Question

Greetings stackoverflow community!

I am working on an Android game that uses Box2D for collision detection and physics. Right now, I’m implementing a basic platformer system. Jumping is in place, but it has a small glitch. The following function is meant to determine if a player is on the ground. Simple, right? Well, it works okay, unless the player is right next to a wall, in which case it’ll just let him constantly jump up, as the function will still return true, even if the wall isn’t directly below the player.

I have tried adding additional x-checks to make sure that I’m colliding with something that’s directly under me, but to no avail.

private boolean isGrounded(float deltaTime) {               
    List<Contact> contactList = world.getContactList();

    for(int i = 0; i < contactList.size(); i++) {
        Contact contact = contactList.get(i);
        if(contact.isTouching() && (contact.getFixtureA() == sensorFixture ||
           contact.getFixtureB() == sensorFixture)) {               

            Vector2 pos = body.getPosition();
            WorldManifold manifold = contact.getWorldManifold();
            boolean below = true;
            Vector2 cpoint;
            for(int j = 0; j < manifold.getNumberOfContactPoints(); j++) {
                cpoint = manifold.getPoints()[j];
                below &= (
                            (cpoint.y < pos.y - 0.15f) 
                            && (cpoint.x > pos.x - w/2 + 3f)
                            && (cpoint.x < pos.x + w/2 - 3f)
                        );

                if(below) return true;
            }

            return false;
        }
    }
    return false;
}

So, does anyone have any pointers towards what I could do to make the isGrounded check more efficient?

Thanks in advance! 🙂

Answer 1

I’m not familiar with Box2d, but my first thought would be to check the orientation of the object you are in contact with. A quick check of the documentation will tell you that WorldManifold has a getNormal which provides a local normal that points from fixtureA to fixtureB. From there you compare the orientation to the X-axis or Y-axis using a dot product. Then you can threshold how steep a surface you want to let the player be able to jump on. You may need to convert the local normal to world space.

Sources: