SIM_contact, for creating dynamic event scenarios #972 #936

trick_sims/SIM_contact/Modified_data/realtime.py

@@ -0,0 +1,10 @@
+
+trick.real_time_enable()
+trick.exec_set_software_frame(0.1)
+trick.itimer_enable()
+
+trick.exec_set_enable_freeze(True)
+trick.exec_set_freeze_command(True)
+
+simControlPanel = trick.SimControlPanel()
+trick.add_external_application(simControlPanel)

trick_sims/SIM_contact/README.md

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+#SIM_contact
+
+SIM_contact simulates the motion, and the collisions of any arbitrary numbers of moving balls.
+
+Each ball is defined by a position, velocity, radius, and mass.
+
+Collisions are purely elastic. In addition to conserving momentum, they also conserve kinetic energy.
+
+A graphics client allows you to see the balls in motion:
+
+![Graphics CLient showing two balls](images/TwoBalls.png)
+
+##Examples
+The configuration for the above display is:
+
+```
+dyn.contact.nballs = 2
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+dyn.contact.balls[0] = trick.make_Ball(0.0, 0.0,  0.5, 0.0, 0.5, 1.0)
+dyn.contact.balls[1] = trick.make_Ball(3.0, 0.0, -0.5, 0.0, 0.5, 1.0)
+```
+
+The initial state of the first ball is pos=<0.0, 0.0>, vel=<0.5,0.0>, radius=0.5, and mass=1.0.
+
+The initial state of the second ball is pos=<3.0, 0.0>, vel=<-0.5,0.0>, radius=0.5, and mass=1.0.

trick_sims/SIM_contact/RUN_test1/input.py

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+execfile("Modified_data/realtime.py")
+
+# Variable Server Data should be copied at top of frame.
+trick.var_set_copy_mode(2)
+
+dyn.contact.nballs = 2
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+
+dyn.contact.balls[0] = trick.make_Ball(0.0, 0.0,  0.5, 0.0, 0.5, 1.0)
+dyn.contact.balls[1] = trick.make_Ball(3.0, 0.0, -0.5, 0.0, 0.5, 1.0)
+
+dyn_integloop.getIntegrator(trick.Euler, 2*dyn.contact.nballs)
+
+
+#==========================================
+# Start the Satellite Graphics Client
+#==========================================
+varServerPort = trick.var_server_get_port();
+BallDisplay_path = "models/graphics/dist/BallDisplay.jar"
+
+if (os.path.isfile(BallDisplay_path)) :
+    BallDisplay_cmd = "java -jar " \
+                   + BallDisplay_path \
+                   + " " + str(varServerPort) + " &" ;
+    print(BallDisplay_cmd)
+    os.system( BallDisplay_cmd);
+else :
+    print('==================================================================================')
+    print('BallDisplay needs to be built. Please \"cd\" into ../models/graphics and type \"make\".')
+    print('==================================================================================')

trick_sims/SIM_contact/RUN_test2/input.py

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+execfile("Modified_data/realtime.py")
+
+# Variable Server Data should be copied at top of frame.
+trick.var_set_copy_mode(2)
+
+dyn.contact.nballs = 5
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+#                                        x,   y,   vx,  vy,  r,   m
+dyn.contact.balls[0] = trick.make_Ball( 0.0, 0.0, 0.67, 0.0, 2.0, 4.0)
+dyn.contact.balls[1] = trick.make_Ball( 5.0, 0.0,-0.67, 0.0, 0.5, 1.0)
+dyn.contact.balls[2] = trick.make_Ball( 7.0, 0.3,-0.67, 0.0, 0.5, 1.0)
+dyn.contact.balls[3] = trick.make_Ball(-7.0, 0.0, 0.67, 0.1, 0.5, 1.0)
+dyn.contact.balls[4] = trick.make_Ball( 9.0, 0.0,-0.67, 0.0, 0.5, 1.0)
+
+dyn_integloop.getIntegrator(trick.Euler, 2*dyn.contact.nballs)
+
+# dyn.contact.nbounds = 5
+# dyn.contact.bounds = trick.TMM_declare_var_1d("Boundary*", dyn.contact.nbounds)
+
+#==========================================
+# Start the Satellite Graphics Client
+#==========================================
+varServerPort = trick.var_server_get_port();
+BallDisplay_path = "models/graphics/dist/BallDisplay.jar"
+
+if (os.path.isfile(BallDisplay_path)) :
+    BallDisplay_cmd = "java -jar " \
+                   + BallDisplay_path \
+                   + " " + str(varServerPort) + " &" ;
+    print(BallDisplay_cmd)
+    os.system( BallDisplay_cmd);
+else :
+    print('==================================================================================')
+    print('BallDisplay needs to be built. Please \"cd\" into ../models/graphics and type \"make\".')
+    print('==================================================================================')

