Box2D / cocos2d animation to a point with rotation

Question

I'm pretty new to Box2D and cocos2d. I'm trying to do something which I thought would be pretty simple, but it is turning out to be more difficult than I had expected and I cannot find a working answer anywhere.

Basically, I want to move a b2body and rotate it to a certain point with animation.

I can get it to the correct position and rotation with:

targetBody->SetTransform(b2Vec2(10.0f,1.0f),10);

But I have no idea how to animate it there over time. I tried using cocos2d animation on the sprite used for the body, but that doesn't do anything. Any ideas?

Answer 1

There are a couple of ways you could do this.

You could use SetTransform every time step to update the position of the body gradually over time, in effect performing the animation yourself. The drawback with this method is that the body is 'teleporting' to the new position rather than moving, so it has no momentum, which means you can get odd results if it hits anything along the way. Still, if you know it will not hit anything or does not need to react to a collision this would be ok.

The other way is to SetLinearVelocity and SetAngularVelocity to give the body proper movement. This will keep the results of a collision realistic, and you don't need to keep updating anything every timestep. On the other hand, you will need to detect when the body has arrived at the desired position and then set the velocities back to zero, otherwise it will just keep moving. For dynamic bodies you will also need to counter gravity somehow, either by setting the gravity scale of the body to zero, or by applying an upwards force to balance gravity, or by changing the body type to kinematic during the move.

Answer 2

In general, you use Box2D to simulate the physical behavior of objects in relation to each other. The rules of mechanics implemented by Box2D dictate how your cocos2d CCSprites move if you continuously update the translation and rotation of your sprites according to their corresponding Box2d b2Body. You will have some kind of repeatedly invoked tick: method in which you step your Box2d world along in time, and in which you update your sprite positions according to simulation results of Box2d.

This pattern corresponds to b2Bodys of type b2_dynamicBody. Physical laws dictate the motion of the body in this case, and your sprites will follow these simulation results. This is why setting a conflicting position of a sprite by means of a CCAction or even directly will be undone almost instantaneously with the next tick:.

Solution 1: kinematic body type

However, there do exist other modes for a b2Body, and one of these is b2_kinematicBody in which the translation is no longer governed by the world but by the velocities or angular speeds you dictate through setters. So it would be one solution to work with body type b2_kinematicBody as in:

b2BodyDef bdef;
bdef.type = b2_kinematicBody;

With this you would manipulate the velocity and angular speed of a b2Body explicitly. In this case, Box2d is still responsible for moving bodies around, but according the velocities you dictate, and without any force effects of collision or gravity applied to this particular body. Also with this strategy, your CCSprites will follow b2Bodys. Setting conflicting positions for your sprites directly or by applying a CCAction would not make sense for the same reason as described above.

Solution 2: decouple sprites from Box2d

An alternative way to animating sprites would be to fully decouple those sprites from Box2d. In this case, you would simply not have any b2Body that governs the position of the sprite you are going to animate. Now, in this case, only you will dictate the position and rotation of your CCSprite, i.e. directly either through its position property or by applying a CCAction.