After coming to rest JMonkey physics objects are frozen in position

Question

This is a very simple extension of HelloPhysics in the JME3 tutorial. When you click on a brick, the brick is removed from the game and the physicsspace. If you remove the bricks fast enough, before the wall settles, it crumbles like you would expect, but if you wait a little bit before removing bricks, nothing happens at all.

I think the physics turn off after things have stopped moving to prevent excessive jittering, and I want to manually start it up again.

package jme3test.helloworld;

import com.jme3.app.SimpleApplication;
import com.jme3.asset.TextureKey;
import com.jme3.bullet.BulletAppState;
import com.jme3.bullet.control.RigidBodyControl;
import com.jme3.collision.CollisionResult;
import com.jme3.collision.CollisionResults;
import com.jme3.font.BitmapText;
import com.jme3.input.KeyInput;
import com.jme3.input.MouseInput;
import com.jme3.input.controls.ActionListener;
import com.jme3.input.controls.KeyTrigger;
import com.jme3.input.controls.MouseButtonTrigger;
import com.jme3.material.Material;
import com.jme3.math.Ray;
import com.jme3.math.Vector2f;
import com.jme3.math.Vector3f;
import com.jme3.scene.Geometry;
import com.jme3.scene.Node;
import com.jme3.scene.shape.Box;
import com.jme3.scene.shape.Sphere;
import com.jme3.scene.shape.Sphere.TextureMode;
import com.jme3.texture.Texture;
import com.jme3.texture.Texture.WrapMode;


/**
 * Example 12 - how to give objects physical properties so they bounce and fall.
 * @author base code by double1984, updated by zathras
 */
class HelloPhysics extends SimpleApplication {

  public static void main(String args[]) {
    HelloPhysics app = new HelloPhysics();
    app.start();
  }

  /** Prepare the Physics Application State (jBullet) */
  private BulletAppState bulletAppState;

  /** Prepare Materials */
  Material wall_mat;
  Material stone_mat;
  Material floor_mat;

  /** Prepare geometries and physical nodes for bricks and cannon balls. */
  Node removables;
  private RigidBodyControl    brick_phy;
  private static final Box    box;
  private RigidBodyControl    ball_phy;
  private static final Sphere sphere;
  private RigidBodyControl    floor_phy;
  private static final Box    floor;

  /** dimensions used for bricks and wall */
  private static final float brickLength = 0.48f;
  private static final float brickWidth  = 0.24f;
  private static final float brickHeight = 0.12f;

  static {
    /** Initialize the cannon ball geometry */
    sphere = new Sphere(32, 32, 0.4f, true, false);
    sphere.setTextureMode(TextureMode.Projected);
    /** Initialize the brick geometry */
    box = new Box(Vector3f.ZERO, brickLength, brickHeight, brickWidth);
    box.scaleTextureCoordinates(new Vector2f(1f, .5f));
    /** Initialize the floor geometry */
    floor = new Box(Vector3f.ZERO, 10f, 0.1f, 5f);
    floor.scaleTextureCoordinates(new Vector2f(3, 6));
  }

  @Override
  public void simpleInitApp() {
    /** Set up Physics Game */
    bulletAppState = new BulletAppState();
    stateManager.attach(bulletAppState);
    //bulletAppState.getPhysicsSpace().enableDebug(assetManager);

    /** Configure cam to look at scene */
    cam.setLocation(new Vector3f(0, 4f, 6f));
    cam.lookAt(new Vector3f(2, 2, 0), Vector3f.UNIT_Y);
    /** Add InputManager action: Left click triggers shooting. */
    inputManager.addMapping("shoot", 
            new MouseButtonTrigger(MouseInput.BUTTON_LEFT));
    inputManager.addListener(actionListener, "shoot");
    removables = new Node("Removables");
    rootNode.attachChild(removables);
    /** Initialize the scene, materials, and physics space */
    initMaterials();
    initWall();
    initFloor();
    initKeys();
    initCrossHairs();
  }
    /** Declaring the "Shoot" action and mapping to its triggers. */
  private void initKeys() {
    inputManager.addMapping("Shoot",
      new KeyTrigger(KeyInput.KEY_SPACE), // trigger 1: spacebar
      new MouseButtonTrigger(MouseInput.BUTTON_LEFT)); // trigger 2: left-button click
    inputManager.addListener(actionListener, "Shoot");
  }
  /** Initialize the materials used in this scene. */
  public void initMaterials() {
    wall_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
    TextureKey key = new TextureKey("Textures/Terrain/BrickWall/BrickWall.jpg");
    key.setGenerateMips(true);
    Texture tex = assetManager.loadTexture(key);
    wall_mat.setTexture("ColorMap", tex);