trick_sims/SIM_contact/RUN_test3/input.py

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+execfile("Modified_data/realtime.py")
+
+# Variable Server Data should be copied at top of frame.
+trick.var_set_copy_mode(2)
+
+dyn.contact.nballs = 5
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+#                                        x,   y,   vx,  vy,  r
+dyn.contact.balls[0] = trick.make_Ball(0.0, 0.0, 0.67, 0.0, 0.5, 5.0)
+dyn.contact.balls[1] = trick.make_Ball(1.5, 0.0, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[2] = trick.make_Ball(3.0, 0.0, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[3] = trick.make_Ball(4.5, 0.0, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[4] = trick.make_Ball(6.0, 0.0, 0.0, 0.0, 0.5, 1.0)
+
+dyn_integloop.getIntegrator(trick.Euler, 2*dyn.contact.nballs)
+
+# dyn.contact.nbounds = 5
+# dyn.contact.bounds = trick.TMM_declare_var_1d("Boundary*", dyn.contact.nbounds)
+
+#==========================================
+# Start the Satellite Graphics Client
+#==========================================
+varServerPort = trick.var_server_get_port();
+BallDisplay_path = "models/graphics/dist/BallDisplay.jar"
+
+if (os.path.isfile(BallDisplay_path)) :
+    BallDisplay_cmd = "java -jar " \
+                   + BallDisplay_path \
+                   + " " + str(varServerPort) + " &" ;
+    print(BallDisplay_cmd)
+    os.system( BallDisplay_cmd);
+else :
+    print('==================================================================================')
+    print('BallDisplay needs to be built. Please \"cd\" into ../models/graphics and type \"make\".')
+    print('==================================================================================')

trick_sims/SIM_contact/RUN_test4/input.py

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+execfile("Modified_data/realtime.py")
+
+# Variable Server Data should be copied at top of frame.
+trick.var_set_copy_mode(2)
+
+dyn.contact.nballs = 7
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+#                                        x,   y,   vx,  vy,  r
+dyn.contact.balls[0] = trick.make_Ball(-5.0, 0.1, 0.5, 0.0, 0.5, 1.0)
+dyn.contact.balls[1] = trick.make_Ball( 0.0, 0.0, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[2] = trick.make_Ball( 1.2, 0.6, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[3] = trick.make_Ball( 1.2,-0.6, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[4] = trick.make_Ball( 2.4, 1.2, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[5] = trick.make_Ball( 2.4, 0.0, 0.0, 0.0, 0.5, 1.0)
+dyn.contact.balls[6] = trick.make_Ball( 2.4,-1.2, 0.0, 0.0, 0.5, 1.0)
+
+dyn_integloop.getIntegrator(trick.Euler, 2*dyn.contact.nballs)
+
+# dyn.contact.nbounds = 5
+# dyn.contact.bounds = trick.TMM_declare_var_1d("Boundary*", dyn.contact.nbounds)
+
+#==========================================
+# Start the Satellite Graphics Client
+#==========================================
+varServerPort = trick.var_server_get_port();
+BallDisplay_path = "models/graphics/dist/BallDisplay.jar"
+
+if (os.path.isfile(BallDisplay_path)) :
+    BallDisplay_cmd = "java -jar " \
+                   + BallDisplay_path \
+                   + " " + str(varServerPort) + " &" ;
+    print(BallDisplay_cmd)
+    os.system( BallDisplay_cmd);
+else :
+    print('==================================================================================')
+    print('BallDisplay needs to be built. Please \"cd\" into ../models/graphics and type \"make\".')
+    print('==================================================================================')

