Box2d setTransform

Question

Set the scene: • Our environment: LibGdx Android & iOS //If you don't know LibGdx, it's fine, just know that this program is being build for iOS and Android

• Our screen has nothing on it, just a Box2d Circle

Okay, so, what do I need to do? I need to make my Box2d Circle to "teleport" to where the screen is tapped... but I need it to have some velocity so if it collides w/ something else it will throw the other object out of its way.

I am currently using body.setTransform() but I have no velocity and people say it is very buggy.

How can I do this? Thank you!

Answer 1

The setTransform method is not buggy itself. It can cause some troubles because of ignoring physics which can appear when :

• using joints - joints provide some constraint and this "violent" change can make bodies behave weird
• collision exists - because the transformed body can be "pushed" into other body

The situation you are describing is totally typical situation when setTransform is being used and I see no reason here to be afraid of it. You don't need any velocity here also.

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However, if you decide to change the "teleporting" mechanism to applying the velocity to move object to the world point you should:

• calculate velocity vector by subtracting target position and body position

   Vector2 velocity = target.sub( body.getPosition() ) ); //where target is Vector2 of touched point 

• limit the velocity to some maximum I guess if you need to (this is optional)

   //optional velocity.nor(); velocity.mul( MAX_VALUE ); //MAX_VALUE is a float 

• set the velocity to the body

   body.setLinearVelocity( velocity ); 

• add to main loop (render method) check if the body is in target position (or in some range - due to precision there is small chance that it will precisely at the target if you will limit the velocity - if not I guess it should be in the target position after one iteration of world.update() )

   if( body.getPosition().sub( target ).len() < SOME_PRECISION ) { body.setLinearVelocity( new Vector2(0, 0) ); } 

You can also take a look at Box2D MouseJoint although I've been never using this and cannot provide any hint here.