    stone_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
    TextureKey key2 = new TextureKey("Textures/Terrain/Rock/Rock.PNG");
    key2.setGenerateMips(true);
    Texture tex2 = assetManager.loadTexture(key2);
    stone_mat.setTexture("ColorMap", tex2);

    floor_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
    TextureKey key3 = new TextureKey("Textures/Terrain/Pond/Pond.jpg");
    key3.setGenerateMips(true);
    Texture tex3 = assetManager.loadTexture(key3);
    tex3.setWrap(WrapMode.Repeat);
    floor_mat.setTexture("ColorMap", tex3);
  }

  /** Make a solid floor and add it to the scene. */
  public void initFloor() {
    Geometry floor_geo = new Geometry("Floor", floor);
    floor_geo.setMaterial(floor_mat);
    floor_geo.setLocalTranslation(0, -0.1f, 0);
    this.rootNode.attachChild(floor_geo);
    /* Make the floor physical with mass 0.0f! */
    floor_phy = new RigidBodyControl(0.0f);
    floor_geo.addControl(floor_phy);
    bulletAppState.getPhysicsSpace().add(floor_phy);
  }

  /** This loop builds a wall out of individual bricks. */
  public void initWall() {
    float startpt = brickLength / 4;
    float height = 0;
    for (int j = 0; j < 15; j++) {
      for (int i = 0; i < 6; i++) {
        Vector3f vt =
         new Vector3f(i * brickLength * 2 + startpt, brickHeight + height, 0);
        makeBrick(vt);
      }
      startpt = -startpt;
      height += 2 * brickHeight;
    }
  }
  /** This method creates one individual physical brick. */
  public void makeBrick(Vector3f loc) {
    /** Create a brick geometry and attach to scene graph. */
    Geometry brick_geo = new Geometry("brick", box);
    brick_geo.setMaterial(wall_mat);
    /** Position the brick geometry  */
    brick_geo.setLocalTranslation(loc);
    /** Make brick physical with a mass > 0.0f. */
    brick_phy = new RigidBodyControl(2f);

    /** Add physical brick to physics space. */
    brick_geo.addControl(brick_phy);
    bulletAppState.getPhysicsSpace().add(brick_phy);
    removables.attachChild(brick_geo);

  }

  /** This method creates one individual physical cannon ball.
   * By defaul, the ball is accelerated and flies
   * from the camera position in the camera direction.*/
   public void makeCannonBall() {
    /** Create a cannon ball geometry and attach to scene graph. */
    Geometry ball_geo = new Geometry("cannon ball", sphere);
    ball_geo.setMaterial(stone_mat);
    rootNode.attachChild(ball_geo);
    /** Position the cannon ball  */
    ball_geo.setLocalTranslation(cam.getLocation());
    /** Make the ball physcial with a mass > 0.0f */
    ball_phy = new RigidBodyControl(1f);
    /** Add physical ball to physics space. */
    ball_geo.addControl(ball_phy);
    bulletAppState.getPhysicsSpace().add(ball_phy);
    /** Accelerate the physcial ball to shoot it. */
    ball_phy.setLinearVelocity(cam.getDirection().mult(25));
  }
    private ActionListener actionListener = new ActionListener() {