trick_sims/SIM_contact/RUN_test5/input.py

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+execfile("Modified_data/realtime.py")
+
+# Variable Server Data should be copied at top of frame.
+trick.var_set_copy_mode(2)
+
+dyn.contact.nballs = 3
+dyn.contact.balls = trick.TMM_declare_var_1d("Ball*", dyn.contact.nballs)
+#                                          x,   y,   vx,  vy,    r,   mass
+dyn.contact.balls[0] = trick.make_Ball( -5.0,  2.0, 1.0, 0.0,  0.5,    1.0)
+dyn.contact.balls[1] = trick.make_Ball(  5.0, 10.2, 0.0, 0.0, 10.0, 1000.0)
+dyn.contact.balls[2] = trick.make_Ball(  5.0,-10.2, 0.0, 0.0, 10.0, 1000.0)
+
+dyn_integloop.getIntegrator(trick.Euler, 2*dyn.contact.nballs)
+
+# dyn.contact.nbounds = 5
+# dyn.contact.bounds = trick.TMM_declare_var_1d("Boundary*", dyn.contact.nbounds)
+
+#==========================================
+# Start the Satellite Graphics Client
+#==========================================
+varServerPort = trick.var_server_get_port();
+BallDisplay_path = "models/graphics/dist/BallDisplay.jar"
+
+if (os.path.isfile(BallDisplay_path)) :
+    BallDisplay_cmd = "java -jar " \
+                   + BallDisplay_path \
+                   + " " + str(varServerPort) + " &" ;
+    print(BallDisplay_cmd)
+    os.system( BallDisplay_cmd);
+else :
+    print('==================================================================================')
+    print('BallDisplay needs to be built. Please \"cd\" into ../models/graphics and type \"make\".')
+    print('==================================================================================')

trick_sims/SIM_contact/S_define

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+/************************************************************
+PURPOSE:
+    ( Simulation of Ball Contact. )
+LIBRARY DEPENDENCIES:
+    ((contact/src/Contact.cpp))
+*************************************************************/
+#include "sim_objects/default_trick_sys.sm"
+##include "contact/include/Contact.hh"
+class ContactSimObject : public Trick::SimObject {
+    public:
+    Contact contact;
+
+    ContactSimObject() {
+        ("default_data")            contact.default_data() ;
+        ("initialization")          contact.state_init() ;
+        ("derivative")              contact.state_deriv() ;
+        ("integration") trick_ret = contact.state_integ() ;
+        ("dynamic_event")           contact.collision() ;
+    }
+};
+
+ContactSimObject dyn;
+IntegLoop dyn_integloop(0.1) dyn;

trick_sims/SIM_contact/S_overrides.mk

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+TRICK_CFLAGS += -Imodels
+TRICK_CXXFLAGS += -Imodels

trick_sims/SIM_contact/models/contact/include/Ball.hh

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+/********************************* TRICK HEADER *******************************
+PURPOSE: ( Simulate balls contacting boundaries. )
+LIBRARY DEPENDENCY:
+    ((Ball.o))
+*******************************************************************************/
+#ifndef _ball_hh_
+#define _ball_hh_
+
+class Ball {
+    public:
+        Ball(){}
+        double pos[2];
+        double vel[2];
+        double mass;
+        double radius;
+        Ball(double x, double y, double vx, double vy, double r, double m);
+};
+
+Ball* make_Ball(double x, double y, double vx, double vy, double r, double m);
+
+#endif

trick_sims/SIM_contact/models/contact/include/Contact.hh

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+/************************************************************************
+PURPOSE: (Simulate a contact.)
+LIBRARY DEPENDENCIES:
+    ((contact/src/Contact.o))
+**************************************************************************/
+#ifndef _contact_hh_
+#define _contact_hh_
+#include "trick/regula_falsi.h"
+#include "Ball.hh"
+
+class Contact {
+
+    public:
+        Contact(){}
+        Ball ** balls;
+        unsigned int nballs;
+        REGULA_FALSI rf ;
+
+        void ballCollision(Ball &b1, Ball &b2);
+        int default_data();
+        int state_init();
+        int state_deriv();
+        int state_integ();
+        double collision();
+
+};
+#endif

