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I'm actually using Processing (a java extrapolation) to create a box 2d animation. I have stripped the problem down as follows:
I would like the boxes that the mouse creates on the screen to collide with the round bodies as a brick might hit a soft wall so that there is a 'little' bit of wall movement and the bricks bounce off a little. At the moment when I start the programme this happens but as I click to add new boxes they increasingly 'spin' off the round bodies widly. I don't know why this is. I have changed the restitution and friction etc on the boxes and the round bodies but to no avail.... Interestingly if I change the code so that there is only one round body I don't get this problem making me think the joint between round bodies might be the issue but I am not sure what to change- I'm using a DistanceJoint but maybe I should use another type of joint? My code as MCVE- my apologies for the length but this really is the minimal version
// A bridge is formed by connected a series of particles with joints
import shiffman.box2d.*;
import org.jbox2d.common.*;
import org.jbox2d.dynamics.joints.*;
import org.jbox2d.collision.shapes.*;
import org.jbox2d.collision.shapes.Shape;
import org.jbox2d.dynamics.*;
import org.jbox2d.dynamics.contacts.*;
import org.jbox2d.particle.ParticleGroupDef;
import org.jbox2d.particle.ParticleSystem;
import org.jbox2d.particle.ParticleType;
import org.jbox2d.particle.ParticleDef;
import org.jbox2d.collision.shapes.CircleShape;
import org.jbox2d.collision.shapes.PolygonShape;
import org.jbox2d.common.Vec2;
import org.jbox2d.dynamics.Body;
import org.jbox2d.dynamics.BodyDef;
import org.jbox2d.dynamics.BodyType;
// A reference to our box2d world
Box2DProcessing box2d;
ParticleSystem particleSystem;
ParticleDef pd=new ParticleDef();
Bridge bridge;
int PARTICLES = 10; // the amount of particles in your bridge
ParticleDef pdef=new ParticleDef();
// A list for all of our rectangles
ArrayList<Box> boxes;
void setup() {
size(1040,1060);
smooth();
// Initialize box2d physics and create the world
box2d = new Box2DProcessing(this);
box2d.createWorld();
// Make the bridge total length,number of points,x start position
bridge = new Bridge(width,PARTICLES,200);
// Create ArrayLists
boxes = new ArrayList<Box>();
}
void draw() {
background(255);
// We must always step through time!
box2d.step();
// When the mouse is clicked, add a new Box object
if (mousePressed) {
Box p = new Box(mouseX,mouseY);
boxes.add(p);
}
// Display all the boxes
for (Box b: boxes) {
b.display();
}
// Boxes that leave the screen, we delete them (note they have to be deleted from both the box2d world and our list
for (int i = boxes.size()-1; i >= 0; i--) {
Box b = boxes.get(i);
if (b.done()) {
boxes.remove(i);
}
}
bridge.display();
fill(0);
float move_1=300;
for (int i=0; i<10; i++){
bridge.particles.get(i).body.setTransform(box2d.coordPixelsToWorld(move_1+10,i*5+70), 0);
}
}
// A rectangular box
class Box {
// We need to keep track of a Body and a width and height
Body body;
float w;
float h;
// Constructor
Box(float x, float y) {
w = random(4,16);
h = random(4,16);
// Add the box to the box2d world
makeBody(new Vec2(x,y),w,h);
}
// This function removes the particle from the box2d world
void killBody() {
box2d.destroyBody(body);
}
// Is the particle ready for deletion?
boolean done() {
// Let's find the screen position of the particle
Vec2 pos = box2d.getBodyPixelCoord(body);
// Is it off the bottom of the screen?