    public void onAction(String name, boolean keyPressed, float tpf) {
      if (name.equals("Shoot") && !keyPressed) {
        // 1. Reset results list.
        CollisionResults results = new CollisionResults();
        // 2. Aim the ray from cam loc to cam direction.
        Ray ray = new Ray(cam.getLocation(), cam.getDirection());
        // 3. Collect intersections between Ray and Shootables in results list.
        removables.collideWith(ray, results);
        // 4. Print the results
        System.out.println("----- Collisions? " + results.size() + "-----");
        for (int i = 0; i < results.size(); i++) {
          // For each hit, we know distance, impact point, name of geometry.
          float dist = results.getCollision(i).getDistance();
          Vector3f pt = results.getCollision(i).getContactPoint();
          String hit = results.getCollision(i).getGeometry().getName();
          System.out.println("* Collision #" + i);
          System.out.println("  You shot " + hit + " at " + pt + ", " + dist + " wu away.");
        }
        // 5. Use the results (we mark the hit object)
        if (results.size() > 0) {
          // The closest collision point is what was truly hit:
          Geometry closest = results.getClosestCollision().getGeometry();
          // Let's interact - we mark the hit with a red dot.

          bulletAppState.getPhysicsSpace().remove(closest.getControl(0));
          removables.detachChild(closest);
          bulletAppState.getPhysicsSpace().clearForces();
          bulletAppState.getPhysicsSpace().applyGravity();
          System.out.println("removing a "+closest.getName());

        } 
      }
    }
  };
  /** A plus sign used as crosshairs to help the player with aiming.*/
  protected void initCrossHairs() {
    guiNode.detachAllChildren();
    guiFont = assetManager.loadFont("Interface/Fonts/Default.fnt");
    BitmapText ch = new BitmapText(guiFont, false);
    ch.setSize(guiFont.getCharSet().getRenderedSize() * 2);
    ch.setText("+");        // fake crosshairs :)
    ch.setLocalTranslation( // center
      settings.getWidth() / 2 - guiFont.getCharSet().getRenderedSize() / 3 * 2,
      settings.getHeight() / 2 + ch.getLineHeight() / 2, 0);
    guiNode.attachChild(ch);
  }
}

Answer 1

Objects which aren't moving sleep

Your initial thought was correct, when an object comes to rest it is put to sleep, this is for a number of reasons; avoiding jitter is one, but it also increases efficiency significantly.

While sleeping an object doesn't experience gravity, so if you remove the object supporting it it doesn't notice.

Usually you don't have to worry about this; if the object experiences an effect "internal to the physics engine" such as being involved with a collision it is automatically woken. However if you make external changes (such as removing an object) you have to worry about waking the object up.

Wake the objects that could have been affected by the external change

You can wake a sleeping object up by calling .activate() on the rigidBodyControl, remember sleep is done on a per object basis so you may need to wake more than 1.

At present the program does not keep a track of all the RigidBodyControl so I have added a HashSet<RigidBodyControl>() to hold them, when a brick is added to the scene it's rigidBodyControl should be added to this HashSet and when it is removed from the scene it should be removed from the hashset.

For an efficient solution you should only wake objects that could have been affected by the removal, but in this case it's probably everything anyway and I have simply woken everything by running through all of the HashSet<RigidBodyControl>() and calling activate() on them.

A complete program that demonstrates these changes is shown below

public class HelloPhysics extends SimpleApplication {
    
    public static void main(String args[]) {
        HelloPhysics app = new HelloPhysics();
        app.start();
    }
    
    /** Prepare the Physics Application State (jBullet) */
    private BulletAppState bulletAppState;
    
    /** Prepare Materials */
    Material wall_mat;
    Material stone_mat;
    Material floor_mat;
    
    /** Prepare geometries and physical nodes for bricks and cannon balls. */
    Node removables;
    
    private static final Box    box;
    private RigidBodyControl    ball_phy;
    private static final Sphere sphere;
    private RigidBodyControl    floor_phy;
    private static final Box    floor;
    
    private static Collection<RigidBodyControl> objectsThatNeedWaking=new HashSet<RigidBodyControl>();
    