trick_sims/SIM_contact/models/contact/src/Ball.cpp

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+/********************************* TRICK HEADER *******************************
+PURPOSE: ( Simulate balls contacting boundaries. )
+LIBRARY DEPENDENCY:
+    ((Ball.o))
+*******************************************************************************/
+#include "../include/Ball.hh"
+#include "trick/memorymanager_c_intf.h"
+#include <new>
+
+Ball::Ball(double x, double y, double vx, double vy, double r, double m) {
+    pos[0] = x;
+    pos[1] = y;
+    vel[0] = vx;
+    vel[1] = vy;
+    radius = r;
+    mass = m;
+}
+
+Ball* make_Ball(double x, double y, double vx, double vy, double r, double m) {
+    Ball* b = (Ball*)TMM_declare_var_s("Ball");
+    return (new (b) Ball(x,y,vx,vy,r,m));
+}

trick_sims/SIM_contact/models/contact/src/Contact.cpp

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+/********************************* TRICK HEADER *******************************
+PURPOSE: ( Simulate balls contacting boundaries. )
+LIBRARY DEPENDENCY:
+    ((Contact.o)
+     (Ball.o))
+*******************************************************************************/
+#include <math.h>
+#include <iostream>
+#include "trick/integrator_c_intf.h"
+#include "../include/Contact.hh"
+
+int Contact::default_data() {
+    balls = NULL;
+    nballs = 0;
+    return (0);
+}
+
+int Contact::state_init() {
+    rf.mode = Decreasing;
+    rf.error_tol = 0.001;
+    return (0);
+}
+
+int Contact::state_deriv() {
+    return(0);
+}
+
+int Contact::state_integ() {
+    int integration_step;
+    for (unsigned int ii = 0; ii < nballs; ii++) {
+        // Be sure to fix load_indexed_state() so it only loads @ intermediate_step == 0
+        load_indexed_state( 2*ii,   balls[ii]->pos[0]);
+        load_indexed_state( 2*ii+1, balls[ii]->pos[1]);
+    }
+    for (unsigned int ii = 0; ii < nballs; ii++) {
+        load_indexed_deriv( 2*ii,   balls[ii]->vel[0]);
+        load_indexed_deriv( 2*ii+1, balls[ii]->vel[1]);
+    }
+    integration_step = integrate();
+    for (unsigned int ii = 0; ii < nballs; ii++) {
+        balls[ii]->pos[0] = unload_indexed_state( 2*ii );
+        balls[ii]->pos[1] = unload_indexed_state( 2*ii+1 );
+    }
+    return(integration_step);
+}
+
+void Contact::ballCollision(Ball &b1, Ball &b2) {
+    double unorm[2];
+    double utang[2];
+    if ((b1.mass > 0.0) && (b2.mass > 0.0)) {
+        unorm[0] = ( b2.pos[0] - b1.pos[0] ) / ( b2.radius + b1.radius );
+        unorm[1] = ( b2.pos[1] - b1.pos[1] ) / ( b2.radius + b1.radius );
+        utang[0] = -unorm[1];
+        utang[1] =  unorm[0];
+        double b1vn = unorm[0] * b1.vel[0] + unorm[1] * b1.vel[1];
+        double b1vt = utang[0] * b1.vel[0] + utang[1] * b1.vel[1];
+        double b2vn = unorm[0] * b2.vel[0] + unorm[1] * b2.vel[1];
+        double b2vt = utang[0] * b2.vel[0] + utang[1] * b2.vel[1];
+        double b1vtAfter = b1vt;
+        double b2vtAfter = b2vt;
+        double b1vnAfter = ( b1vn * (b1.mass - b2.mass) + 2.0 * b2.mass * b2vn ) / ( b1.mass + b2.mass );
+        double b2vnAfter = ( b2vn * (b2.mass - b1.mass) + 2.0 * b1.mass * b1vn ) / ( b1.mass + b2.mass );
+        b1.vel[0] = b1vnAfter * unorm[0] + b1vtAfter * utang[0];
+        b1.vel[1] = b1vnAfter * unorm[1] + b1vtAfter * utang[1];
+        b2.vel[0] = b2vnAfter * unorm[0] + b2vtAfter * utang[0];
+        b2.vel[1] = b2vnAfter * unorm[1] + b2vtAfter * utang[1];
+    } else {
+        std::cout << "ERROR: Balls must have positive mass." << std::endl;
+    }
+}
+
+double Contact::collision() {
+    double tgo ; /* time-to-go */
+    double now ; /* current integration time. */
+    int first, second;
+
+    double min_dist = 1000000.0;
+    for (unsigned int ii = 0; ii < nballs; ii++) {
+        for (unsigned int jj = ii+1; jj < nballs; jj++) {
+            double xdiff = balls[ii]->pos[0] - balls[jj]->pos[0];
+            double ydiff = balls[ii]->pos[1] - balls[jj]->pos[1];
+            double dist = sqrt( xdiff * xdiff + ydiff * ydiff) - (balls[ii]->radius + balls[jj]->radius);
+            if (dist < min_dist) {
+                min_dist = dist;
+                first = ii;
+                second = jj;
+            }
+        }
+    }
+    rf.error = min_dist;
+
+    now = get_integ_time() ;               /* Get the current integration time */
+    tgo = regula_falsi( now, &rf ) ;       /* Estimate remaining integration time. */
+    if (tgo == 0.0) {                      /* If we are at the event, it's action time! */
+        now = get_integ_time() ;
+        ballCollision(*balls[first], *balls[second]);
+        std::cout << "Ball["<<first<<"] and Ball["<<second<<"] collide at t = "<<now<<"." << std::endl;
+        reset_regula_falsi( now, &rf );
+    }
+    return (tgo) ;
+}

