Understanding anchor point in PhysicsJointFixed

cocos2d-x C++
1

I’m developing a little prototype game using the built in physics in v.3 and am having difficulty understanding how PhysicsFixedJoint should work.

I have a large circular body (‘wheel’) that is fixed in the world with a PhysicsJointPin at its centre and can freely rotate. I want to fix a smaller circular body (‘ball’) to be touching the outside of this body, so that they will become locked together.

This screenshot shows how this looks so far http://i1293.photobucket.com/albums/b589/stevebarnegren/Screen%20Shot%202015-06-08%20at%2012.52.08_zpsljbrijij.png

It sort of works, but as you can see the green dot representing the joint is in a funny position. Sometimes this joint will differ greatly from the position of the ball, causing the physics to go haywire.

I suspect that it is to do with the third argument when constructing the joint (anchor point):

cocos2d::PhysicsJointFixed *joint = cocos2d::PhysicsJointFixed::construct(ball->getPhysicsBody(),
                                                                              wheel->getPhysicsBody(),
                                                                              Vec2(wheelPos.x - jointXDist, wheelPos.y - jointYDist));

This places the anchor point at the point where the ball and wheel touch, is that correct? I have double checked the maths by placing drawNodes at that point, but the joint debug dot never shows up at the same position.

Any help would be greatly appreciated!

2

Hi, did you ever fix this? I’m having similar problems; can’t get PhysicsJointFixed to work properly at all. The joint always appears at a crazy position.

3

No, I never got it working!

I can’t offer you any alternative I’m afraid, because I abandoned the project shortly after this, so I never found a solution.

I tried a bunch of things, but I could get the joints to consistently work. It put me off using the built in physics a bit tbh, although it might just have been my misunderstanding of how they should behave.

4

Ah shame - thanks for replying though! I already have the integrated physics quite heavily used in my game, only using spring joints though which appear to be fine… Gonna have a look at the test projects and see if I can work this out.