How to Run xrel
If you've succeeded in building
xrel, running it is even easier, although you do have
many options to control its behavior.
You can get a brief summary of the options below by giving the
which will also show you the defaults for options that control
Many options take an additional argument; the value that the
argument represents is shown [in brackets] after the option name
- If specified, xrel will allocate a private
colormap on a PseudoColor display. Has no effect on other types
of displays. Overrides -colors.
- Resize the window to fill the entire screen. This uses
override_redirect to circumvent your window manager, so it's
slightly dangerous because there's a chance it could prevent
access to any of your other windows if the program freaks out.
But it looks much cooler in a dark room.
- -colors [number of colors]
- Change the number of colors xrel tries to allocate on a
PseudoColor display. If the requested number can't be allocated,
it will use as many as it can successfully allocate.
- -gamma [gamma correction factor]
- Change the gamma correction factor. This affects the relative
brightness of different intensities of a given color. Lower gamma
values will make stars appear brighter and colors appear less
saturated; higher values make stars appear dimmer and colors more
intense. The default of 0.5 is about right for most displays.
Most displays are likely to have gammas between 0.3 and 0.8.
- Display a color test pattern instead of the simulation. Use
with -size to get bigger patches of color. The color
test pattern shows colors across several intensities and
- -size [dot radius]
- Change the size of dots drawn to represent stars. -size
1 shows stars as single pixels; larger values correspond
approximately to the radius of the circle drawn for a star.
- -viewangle [angle]
- Change the view angle covered by the display. The view angle
is the visual angle subtended by the narrowest dimension of the
- -fps [frames per second]
- Sets a maximum limit on the number of frames drawn per
second. This may help you to get a consistent update rate or
to limit the amount of CPU time and network bandwidth required for
display updates. -fps 0, the default, means to draw at
the maximum possible rate.
- -stars [number of stars in universe]
- Controls the number of stars in the "universe" of the
simulation. Using more stars will slow down the simulation but
give you more to look at.
- -chunk [chunking factor for drawing]
- Since xrel mostly draws dots into its window,
and since X protocol requests for drawing dots have a fair
amount of overhead, by default xrel "chunks" drawing
requests together to reduce this overhead. This can make
the update rate 1.2 to over 3 times faster than if
"chunking" is not done (try "-chunk 1" and see for
yourself). The default gives good results for typical
cases. Increasing this number may improve drawing
performance somewhat but can also cause a perceptible
flickering; decreasing this number will help reduce the
flicker but will also reduce the update rate.
- -seed [random number seed]
- Lets you set the random number seed used in random star
generation, so you can duplicate the same random star field in
different runs. If not specified, a semi-random seed is used and
printed out for future reference.
- -lattice [spacing]
- Rather than generate stars in random locations, create a cubic
lattice with alternating layers of red and blue stars [spacing]
light-years apart. This may help you get a more quantitative
picture of relativistic effects.
- -velocity [initial velocity]
- Set the initial velocity of your ship, as a fraction of c (in
other words, you need to give a value greater than -1.0 and less
- -accel [initial acceleration]
- Sets the acceleration of your ship, in meters per second
squared (9.8 = 1 Earth gravity).
- -limit [speed limit]
- When accelerating, stop accelerating if the ship's speed
exceeds the specified limit (specified as a fraction of c). Note
that this says "speed", not "velocity"; if you specify a limit of
0.9, your acceleration stops if you go above 0.9c or -0.9c.
- Rather than stop accelerating at the specified limit, reverse
acceleration instead. This simulates the appearance of a round
trip between two different points in space.
- -timescale [ratio of simulation time to real time]
- Controls the rate at which simulation time elapses. The
default, -timescale 86400, means that one second of
real time equals one day of simulation time. Increasing the
timescale speeds up the simulation. Very large timescales and
very high velocities may produce weird results. Very small
timescales produce extremely slow simulations.
Since xrel uses the X Toolkit command-line parser, you can use
other standard X Toolkit options too -- for example, you can specify
an alternate display with -display foo:0.0 or an
alternate window size with -geometry 800x600+0+0.
Consult your X window system reference for more details.
Changing defaults with X resources
Each command-line option has a corresponding X resource, so if
you have defaults you'd like to change (say, gamma correction or
star size) you can set those X resources to change them.
xrel's application name is "xrel", and its class name is
"Xrel". Each of its resources has the same name as the
corresponding command-line option without the leading '-', so the
X resource that controls gamma is called "gamma", for example.
To change the default star size, gamma correction, and initial
velocity and acceleration parameters of xrel for every time you
run it, you could put something like this in
your .Xresources or .Xdefaults file:
Note that command-line options always override X resources.
You can rotate your view left or right in 90-degree increments
using mouse button 1. Pressing the button in the left half of the
window rotates left; in the right half of the window rotates
right. Pressing the 'r' key rotates 90 degrees right.
Pressing mouse button 2 or the 'p' key pauses the simulation.
You can still rotate your view with the simulation paused.
Pressing mouse button 2 again resumes the simulation.
Pressing mouse button 3 or the 'q' key quits the
The display window
The display window is simple. It shows you a view determined
by your current view angle, and three lines in the upper left
corner display your elapsed simulation time, your distance from
the origin (where you started the simulation), and your current
velocity relative to the stars.
- The compiled-in defaults simulate acceleration to 0.95 c at 1
Earth gravity (9.8 meters/second^2), with one real second
representing one day of simulation time. Once you reach 0.95 c
you continue to cruise through space at that rate.
- xrel -bounce
- Same as above, except that at +/- 0.95 c you reverse
acceleration. Interestingly enough, this approximates the profile
of a trip to Alpha Centauri -- you travel about 4.3 light-years
between when you start at rest and when you come to rest again
(relative to the stars).
- xrel -acc 0.1 -times 1e8 -bounce
probably noticed that you don't see any stars pass you in
the above examples. That's because 1 Earth gravity of
acceleration gets you up to near c within a distance that's
fairly short compared to the distance to the stars near
you, making it hard to see any parallax. With these
parameters you accelerate much more slowly (hence the
larger timescale to keep you from dying of boredom), travel
farther (also more slowly), and actually get to see more
stars pass you.
Return to Relativistic Starflight
Last modified: Wed Feb 23 15:03:13 PST 2000