trick_sims/SIM_contact/models/graphics/Makefile

@@ -0,0 +1,36 @@
+SHELL = /bin/sh
+
+PROJECT_NAME = BallDisplay
+SRC_DIR = src
+BUILD_DIR = build
+CLASSES_DIR = $(BUILD_DIR)/classes
+JAR_DIR = dist
+MAIN_CLASS = BallDisplay
+
+all: jar
+
+clean:
+	rm -rf $(BUILD_DIR)
+	rm -f manifest
+
+spotless: clean
+	rm -rf dist
+
+$(CLASSES_DIR):
+	@ mkdir -p $(CLASSES_DIR)
+
+compile: | $(CLASSES_DIR)
+	javac -sourcepath $(SRC_DIR) -d $(CLASSES_DIR) $(SRC_DIR)/BallDisplay.java
+
+manifest:
+	@ echo "Main-Class: $(MAIN_CLASS)" > $@
+
+$(JAR_DIR):
+	@ mkdir -p $(JAR_DIR)
+
+jar: compile manifest | $(JAR_DIR)
+	jar cvfm $(JAR_DIR)/$(PROJECT_NAME).jar manifest -C $(CLASSES_DIR) .
+	@ echo "-------------------------------------------------------------------------------"
+	@ echo "                              BUILD COMPLETE"
+	@ echo "The Java jar file (the Java Executable) is located at: $(JAR_DIR)/$(PROJECT_NAME).jar"
+	@ echo "-------------------------------------------------------------------------------"

