
|
.IR galaxy (6)
|
|
|
|
|
|
.SH HISTORY
|
|
|
|
|
|
.I Galaxy
|
|
|
|
|
|
and
|
|
|
|
|
|
.I mkgalaxy
|
|
|
|
|
|
first appeared in 9front (Feb, 2017).
|
