Huge impulse applied on collision

General Box2D issues or C++ specific issues
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Joined: Sat Jul 25, 2015 8:07 am

Huge impulse applied on collision

Postby tcpie » Sat Jul 25, 2015 9:26 am

Hey all,

I'm starting on a top-down game with Box2D as physics engine. A few months ago I had a first stab at it and I recall it worked quite well (unfortunately I can't access my old code :-(. )

Now, however, when my dynamic bodies collide, they tend to get pushed back with a disproportionately large amount of force! Say, box A hits box B very slowly, both upon collision both will get propelled away from eachother very fast. I do not know why this is happening.

In my entity constructor, I create my physics body as such:

Code: Select all

   b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position = start_pos;
    this->physics_body = this->physics_world->CreateBody(&bodyDef);

    b2PolygonShape dynamicBox;
    dynamicBox.SetAsBox(0.5f, 0.5f);

    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicBox;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.9f;


My main loop looks something like this:

Code: Select all

accumulator = 0;
this->tick_rate = 60;

while (true) {
   double dt_desired = (1.0)/(this->tick_rate);

   sf::Time dt = clock.restart();

   accumulator += dt.asSeconds();

   while (accumulator >= dt_desired) {
      this->physics_world->Step(dt_desired, 8, 3);

      accumulator -= dt_desired;

I move my object by applying a linear impulse (this code is called every tick):

Code: Select all

b2Vec2 move = p->GetCurAction().move_vec;

if (move == b2Vec2_zero)

float max_speed = 5.f;
float max_impulse = 3.f;

if (move.Length() > 1.f)

b2Vec2 desired_vel = b2Vec2(move.x * max_speed, move.y * max_speed);
b2Vec2 cur_vel = this->physics_body->GetLinearVelocity();

b2Vec2 diff = desired_vel - cur_vel;

if (diff.Length() > max_impulse) {
   diff *= max_impulse;

this->physics_body->ApplyLinearImpulse(diff, b2Vec2_zero, true);

I recorded a video so you get more of an idea what it looks like: ... .

I thought this would probably be something more people ran into, but a quite elaborate search on google turns up with nothing. Does anyone know what I am doing wronge? I'm using SFML for drawing and keyboard input, my platform is Windows 7, 64 bit and I compile with MinGW.

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