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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



 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.





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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



      -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.





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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



 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.




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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



           - 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



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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



           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



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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



           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



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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



           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



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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



           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



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 XSPRINGIES(1)                                                 XSPRINGIES(1)
                              September 1, 1992



              <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.  :-)






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