XSPRINGIES(1)XSPRINGIES(1)NAMExspringies - a mass and spring simulation system for X windows
SYNTAXxspringies [ options ]
DESCRIPTIONxspringies is a simulator which allows you to interactively create and
edit a system of masses and springs. The parameters of the masses and
springs (such as mass elasticity and spring K) as well as those of the
surrounding system (such as air viscosity and gravity) can be changed.
These systems can be loaded and saved into a file.
I guess you could use xspringies for real work, but it's really
intended to be a time waster.
OPTIONS-bg color
Specifies the color of the window background. The default is
black.
-d dispname
Specifies the display screen on which xspringies displays its
window. If the display option is not specified, xspringies uses
the display screen specified by your DISPLAY environment vari‐
able.
-display dispname
This option is the same as the -d option.
-fg color
Specifies the foreground color of the window. The default is
white.
-geometry geom
Specifies the width, height, and location of the xspringies win‐
dow. If the geometry option is not specified, xspringies uses
default values. The geometry option has the format
[=][widthxheight][+/-xoffset+/-yoffset].
-nbb This option turns off the bounding-box optimization. To produce
smooth animation, xspringies redraws the smallest screen region
which contains all objects. For smaller objects, the perfor‐
mance improvement is very noticible -- this is the bounding-box
optimization. By disabling it with this option, xspringies
redraws the entire display window. On slower machines or larger
window sizes, this can produce slow results. The main reason
for using this option is if the bounding-box changes size
rapidly, causing uneven animation.
-hl color
Specifies the color of the button and selection highlights in
the window. This defaults to the foreground color, or to green
on color displays.
-rv Specifies that the foreground and background colors be reversed.
-st thickness
Specifies the thickness of the springs in pixels (0 is the
default value)
SUMMARY OF OPERATION
The left side of the xspringies window contains the controls, and the
right side contains the display. Masses can be created and placed with
the mouse when in Mass mode, and springs can be created when in Spring
mode. Temporary springs that connect the mouse and any mass can be
used to pull on objects. Masses and springs can be selected in edit
mode, and moved around. Parameters of the masses and springs (such as
Mass or Elasticity) can be set upon creation or if they are selected.
Forces (such as gravity) can be enabled by pressing the appropriate
force button with customizable parameters. Environment parameters such
as viscosity of the medium and stickiness of the walls can also be set.
Each of the four walls can be disabled.
The animation/simulation is activated by pressing the GO! button.
The entire system (masses, springs and parameters) can be loaded and
saved to files. Xspringies comes with many demonstration files.
CONTROLS
There are three types of controls (widgets) in xspringies. There are
push buttons, checkboxes and sliders. The push buttons and checkboxes
act in the obvious manner. If you click on them, they get activated.
Sliders are a little more compilicated. They consist of a left arrow
button, a right arrow button, and a text box. The text box displays
the current value. Clicking on this text box causes it to become high‐
lighted. All text input then goes to the text box. After entering a
value, return accepts it, and escape cancels.
The value displayed can also be changed by pressing the arrow but‐
tons. Using the Left mouse button causes the value to be incremented
or decremented by one step. The Middle mouse button is the same as the
left mouse button, but holding it down causes it to scan by one step at
a time. The Right mouse button scans like the Middle mouse button
except that it scans 10 steps at a time.
EDITING MODES
There are three main modes in xspringies. These are Edit, Mass and
Spring. In Edit mode, you can select, move and throw objects. In Mass
mode, you can create masses. In Spring mode, you can create springs
and tug on an object with a spring connected to the mouse.
More specifically,
Edit mode:
- Left mouse button selects objects.
If you click on or near an object, it becomes selected, and
all other objects become unselected. If you hold down shift
while clicking, the object becomes selected (or unselected if it
was already selected), and all other objects remain the same.
If you do not click near an object, dragging the mouse causes
a selection box to appear. Anything within the selection box
when the mouse is released becomes selected. All other masses
become unselected, unless the shift key was held down for the
initial click.
- Middle mouse button moves objects.
All selected objects move with the mouse. The masses are
frozen in their positions after the initial click. They con‐
tinue to move relative to the mouse movement until the middle
button is released.
- Right mouse button throws objects.
This acts the same way as moving objects with the middle but‐
ton, except for the fact that the mouse velocity is transferred
to all selected objects when the right mouse button is released.
Note: a good way to stop an object from moving is to simply
select it and click the right mouse button.
Mass mode:
When you click with the mouse, a mass appears. The mass takes
on the values of the Mass and Elasticity sliders. The mass is
placed when the mouse button is released.
Spring mode:
When you click with the mouse, if there is a mass nearby, one
end of a spring is connected to it. The other end of the spring
is connected to the cursor until the mouse button is released.
If no mass is nearby when the spring is released, the spring is
discarded. The new spring takes on the values of the Kspring
and Kdamp sliders. The rest length of the spring is equal to
the length of the spring when it was created.
- Left mouse button adds a spring between two masses while
actively affecting the first mass.
- Middle mouse button adds a spring between the first mass and
the cursor, actively affecting the first mass. The spring is
discarded when the mouse button is released.
- Right mouse button adds a spring between two masses. The
first mass is not affected by the spring until the spring is in
place after the mouse is released.
OPERATION
Masses and Springs
Accelerations on the masses are calculated according to grav‐
ity (and other special forces), viscous forces, and by forces
from the springs. When a mass collides with a wall, its veloc‐
ity in the direction of the wall is reversed. The resulting
velocity is multipled by the Elasticity of the mass. So, an
elasticity of 0.0 causes an inelastic collision (it stops on the
wall), and an elasticity of 1.0 results in an elastic collision.
If a mass is fixed, all forces on it are ignored. It simply
does not move. Think of it as a nail (a really good one).
The Mass and Elasticity of a mass can be changed by selecting
the mass and changing the values on the corresponding sliders.
To make a mass fixed or unfixed, check or uncheck the Fixed Mass
checkbox while the mass is selected.
A spring has three parameters associated with it. Kspring,
Kdamp and rest length. The spring force is calculated as fol‐
lows (according to Hooke's law):
F = - Kspring * (length - rest length) - Kdamp * (velocity
in spring direction)
To change the Kspring or Kdamp of a spring, change the values
of the sliders when the spring is selected. Pressing the Set
Rest Length button changes the rest length of a selected spring
to its current length.
Forces and sticky stuff
There are four special forces. They can be turned on and off
by clicking their appropriate box. When highlighted, the force
is on. Each of these forces has two parameters associated with
it (for example, Magnitude and Direction for gravity). Only one
forces's parameters are displayed at a time (below the force
buttons). Which particular force is shown by a darker box
around that force. This force selector box is moved to another
force whenever a force is turned on.
Some of the forces are applied relative to some specified ori‐
gin, or center point. By default, this center point is the cen‐
ter of the screen. It can be changed to be any one particular
mass by selecting a single mass, and pushing the Set Center but‐
ton. If no masses are selected, the current center is changed
to be the center of the screen.
Center points are marked by a box around the center mass.
There are four forces that can be enabled. The first one,
Gravity, acts in the familiar manner. It accelerates masses by
the value specified by Gravity in a direction specified by
Direction. The Direction is measured in degrees, with 0.0
degrees being down, increasing counter-clockwise.
The second force is a bit strange, and isn't real. Its a
force which attracts the center of mass of all the objects
toward the center point. It has a Magnitude and a Damping coef‐
ficient.
The third force is a force which attracts all masses toward
the center point. This force has a Magnitude and an Exponent
associated with it. The Exponent is simply how the force rela‐
tion works. A value of 2.0 means inverse-square force (the
force is inversely proportional to the distance squared). A
value of 0.0 is a constant force independent of position. If
the Magnitude of this force is negative, it becomes a repulsion
force.
The fourth force is a wall repulsion force. Masses are
repelled by a force from each wall that is on. This force has a
Magnitude and Exponent associated with it. The Exponent behaves
similarly to that of the third force.
For the most part, most everything obeys f = ma. The only
exceptions are wall bounces and wall stickiness. Another
unphysical aspect is found in some of the special forces (the
second and third ones). If a center point exists, that mass
does not receive any force response from other masses due to the
special force. In other words, these two special forces are not
equal and opposite forces. They're pretty much just unreal.
Viscosity is a viscous drag force which applies a resistive
force on the masses proportional to their velocity.
Stickiness is not a real force. When a mass hits a wall, it
loses part of its velocity component in the direction of the
wall (in an amount proportional to the Stickiness). If it loses
all of this component, it remains stuck to the wall. It will
remain stuck to the wall until a force (which exceeds an amount
proportional to the Stickiness) pulls it off the wall.
Numerics
The internals of xspringies consist of a fourth order Runge-
Kutta (RK4) differental equation solver. Consult a Numerical
Methods text for more information. The Time Step that is used
by this solver (the dt) can be set using the slider.
The solver can be selectively made into an adaptive RK4 solver
using the Adaptive Time Step checkbox. An adaptive solver
chooses the best time step value according to an error calcula‐
tion. The error is not allowed to exceed the Precision value.
Lower precision values result in smaller time steps. While this
is more accurate, the simulation runs slower.
You will notice that some objects will tend to "blow up" eas‐
ily. This is because the objects are unstable, or are sensitive
to small numerical errors. An object will tend to "blow up"
less with smaller time steps. By using an adaptive solver, the
simulation can be made more accurate only when necessary. This
results in a more stable system which runs at a reasonable
speed.
Walls
There are four walls. In case you haven't guessed by now,
they are the top, left, right and bottom walls. They are
located at the window boundaries; they move with window resizes.
Individual walls can be enabled and disabled by checking the
corresponding checkboxes.
Walls are only one-way. An object moving from the screen
toward a wall will bounce off the wall. But an object moving
from off screen toward the screen will pass through the walls.
Nearby is the button to enable mass-to-mass collisions. Note
that for large numbers of masses, this can get pretty slow.
Saving and Restoring State
The state of the world (at least for xspringies) includes all
of the masses and springs, as well as the system parameters. By
pushing the Save State button, the current system state is
saved. By pushing the Restore State button the previously
saved system state is restored (if no previous save was made,
the initial state is used).
This is useful for temporarily saving a system configuration
that you do not feel like setting up again (or saving to a
file), that you might disturb with experimentation. If you
break it, you can Restore State any number of times you like.
The Reset button resets xspringies to its initial configura‐
tion. All the masses and springs are removed, and the default
system parameters are used.
Saving and Loading Files
The system state can be saved to and loaded from files. By
pushing the Load File button, a previously saved file can be
loaded. This will load up all the masses and springs that were
saved, as well as the system parameters. Any previous state
before the load is cleared. Pushing the Save File button saves
this information in the designated file. The Insert File button
is much like the Load File button, except that the current state
is not cleared. Instead, only the masses and springs are loaded
(the system paramaters are not changed), and are added to the
current collection. If no objects are selected when the Insert
File button is pressed, then all the objects in the file are
selected after they are loaded.
The filenames are entered in the text window, which is located
at the bottom right of the window. For consistency, the file‐
names should terminate with ".xsp". When a file is loaded or
saved, this extension is automatically added if not added by the
user. Standard emacs-like editing features are present. The
following key controls can be used:
control-B move cursor backward
control-F move cursor forward
control-A goto beginning of line
control-E goto end of line
control-K kill to end of line
control-Y yank from kill buffer
control-D delete character under cursor
control-U erase all input
control-T transpose character under cursor with previous
character
Escape exit from filename edit mode
By default, the directory which contains the xspringies files
is present automatically. If the environment variable SPRINGDIR
is set, then the default directory is changed to reflect it.
If a file error occurs (for example, the file does not exist),
a beep is emitted.
Other stuff
The GO! button simply turns on and off the animation. With
GO! turned off, xspringies uses up little CPU time.
The Show Springs checkbox controls whether or not the springs
are drawn. If there are a lot of springs, animation may go
faster with this option on. Sometimes an object will even look
better with only the masses.
When placing masses or springs, objects can be placed in a
gridlike fashion if the Grid Snap checkbox is enabled. Masses
will be separated (vertically and horizontally) by the amount
specified by the Grid Snap slider.
When the Duplicate button is pushed, all selected masses and
springs are duplicated. The copy is left in the same place,
unselected.
By pushing the Select All button, all masses and springs are
selected.
By pushing the Delete button, or pressing the Delete key, all
selected objects are deleted. Note that if a mass is deleted,
all attached springs are also deleted (even if they were not
selected).
The Quit button quits the program. This same effect is found
by pressing the Q key.
FILE FORMAT
The xspringies file format is human readable. Each line consists of
a four letter command string, followed by the parameters for that com‐
mand. The file must start with the command "#1.0". (This 1.0 refers
to file format version 1.0, and not the version of xspringies being
run) Each of the commands below is on a line by itself. There are no
blank lines allowed. The file must end in a newline.
The file consists of the following commands:
cmas <current Mass value>
elas <current Elasticity value>
kspr <current Kspring value>
kdmp <current Kdamp value>
fixm <boolean value for Fixed Mass>
shws <boolean value for Show Springs>
cent <mass id number of center mass>
If there is no center mass (i.e. - center of screen is to be
used), then the value of -1 is used.
frce <force id number> <boolean active> <parameter #1 value>
<parameter #2 value>
The <force id number> sequence is as follows:
0 - Gravity
1 - Center of mass attraction force
2 - Center attraction force
3 - Wall repulsion force
visc <current Viscosity value>
stck <current Stickiness value>
step <current Time Step value>
prec <current Precision value>
adpt <boolean value for Adaptive Time Step>
gsnp <current Grid Snap value> <boolean enable>
wall <boolean top> <boolean left> <boolean right> <boolean bottom>
mass <mass id number> <x position value> <y position value>
<x velocity value> <y velocity value> <mass value> <elastic‐
ity value>
For each mass, the <mass id number> must be unique. They do
not need to be in any order. If a mass is fixed, then the
<mass value> field is negated.
spng <spring id number> <mass #1 id number> <mass #2 id number>
<Kspring value> <Kdamp value> <rest length value>
For each spring, the <spring id number> must be unique. They
do not need to be in any order. The order of the <mass id num‐
ber>'s is not important.
All values are floating point numbers. All id numbers are positive
integers, and all boolean values are non-zero/zero for True/False. It
is possible to feed xspringies bogus values. It may produce interest‐
ing or amusing side effects, but will most likely cause an object to
explode or xspringies to crash.
AUTHOR
Douglas DeCarlo (dmd@gradient.cis.upenn.edu)
Please send demo files, comments, suggestions, bug reports, bug fixes
and enhancements.
With thanks to:
Elliott Evans
Bitmap slave.
Nathan Loofbourrow
I bothered him a whole lot about the user interface.
Drew Olbrich
The blame for "stickiness" falls on him, as well as a few
other things.
Andy Witkin
For teaching a really great physically based modeling course
at CMU. Many ideas (both methods and interface) came from
that class.
And thanks to the many other people who helped in testing
xspringies and make some of the neat demo files, including James
Helfrich, Brian Kelley, Patrick Lopez, Chris Newman and Jef
Poskanzer.
RESTRICTIONSxspringies runs faster on a monochrome display than on a color display
(usually).
You probably don't want to run xspringies on a slow machine or a
machine which does slow bit-blitting operations. Well, I guess you
could... But you would be sorry for even trying.
Here is a good rule: If xspringies isn't fun to use, then your machine
is either too slow, or it is overloaded. Or maybe you just aren't a
fun person. :-)
September 1, 1992 XSPRINGIES(1)