    /** dimensions used for bricks and wall */
    private static final float brickLength = 0.48f;
    private static final float brickWidth  = 0.24f;
    private static final float brickHeight = 0.12f;
    
    static {
        /** Initialize the cannon ball geometry */
        sphere = new Sphere(32, 32, 0.4f, true, false);
        sphere.setTextureMode(TextureMode.Projected);
        /** Initialize the brick geometry */
        box = new Box(Vector3f.ZERO, brickLength, brickHeight, brickWidth);
        box.scaleTextureCoordinates(new Vector2f(1f, .5f));
        /** Initialize the floor geometry */
        floor = new Box(Vector3f.ZERO, 10f, 0.1f, 5f);
        floor.scaleTextureCoordinates(new Vector2f(3, 6));
    }
    
    @Override
    public void simpleInitApp() {
        /** Set up Physics Game */
        bulletAppState = new BulletAppState();
        stateManager.attach(bulletAppState);
        //bulletAppState.getPhysicsSpace().enableDebug(assetManager);
        
        /** Configure cam to look at scene */
        cam.setLocation(new Vector3f(0, 4f, 6f));
        cam.lookAt(new Vector3f(2, 2, 0), Vector3f.UNIT_Y);
        /** Add InputManager action: Left click triggers shooting. */
        inputManager.addMapping("shoot",
                new MouseButtonTrigger(MouseInput.BUTTON_LEFT));
        inputManager.addListener(actionListener, "shoot");
        removables = new Node("Removables");
        rootNode.attachChild(removables);
        /** Initialize the scene, materials, and physics space */
        initMaterials();
        initWall();
        initFloor();
        initKeys();
        initCrossHairs();
    }
    /** Declaring the "Shoot" action and mapping to its triggers. */
    private void initKeys() {
        inputManager.addMapping("Shoot",
                new KeyTrigger(KeyInput.KEY_SPACE), // trigger 1: spacebar
                new MouseButtonTrigger(MouseInput.BUTTON_LEFT)); // trigger 2: left-button click
        inputManager.addListener(actionListener, "Shoot");
    }
    /** Initialize the materials used in this scene. */
    public void initMaterials() {
        wall_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
        TextureKey key = new TextureKey("Textures/Terrain/BrickWall/BrickWall.jpg");
        key.setGenerateMips(true);
        Texture tex = assetManager.loadTexture(key);
        wall_mat.setTexture("ColorMap", tex);
        
        stone_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
        TextureKey key2 = new TextureKey("Textures/Terrain/Rock/Rock.PNG");
        key2.setGenerateMips(true);
        Texture tex2 = assetManager.loadTexture(key2);
        stone_mat.setTexture("ColorMap", tex2);
        
        floor_mat = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
        TextureKey key3 = new TextureKey("Textures/Terrain/Pond/Pond.jpg");
        key3.setGenerateMips(true);
        Texture tex3 = assetManager.loadTexture(key3);
        tex3.setWrap(WrapMode.Repeat);
        floor_mat.setTexture("ColorMap", tex3);
    }
    
    /** Make a solid floor and add it to the scene. */
    public void initFloor() {
        Geometry floor_geo = new Geometry("Floor", floor);
        floor_geo.setMaterial(floor_mat);
        floor_geo.setLocalTranslation(0, -0.1f, 0);
        this.rootNode.attachChild(floor_geo);
        /* Make the floor physical with mass 0.0f! */
        floor_phy = new RigidBodyControl(0.0f);
        floor_geo.addControl(floor_phy);
        bulletAppState.getPhysicsSpace().add(floor_phy);
    }
    
    /** This loop builds a wall out of individual bricks. */
    public void initWall() {
        float startpt = brickLength / 4;
        float height = 0;
        for (int j = 0; j < 15; j++) {
            for (int i = 0; i < 6; i++) {
                Vector3f vt =
                        new Vector3f(i * brickLength * 2 + startpt, brickHeight + height, 0);
                makeBrick(vt);
            }
            startpt = -startpt;
            height += 2 * brickHeight;
        }
    }
    /** This method creates one individual physical brick. */
    public void makeBrick(Vector3f loc) {
        /** Create a brick geometry and attach to scene graph. */
        Geometry brick_geo = new Geometry("brick", box);
        brick_geo.setMaterial(wall_mat);
        /** Position the brick geometry  */
        brick_geo.setLocalTranslation(loc);
        /** Make brick physical with a mass > 0.0f. */
        RigidBodyControl brick_phy = new RigidBodyControl(2f);
        
        objectsThatNeedWaking.add(brick_phy);
        
