XSPRINGIES(1)

NAME
xspringies - a mass and spring simulation system for X windows

SYNTAX
xspringies [ options ]

DESCRIPTION
xspringies 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 variable.

-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 window. 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 performance 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 highlighted. 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 buttons. 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 continue 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 button, 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 gravity (and other special forces), viscous forces, and by forces from the springs. When a mass collides with a wall, its velocity 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 follows (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 origin, or center point. By default, this center point is the center of the screen. It can be changed to be any one particular mass by selecting a single mass, and pushing the Set Center button. 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 coefficient.

    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 relation 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 calculation. 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" easily. 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.

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 configuration. 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 filenames 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 command. 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> <elasticity 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 number>'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 interesting 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.

RESTRICTIONS
xspringies 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. :-)