Box2D bodies moving for no reason?

Question

I am using Box2D with Cocos2D in my app. So pretty much in my app I make the bodies follow my CCSprites even though people recommend against it. Also I only use Box2D for collision detection so I just attach a body to my sprites and use a b2ContactListener and nothing else. Anyway this setup works for the most part except this little issue.

So my bodies follow my sprites like this in the game loop:

for(b2Body *b = world->GetBodyList(); b; b=b->GetNext()) {
        if (b->GetUserData() != NULL) {
            CCSprite *sprite = (CCSprite *)b->GetUserData();
                b2Vec2 b2Position = b2Vec2(sprite.position.x/PTM_RATIO, sprite.position.y/PTM_RATIO);
                float32 b2Angle = -1 * CC_DEGREES_TO_RADIANS(sprite.rotation);
                b->SetTransform(b2Position, b2Angle);
        }
    }

Also I create my b2World like this (if it makes any difference):

world = new b2World(b2Vec2(0.0f, 0.0f));
world->SetAllowSleeping(NO);
_contactListener = new MyContactListener();
world->SetContactListener(_contactListener);

This issue is, whenever my CCSprites collide (technically my b2Bodys are colliding too), the b2Body on each CCSprite seems to move a bit as if a tiny force just hit it so they have a "recoil" if that makes sense. They move farther away from each other when the hit occurs even though no forces are involved.

Another example is when one of my bodies sits on top of another body, the body that is being stood on seems to tilt a bit when I am on the edge of it which is odd because I don't tilt my sprites.

Lastly, my bodies seem to be a bit behind on where the sprite is at that exact moment and I am not sure if that is also related to this issue but anyway is there any reason why this would be happening? Is there any way to make it so the bodies are not affected by forces and are solely used for collisions?

Thanks!

I believe it goes along the lines of: If two bodies hit and they start moving away from each other, the two bodies must have hit each other with a force. The force on each of them set them in motion, and a body in motion stays in motion unless acted upon. If they are in motion they will not stop because nothing else is acting on them. I think there is likely a force involved. — Dair
Somewhat, when the bodies hit each other, the bodies move differently in an odd direction but soon snap back to the position they used to be in when it reality they shouldn't be moving in that odd direction overall. They should just be following the sprite they are attached to. I must also add nowhere in my project do I apply any forces or any other Box2D related code except for the b2ContactListener. — SimplyKiwi
Now I am not sure if this is an issue or not, although I do see this behavior when debugging my b2World it may not be the reason the following happens. Now in my game, my bodies are a bit small and they do move fast and sometimes when they should have a collision it misses for an unknown reason although all code is correct it could be because of this or for another reason which I am unsure of. Any tips/ideas/suggestions would be appreciated! — SimplyKiwi
I am not sure if this is the best way but if someone wants to confirm this: Have you tried setting both of the objects to static when the collision happens? objectA->setType(b2_staticBody) — Dair
Or better yet set their velocity to zero for both of them when they collide? — Dair

Ken Toh Ken Toh · Accepted Answer · 2012-11-13T05:02:02

I believe that the bodies are triggering a collision response upon collision, which causes them to momentarily move apart a little, before snapping back into position when you force their b2transforms to sync with the sprite's positions/angles in the next game tick.

Since you are using Box2D purely for collision detection, did you set the isSensor property of your fixture definition to true?

b2FixtureDef shapeDef;
shapeDef.isSensor = true;

By specially marking the fixture as a sensor, it will never physically collide with anything, but you will still be notified when an overlap of bodies occur, via the contact listener.

Box2D bodies moving for no reason?

1 Answers