ref: 297bf25f612d2dde0d28d7feaae0ea381be114a7
dir: /sys/man/1/galaxy/
.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 .EE .PP Cool patterns made by a square galaxy: .IP .EX games/mkgalaxy -sq -av 20 5000 | games/galaxy -i .EE .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).