GALAXY(1) GALAXY(1)
NAME
galaxy, mkgalaxy - galactic n-body simulator
SYNOPSIS
games/galaxy [ options ] [ -i ] [ file ]
games/mkgalaxy [ options ] [ -i ] [ -f file ] size
DESCRIPTION
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 galaxy(6)) from standard input
using the -i command-line option or from a file using the -f
option. If no file is read then the simulator starts with an
empty universe.
Mouse commands
Holding mouse button 1 while dragging repositions the visi-
ble region of the galaxy. Holding mouse button 2 while drag-
ging up or down zooms the visible region of the galaxy in or
out, respectively. Mouse button 3 opens a menu with the
following options:
new body
Creates a new galactic body. Holding button 1 posi-
tions 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 where
the center of mass of the galaxy is at rest.
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.
gravity
Prompts for a floating point value to change the gravi-
tational constant. E.g. a value of 2 will double the
force exerted by gravity and a value of 0.5 will halve
it.
save Prompts for a file name to save the current galaxy as a
galaxy(6) file.
load Prompts for a file name to load the galaxy from the
galaxy(6) file.
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exit Exits the simulator.
Keyboard commands
The following keys are recognized as commands:
a Show accelerations as vectors.
v Show velocities as vectors.
s Show statistics such as the number of bodies being sim-
ulated, the maximum depth of the quad-tree, and the
average number of calculations made per body.
q Exit the simulator.
space
Pause and unpause the simulator.
del Exit the simulator.
Command-line options
Certain aspects of the galaxy simulator are controlled by
the following options:
-G gravity
Sets the gravitational constant to gravity. The default
value is 1.
-f file
Reads the galaxy file file (see galaxy(6)).
-i Reads a galaxy file from standard input.
-p procs
Specifies the number of extra processes to use in order
to calculate the gravitational force on each body in
parallel. The default value is $NPROC-1.
-t throttle
Causes the process that calculates forces to relinquish
the processor for throttle milliseconds after each cal-
culation.
-ε softening
Sets the softening factor to prevent gravitational sin-
gularities during collisions or near-collisions. The
default value is 500.
Mkgalaxy
Mkgalaxy is a utility to create galaxies for simulation.
Galaxies can be assembled incrementally by reading an exist-
ing galaxy file from standard input with the -i command-line
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option or from a file with the -f option. Mkgalaxy then
writes to standard output a galaxy(6) file with a galaxy of
the given size together with the previously read galaxy.
Galaxies generated by mkgalaxy have characteristics deter-
mined by the following options:
-d distance
Distance determines the spacing between bodies. The
default value is 100.
-s size
Bodies have the given size. The default value is 25.
-v velocity
Bodies have the given velocity in a random direction.
The default value is 0.
-av angular velocity
Bodies have the given angular velocity relative to the
center of mass of the new galaxy being generated. The
default value is 0.
-gv x,y
The entire galaxy being generated is given the direc-
tional velocity determined by the vector (x,y). The
default value is (0, 0).
-o x,y
The entire galaxy being generated is offset by the vec-
tor (x,y). The default value is (0, 0).
-sq The galaxy being generated is a square. Without this
option, the galaxy will be circular.
The arguments to the -d, -s, -v, and -av arguments have the
form s or s±r where s and r are double-precision floating
point numbers. S is the base value and r if given deter-
mines a range in which the value will vary randomly from the
base.
EXAMPLES
Two rotating circles destroy each other:
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
Cool patterns made by a square galaxy:
games/mkgalaxy -sq -av 20 5000 | games/galaxy -i
SOURCE
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GALAXY(1) GALAXY(1)
/sys/src/games/galaxy
SEE ALSO
J. Barnes & P. Hut (December 1986). "A hierarchical O(N log
N) force-calculation algorithm". Nature. 324 (4): 446–449.
galaxy(6)
HISTORY
Galaxy and mkgalaxy first appeared in 9front (Feb, 2017).
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