Box2D program crashes when using multiple fixtures

Question

This should be a quick question, with hopefully a quick answer. :) I am using box2d for a side scroller project I'm doing. All's going good, thanks for asking. :D Anyways, my character needed to differently sized boxes used for bounding. I thought it would be as simple as adding a new fixture to the body, but it didn't work. It crashes and I get this:

java: ./Box2D/Collision/b2Distance.h:103: const b2Vec2& b2DistanceProxy::GetVertex(int32) const: Assertion `0 <= index && index < m_count' failed.

I have no idea what that means. Here is the code I use to create the body and fixtures:

        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyType.DynamicBody;
        bodyDef.position.set(level.character.position);
        Body body = b2world.createBody(bodyDef);
        level.character.body = body;

        PolygonShape polygonShapeHead = new PolygonShape();
        origin.x = level.character.circleBoundOrigin.x * 2.0f;
        origin.y = level.character.circleBoundOrigin.y * 3.0f;
        polygonShapeHead.setAsBox(level.character.circleBoundOrigin.x,
                level.character.circleBoundOrigin.y, origin, 0);
        FixtureDef fixtureDefHead = new FixtureDef();
        fixtureDefHead.shape = polygonShapeHead;
        fixtureDefHead.friction = level.character.friction.x;
        body.createFixture(fixtureDefHead);

        polygonShapeHead.dispose();

        PolygonShape polygonShapeBod = new PolygonShape();
        origin = level.character.rectBoundOrigin;
        polygonShapeHead.setAsBox(level.character.rectBoundOrigin.x,
                level.character.rectBoundOrigin.y, origin, 0);
        FixtureDef fixtureDefBod = new FixtureDef();
        fixtureDefBod.shape = polygonShapeBod;
        fixtureDefBod.friction = level.character.friction.x;
        body.createFixture(fixtureDefBod);

        polygonShapeBod.dispose();

As far as I know, I'm doing everything properly. So why is it not working? Why does it crash? I'm using fixturedef. Could that possibly be the problem. Thanks!

Do you know what line the error occurs on in the code you've given us? — kabb
No, unfortunately. The only thing that appeared in the console box was the message above. — josephoneill
I just ran it, and it worked. Ran it again, same message. Sometimes it works, sometimes it doesn't. What's going on. :| — josephoneill
Could it be that your setAsBox() does set the vertices incorrect? Like width=5, height=0. or width=-2, height=3? — noone
No. :/ Everything is proper. The strange thing is, every 1/20 tries, it'll actually run. It runs just how I need it too, with boxes in the correct spots. Could it possibly be not an error in my code, but a build error? — josephoneill

JMD83 JMD83 · Accepted Answer · 2014-05-20T19:58:17

This happens when you add/destroy obects during world.step(). If you're dynamically adding/destroying objects you need to queue them up and ensure you make your changes before or after you call world.step().

It's discussed in this tutorial series. http://youtu.be/ACQaU2Vr1ao?t=15m26s

Box2D program crashes when using multiple fixtures

1 Answers