trick_sims/SIM_contact/models/graphics/src/BallDisplay.java

@@ -0,0 +1,280 @@
+/*
+ * Trick
+ * 2020 (c) National Aeronautics and Space Administration (NASA)
+ *
+ * @author penn
+ */
+
+import java.awt.Color;
+import java.awt.Graphics2D;
+import java.awt.Graphics;
+import java.awt.RenderingHints;
+import java.io.BufferedOutputStream;
+import java.io.BufferedReader;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.net.Socket;
+import javax.swing.BoxLayout;
+import javax.swing.JFrame;
+import javax.swing.JPanel;
+
+class Ball {
+    public Color color;
+    public double x;
+    public double y;
+    public double radius;
+    public int identity;
+    public Ball (int id) {
+        identity = id;
+        x = 0.0;
+        y = 0.0;
+        radius = 0.5;
+        if (id == 0) {
+            color = Color.GREEN;
+        } else {
+            color = Color.LIGHT_GRAY;
+        }
+    }
+}
+
+class RangeView extends JPanel {
+
+    private int scale;
+    private Color backGroundColor;
+
+    // Origin of world coordinates in jpanel coordinates.
+    private int worldOriginX;
+    private int worldOriginY;
+
+    public Ball[] balls;
+
+    /**
+     * Class constructor.
+     */
+    public RangeView( int mapScale, int numberOfBalls) {
+        setScale(mapScale);
+        backGroundColor = Color.WHITE;
+        balls = new Ball[numberOfBalls];
+        for (int ii=0 ; ii<numberOfBalls ; ii++) {
+            balls[ii] = new Ball(ii);
+        }
+    }
+
+    public void setScale (int mapScale) {
+        if (mapScale < 4) {
+            scale = 4;
+        } else if (mapScale > 128) {
+            scale = 128;
+        } else {
+            scale = mapScale;
+        }
+        repaint();
+    }
+
+    public int getScale() {
+        return scale;
+    }
+
+    public void drawCenteredCircle(Graphics2D g, int x, int y, int d) {
+        x = x-(d/2);
+        y = y-(d/2);
+        g.fillOval(x,y,d,d);
+    }
+
+    private void doDrawing(Graphics g) {
+        Graphics2D g2d = (Graphics2D) g;
+
+        RenderingHints rh = new RenderingHints(
+                RenderingHints.KEY_ANTIALIASING,
+                RenderingHints.VALUE_ANTIALIAS_ON);
+
+        rh.put(RenderingHints.KEY_RENDERING,
+               RenderingHints.VALUE_RENDER_QUALITY);
+
+        int width  = getWidth();
+        int height = getHeight();
+
+        worldOriginX = (width/2);
+        worldOriginY = (height/2);
+
+        // Draw Background
+        g2d.setPaint(backGroundColor);
+        g2d.fillRect(0, 0, width, height);
+
+        //  Draw balls
+        for (int ii = 0; ii < balls.length ; ii++) {
+            g2d.setPaint(balls[ii].color);
+            int bx = (int)(worldOriginX + scale * balls[ii].x);
+            int by = (int)(worldOriginY - scale * balls[ii].y);
+            drawCenteredCircle(g2d, bx, by, (int)(scale * 2 * balls[ii].radius));
+            g2d.setPaint(Color.BLACK);
+            g2d.drawString ( String.format("%d",ii), bx,by);
+        }
+
+        g2d.drawString ( String.format("SCALE: %d pixels/meter",scale), 20,20);
+
+    }
+
+    @Override
+    public void paintComponent(Graphics g) {
+        super.paintComponent(g);
+        doDrawing(g);
+    }
+}
+
+public class BallDisplay extends JFrame {
+
+    private RangeView rangeView;
+    private BufferedReader in;
+    private DataOutputStream out;
+    private JPanel panelGroup0;
+    private JPanel panelGroup1;
+
+    public BallDisplay() {
+
+        rangeView = null;
+        setTitle("Lander Range");
+        setSize(800, 500);
+        setLocationRelativeTo(null);
+        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
+        setFocusable(true);
+    }
+
+    public void connectToServer(String host, int port ) throws IOException {
+        Socket socket = new Socket(host, port);
+        in = new BufferedReader( new InputStreamReader( socket.getInputStream()));
+        out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));
+    }
+
+    public void setRangeView (RangeView arena) {
+        rangeView = arena;
+        panelGroup1 = new JPanel();
+        panelGroup1.setLayout(new BoxLayout(panelGroup1, BoxLayout.X_AXIS));
+        panelGroup1.add(rangeView);
+        add(panelGroup1);
+    }
+
+    private static void  printHelpText() {
+        System.out.println(
+            "----------------------------------------------------------------------\n"