        
        /** Add physical brick to physics space. */
        brick_geo.addControl(brick_phy);
        
        
        bulletAppState.getPhysicsSpace().add(brick_phy);
        removables.attachChild(brick_geo);
        
    }
    
    /** This method creates one individual physical cannon ball.
     * By defaul, the ball is accelerated and flies
     * from the camera position in the camera direction.*/
    public void makeCannonBall() {
        /** Create a cannon ball geometry and attach to scene graph. */
        Geometry ball_geo = new Geometry("cannon ball", sphere);
        ball_geo.setMaterial(stone_mat);
        rootNode.attachChild(ball_geo);
        /** Position the cannon ball  */
        ball_geo.setLocalTranslation(cam.getLocation());
        /** Make the ball physcial with a mass > 0.0f */
        ball_phy = new RigidBodyControl(1f);
        /** Add physical ball to physics space. */
        ball_geo.addControl(ball_phy);
        bulletAppState.getPhysicsSpace().add(ball_phy);
        /** Accelerate the physcial ball to shoot it. */
        ball_phy.setLinearVelocity(cam.getDirection().mult(25));
    }
    private ActionListener actionListener = new ActionListener() {
        
        public void onAction(String name, boolean keyPressed, float tpf) {
            if (name.equals("Shoot") && !keyPressed) {
                // 1. Reset results list.
                CollisionResults results = new CollisionResults();
                // 2. Aim the ray from cam loc to cam direction.
                Ray ray = new Ray(cam.getLocation(), cam.getDirection());
                // 3. Collect intersections between Ray and Shootables in results list.
                removables.collideWith(ray, results);
                // 4. Print the results
                System.out.println("----- Collisions? " + results.size() + "-----");
                for (int i = 0; i < results.size(); i++) {
                    // For each hit, we know distance, impact point, name of geometry.
                    float dist = results.getCollision(i).getDistance();
                    Vector3f pt = results.getCollision(i).getContactPoint();
                    String hit = results.getCollision(i).getGeometry().getName();
                    System.out.println("* Collision #" + i);
                    System.out.println("  You shot " + hit + " at " + pt + ", " + dist + " wu away.");
                }
                // 5. Use the results (we mark the hit object)
                if (results.size() > 0) {
                    // The closest collision point is what was truly hit:
                    Geometry closest = results.getClosestCollision().getGeometry();
                    // Let's interact - we mark the hit with a red dot.
                    
                    bulletAppState.getPhysicsSpace().remove(closest.getControl(0));
                    objectsThatNeedWaking.remove(closest.getControl(0));
                    removables.detachChild(closest);
                    bulletAppState.getPhysicsSpace().clearForces();
                    bulletAppState.getPhysicsSpace().applyGravity();
                    System.out.println("removing a "+closest.getName());
                    
                    for(RigidBodyControl wakeMeUp: objectsThatNeedWaking){
                        wakeMeUp.activate();
                    }
                    
                }
            }
        }
    };
    /** A plus sign used as crosshairs to help the player with aiming.*/
    protected void initCrossHairs() {
        guiNode.detachAllChildren();
        guiFont = assetManager.loadFont("Interface/Fonts/Default.fnt");
        BitmapText ch = new BitmapText(guiFont, false);
        ch.setSize(guiFont.getCharSet().getRenderedSize() * 2);
        ch.setText("+");        // fake crosshairs :)
        ch.setLocalTranslation( // center
                settings.getWidth() / 2 - guiFont.getCharSet().getRenderedSize() / 3 * 2,
                settings.getHeight() / 2 + ch.getLineHeight() / 2, 0);
        guiNode.attachChild(ch);
    }
}