Box2D - When creating a body at runtime, the body does not collide

Question

I've been working on a game with destructible environments and I've come up with a solution where I check for possible destruction within my ContactListener object. Obviously because this is taking place within Step(), I postpone processing the destruction until the moment after the step. I do this by pooling "destruction events" that need to be processed within the contact listener, and then immediately after Step() calling something like contactListener->processDestructionEvents();.

The way I do this is by capturing the colliding fixtures within the beginContact event and then determining the angle of impact, then using that angle to raycast on the fixture itself. I then grap the vertices from the b2PolygonShape of the fixture, then generate two new shapes which are split based on the impact and exit points of the ray. The original fixture is destroyed on the body, and then a new fixture is generated for the first new shape and added to the original body. For the second shape, a new body is generated and that shape is added to this new body.

Anyway everything works great, in debug view I can see that the new shapes have been generated and are all in place, as they should be. However, I get really screwed up behavior at this point. As soon as this process is complete, neither the original nor the newly generated body will collide with anything. If I enable continuous physics, SOMETIMES a dynamic object will collide with one of the edges of these bodies/fixtures, but not always. I'm wondering if it's something I'm doing wrong in my approach to rebuilding bodies/fixtures at runtime. Here is the code for generating the new objects, any help would be greatly appreciated.

void PhysicsContactListener::processDestructionEvents() {
   if(!hasDestructionEvents) {return;}

   for(destructionEventsIterator = destructionEvents.begin(); destructionEventsIterator != destructionEvents.end(); ++destructionEventsIterator) {

      b2Filter f1, f2;
      f1.groupIndex = destructionEventsIterator->originalFixture->GetFilterData().groupIndex;
      f1.categoryBits = destructionEventsIterator->originalFixture->GetFilterData().categoryBits;
      f1.maskBits = destructionEventsIterator->originalFixture->GetFilterData().maskBits;

      f2.groupIndex = destructionEventsIterator->originalFixture->GetFilterData().groupIndex;
      f2.categoryBits = destructionEventsIterator->originalFixture->GetFilterData().categoryBits;
      f2.maskBits = destructionEventsIterator->originalFixture->GetFilterData().maskBits;

      b2PolygonShape newShape0 = destructionEventsIterator->newFixtures[0];

      b2FixtureDef fixture0Def;
      fixture0Def.shape = &newShape0;
      fixture0Def.density = 1.0f;
      fixture0Def.restitution = 0.2f;

      b2Fixture* fixture1 = destructionEventsIterator->hostBody->CreateFixture(&fixture0Def);
      fixture1->SetFilterData(f1);
      //destructionEventsIterator->hostBody->SetAwake(true);
      destructionEventsIterator->hostBody->ResetMassData();
      //destructionEventsIterator->hostBody->SetActive(true);
      destructionEventsIterator->hostBody->SetTransform(destructionEventsIterator->hostBody->GetPosition(), destructionEventsIterator->hostBody->GetAngle());

      b2BodyDef bd;
      bd.position = destructionEventsIterator->hostBody->GetPosition();
      bd.angle = destructionEventsIterator->hostBody->GetAngle();
      bd.type = destructionEventsIterator->hostBody->GetType();

      b2Body* newBody = destructionEventsIterator->hostBody->GetWorld()->CreateBody(&bd);
      b2PolygonShape* newShape1 = (b2PolygonShape*)(&destructionEventsIterator->newFixtures[1]);
      b2Fixture* fixture2 = newBody->CreateFixture(newShape1, destructionEventsIterator->hostBodyDensity);
      fixture2->SetFilterData(f2);
      newBody->SetAngularVelocity(destructionEventsIterator->hostBody->GetAngularVelocity());
      newBody->SetLinearVelocity(destructionEventsIterator->hostBody->GetLinearVelocity());
      //newBody->SetAwake(true);
      newBody->ResetMassData();
      //newBody->SetActive(true);
      newBody->SetTransform(destructionEventsIterator->hostBody->GetPosition(), destructionEventsIterator->hostBody->GetAngle());

      destructionEventsIterator->hostBody->DestroyFixture(destructionEventsIterator->originalFixture);

   }

Just to be sure, neither body will collide with anything means that they fall through the ground and down down into the abyss...? Or the two bodies just do not collide with each other? — iforce2d
That's correct they won't collide with anything at all. If I set them as static, nothing. If I set them as dynamic, they just fall into the abyss. — user562566

iforce2d iforce2d · Accepted Answer · 2012-02-21T22:06:12

The two pieces don't collide with each other? Take a look at the categoryBits and maskBits values that each fixture ends up with - looks like each piece is given the same values for these. My guess is you are just overlooking the fact that these masks are checked against each other both ways, eg. from the Box2D source code:

bool collide =
      (filterA.maskBits & filterB.categoryBits) != 0 &&
      (filterA.categoryBits & filterB.maskBits) != 0;

On the other hand if you mean the pieces collide with nothing at all and simply fall through the ground and down forever except for SOMETIMES, then I might suspect an incorrect polygon winding.

btw a b2Filter holds only primitives so you could assign those directly:

b2Filter f1 = destructionEventsIterator->originalFixture->GetFilterData();

...also, the first SetTransform and the second ResetMassData are redundant.

Box2D - When creating a body at runtime, the body does not collide

1 Answers