+          + "usage: java jar BallDisplay.jar <port-number>\n"
+          + "----------------------------------------------------------------------\n"
+          );
+    }
+
+    public static void main(String[] args) throws IOException, InterruptedException {
+
+        String host = "localHost";
+        int port = 0;
+        boolean boom = false;
+
+        // Handle program arguments.
+        int ii = 0;
+        while (ii < args.length) {
+            switch (args[ii]) {
+                case "-help" :
+                case "--help" : {
+                    printHelpText();
+                    System.exit(0);
+                } break;
+                default : {
+                    port = (Integer.parseInt(args[ii]));
+                } break;
+            }
+            ++ii;
+        }
+
+        boolean go = true;
+        double dt = 0.100;  // Time between updates (seconds).
+        int mapScale = 16 ; // pixels per meter.
+        int nballs = 7;
+
+        if (port == 0) {
+            System.out.println("No variable server port specified.");
+            printHelpText();
+            System.exit(0);
+        }
+
+        BallDisplay ballDisplay = new BallDisplay();
+
+        // Connect to the Trick simulation's variable server.
+        System.out.println("Connecting to: " + host + ":" + port);
+        ballDisplay.connectToServer(host, port);
+        ballDisplay.out.writeBytes("trick.var_set_client_tag(\"BallDisplay\") \n");
+        ballDisplay.out.flush();
+
+
+        // Get the number of  balls.
+        ballDisplay.out.writeBytes(
+            "trick.var_add(\"dyn.contact.nballs\")\n" +
+            "trick.var_send() \n" +
+            "trick.var_clear() \n");
+        ballDisplay.out.flush();
+        try {
+            String line;
+            String field[];
+            line = ballDisplay.in.readLine();
+            field = line.split("\t");
+            nballs = Integer.parseInt( field[1]);
+        } catch (IOException | NullPointerException e ) {
+            go = false;
+        }
+
+        RangeView rangeView = new RangeView(mapScale, nballs);
+        ballDisplay.setRangeView(rangeView);
+        ballDisplay.setVisible(true);
+
+        // Get the Radii of the balls.
+        for ( ii = 0; ii < nballs; ii ++) {
+            ballDisplay.out.writeBytes( String.format("trick.var_add(\"dyn.contact.balls[%d][0].radius\")\n", ii));
+        }
+        ballDisplay.out.flush();
+        ballDisplay.out.writeBytes(
+            "trick.var_send() \n" +
+            "trick.var_clear() \n");
+        ballDisplay.out.flush();
+        try {
+            String line;
+            String field[];
+            line = ballDisplay.in.readLine();
+            field = line.split("\t");
+            for ( ii=0; ii < nballs; ii++) {
+                rangeView.balls[ii].radius = Double.parseDouble( field[ii+1]);
+            }
+        } catch (IOException | NullPointerException e ) {
+            go = false;
+        }
+
+        // Get the Positions of the balls, and update the display, periodically.
+        ballDisplay.out.writeBytes( "trick.var_pause() \n");
+        for ( ii = 0; ii < nballs; ii ++) {
+            ballDisplay.out.writeBytes(
+                  String.format("trick.var_add(\"dyn.contact.balls[%d][0].pos[0]\")\n", ii)
+                + String.format("trick.var_add(\"dyn.contact.balls[%d][0].pos[1]\")\n", ii)
+            );
+        }
+        ballDisplay.out.writeBytes("trick.var_ascii() \n" +
+                                   String.format("trick.var_cycle(%.3f)\n", dt) +
+                                   "trick.var_unpause() \n" );
+        ballDisplay.out.flush();
+        while (go) {
+            try {
+                String line;
+                String field[];
+                line = ballDisplay.in.readLine();
+                // System.out.println("Sim->Client:" + line);
+                field = line.split("\t");
+                for ( ii=0; ii < nballs; ii++) {
+                    rangeView.balls[ii].x = Double.parseDouble( field[2*ii+1]);
+                    rangeView.balls[ii].y = Double.parseDouble( field[2*ii+2]);
+                }
+            } catch (IOException | NullPointerException e ) {
+                go = false;
+            }
+            //  Update the scene.
+            rangeView.repaint();
+        } // while
+
+    } // main
+} // class