if (pos.y > height+w*h) {
killBody();
return true;
}
return false;
}
// Drawing the box
void display() {
// We look at each body and get its screen position
Vec2 pos = box2d.getBodyPixelCoord(body);
// Get its angle of rotation
float a = body.getAngle();
rectMode(CENTER);
pushMatrix();
translate(pos.x,pos.y);
rotate(-a);
fill(175);
stroke(0);
rect(0,0,w,h);
popMatrix();
}
// This function adds the rectangle to the box2d world
void makeBody(Vec2 center, float w_, float h_) {
// Define a polygon (this is what we use for a rectangle)
PolygonShape sd = new PolygonShape();
float box2dW = box2d.scalarPixelsToWorld(w_/2);
float box2dH = box2d.scalarPixelsToWorld(h_/2);
sd.setAsBox(box2dW, box2dH);
// Define a fixture
FixtureDef fd = new FixtureDef();
fd.shape = sd;
// Parameters that affect physics
fd.density = 1;
fd.friction = 300;
fd.restitution = 0.0;
// Define the body and make it from the shape
BodyDef bd = new BodyDef();
bd.type = BodyType.DYNAMIC;
bd.position.set(box2d.coordPixelsToWorld(center));
body = box2d.createBody(bd);
body.createFixture(fd);
// Give it some initial random velocity
body.setLinearVelocity(new Vec2(random(-5, 5), random(2, 5)));
}
}
// The Nature of Code
// <http://www.shiffman.net/teaching/nature>
// Spring 2011
// Box2DProcessing example
// Series of Particles connected with distance joints
class Bridge {
// Bridge properties
float totalLength; // How long
int numPoints; // How many points in a line
int NewCurvepoints; // How many points in a curve for the top of the stomach
// Our chain is a list of particles
ArrayList<Particle> particles;
// Chain constructor
Bridge(float l, int n,int start) {
totalLength = l;
numPoints = n;
NewCurvepoints=n;
particles = new ArrayList();
float len = totalLength / numPoints;
// Add particles to the chain
for(int i=0; i < PARTICLES; i++) {
// Make a new particle
Particle p = null;
//x,y,radius,fixed
p = new Particle(start,i*len,7,3);
particles.add(p);
if (i > 0) {
DistanceJointDef djd = new DistanceJointDef();
Particle previous = particles.get(i-1);
djd.bodyA = previous.body;
djd.bodyB = p.body;
djd.length = box2d.scalarPixelsToWorld(len/40);
line(djd.bodyA.getPosition().x,djd.bodyA.getPosition().y,djd.bodyB.getPosition().x,djd.bodyB.getPosition().y);
djd.frequencyHz = 0;
djd.dampingRatio = 0;
DistanceJoint dj = (DistanceJoint) box2d.world.createJoint(djd);
}
}
}
// // Draw the bridge
void display() { for(int i=0; i < particles.size()-1; i++) {
Vec2 pos1 = box2d.getBodyPixelCoord(particles.get(i).body);
Vec2 pos2 = box2d.getBodyPixelCoord(particles.get(i+1).body);
stroke(0);
strokeWeight(2);
line(pos1.x,pos1.y,pos2.x,pos2.y);
}
for (Particle p: particles) {
p.display();
}
}
}
class Particle {
// We need to keep track of a Body and a radius
Body body;
float r;
color col;
Particle(float x, float y, float r_, int fixed) {
r = r_;
// Define a body
BodyDef bd = new BodyDef();
bd.fixedRotation=true;
if (fixed==1) bd.type = BodyType.STATIC;
else if (fixed==2) bd.type = BodyType.KINEMATIC;
else if (fixed==3) bd.type = BodyType.DYNAMIC;
// Set its position
bd.position = box2d.coordPixelsToWorld(x,y);
body = box2d.world.createBody(bd);
// Make the body's shape a circle
CircleShape cs = new CircleShape();
cs.m_radius = box2d.scalarPixelsToWorld(r);
FixtureDef fd = new FixtureDef();
fd.shape = cs;
// Parameters that affect physics
fd.density = 0;
fd.friction = 0;
fd.restitution = -90;
body.createFixture(fd);
col = color(175);
//determine how it moves
}
//
void display() {
// We look at each body and get its screen position
Vec2 pos = box2d.getBodyPixelCoord(body);
// Get its angle of rotation
float a = body.getAngle();
pushMatrix();
translate(pos.x,pos.y);
rotate(a);
fill(col);
stroke(0);
strokeWeight(1);
ellipse(0,0,r*2,r*2);
// Let's add a line so we can see the rotation
line(0,0,r,0);
popMatrix();
}
}