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Im making a 2D game with box2D physics, and i want to implement a parent-child system between the objects. The child's position will be relative to it parent. For example, father object position is (10, 0), the relative child's position is (0, 1) and the result child's position is (10, 1).

I was thinking about how to implement something like that, and I came to the conclusion that adding to each component of the child's position vector the parent's current position minus parent's previous frame position, this effect was achieved.

That worked fine in a normal game, but with box2d physics, there is some "delay" and children position seems to update slower than the father.

There is a proper way to achieve this effect more accurately?

Vestelion
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  • This is completely impossible to answer without any code. At a guess I would say that Box2D is not aware of the parent-child relationship, and may be updating the child position before the parent? – Botje Sep 08 '22 at 13:28
  • TBH this might well be closed as a duplicate of https://stackoverflow.com/questions/35667232/how-to-combine-box2d-bodies, but I'd leave it to the author to decide if it's similar enough. – Bartek Banachewicz Sep 08 '22 at 13:35
  • A delay in updating a position would be related to how you process the position update, not how you calculate the position. Adding two pairs of integers is not going to take a noticeable amount of time. – JaMiT Sep 08 '22 at 13:45

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What you're describing is known in the physics simulation world as a constraint. It simply means that the constrained object isn't free to move like it wants to, but instead there are some restrictions. A special case of a constraint is a joint that links two bodies together - and Box2D supports them. In your specific case, if you want the child to always be exactly in the same spot as the parent, you probably want a "weld" joint.

That being said, if you want a 100% interlock between parent and child, you might want to create one compound shape by either composing Fixtures or using ChainShape; that will guarantee that the system will never consider a possibility of them moving relative to one another, at least not with regards to dynamic forces.

Bartek Banachewicz
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