Configurable Joint 2D Equivalent? Possible "RopeJoint?"

This is also on Unity Answers: http://answers.unity3d.com/questions/593111/configurable-joint-2d-equivalent.html
Just realized that there was a dedicated 2D section on the forums so I felt it worth posting here.


I'm doing a game with similar mechanics to "Cut the Rope" - I actually have a system that works quite well - I use a Configurable Joint on each Rope Anchor, and connect it to a "Player Object", and set its linear limit to the length of the rope. I then use a Verlet method to generate a rope mesh which sags etc (but the physics are all controlled by the Config Joint).

However, it's a 2D game, using the 3D physics (rigidbodies etc) with axis restrictions- and 4.3 dropped, I'm trying to convert my system over, so I can use 2d collides etc, and make things more efficient.

I'm running into an issue with my rope system in that I'm not sure how to replace the configurable joint I use with a 2D equivalent.

My first thought was to use a Slider Joint 2D, to handle the variable length, but that didn't produce good results.

I then tried to use it in conjunction with a Hinge Joint 2D (with the hinge as the parent), which worked a bit better, and I could get a pendulum motion going, but would still have issues when the "Player Object" would fall from above, with the Hinge Joint at some angle - the Slider Joint wouldn't seem to contract, but instead the force would make the Hinge Joint rotate faster - which kind of makes physical sense if I think of a Slider which can only slide on one axis.

The Configurable Joint actually has its angular motion locked, and just moves along X and Y (I don't completely understand this...but it works).

Is there a 2D Joint or some combination that would allow movement along X and Y similar to a config joint, or some other method I could use to make this work?


http://www.box2d.org/manual.html#_Toc258082974 - seems like a Box2D RopeJoint (8.13) would do the trick, but that didn't seem to get brought in?

Here is my code to initialize the Config Joint:

Code (csharp):

1.  //Adding a configurable joint to the rope anchor
2.         cj = anchorObject.AddComponent<ConfigurableJoint>();
3.  
4.         cj.anchor = Vector3.zero;
5.         cj.axis = Vector3.zero;
6.         cj.autoConfigureConnectedAnchor = false;
7.         cj.connectedAnchor = Vector3.zero;
8.         cj.secondaryAxis = Vector3.zero;
9.  
10.         cj.linearLimit = new SoftJointLimit() { limit = targetLength, bounciness = 0, damper = 0, spring = 0 };
11.  
12.        //We want to limit motion along the X and Y axis, to a maximum distance of "targetLength", and of course lock motion on the Z axis
13.         cj.xMotion = ConfigurableJointMotion.Limited;
14.         cj.yMotion = ConfigurableJointMotion.Limited;
15.         cj.zMotion = ConfigurableJointMotion.Locked;
16.  
17.        //The configurable joint doesn't need to rotate, but makes no difference if it's set to Locked or Free (don't really understand why at the moment)
18.        cj.angularXMotion = ConfigurableJointMotion.Locked;
19.        cj.angularYMotion = ConfigurableJointMotion.Locked;
20.        cj.angularZMotion = ConfigurableJointMotion.Locked;
21.  
22.        //Attach the configurable joint to the player's rigidbody
23.        cj.connectedBody = connectedObject.rigidbody;

 

Looks like Chipmunk has this too. http://chipmunk-physics.net/release/ChipmunkLatest-API-Reference/interface_chipmunk_slide_joint.html "Slide joints hold the distance between points on two bodies between a minimum and a maximum."

They have a unity integration but I'd rather not abandon all my colliders and unity physics etc...

Hopefully can get something that works with Unity Box2d within 4.x. As of right now I'm forced to use expensive mesh colliders for my terrain on my mobile game etc. I guess it's just the same as before 4.3, but now with all these 2D googies I want to take part!

 

Lonely bump... sigh

 

Well its just a matter of waiting until the Unity guys implement all of the features of Box2D. Right now its a partial Box2D implementation in Unity.

 

Well that's what I hope, but is this on a roadmap somewhere? This is a core functionality of my game, so knowing if this feature will get brought into Unity2D or not will help me greatly in planning for it.

I'd rather not switch all my physics completely over to Chipmunk or try and use some 3rd party Box2d port. Right now it's working with 3d PhysX so I'm thinking "if it ain't broke" but obviously 2d will be a huge performance boost on mobile. If it's on the roadmap for 4.x (and not on the same roadmap as UnityGUI...) then I'd be comfortable waiting because I won't be launching until late 2014 is my guess.

Would just like some official Unity confirmation if possible.

 

I remember in one of these videos, I heard ThomasJ saying that more Box2D features are coming...

This video...
http://www.youtube.com/watch?v=B1F6fi04qw8

Or this video...
http://www.youtube.com/watch?v=CFT_abC4oq4

 

Good catch, it's here: http://www.youtube.com/watch?v=CFT_abC4oq4#t=2508

I heard: "No Weld joint, pully joint ,mouse joint, gear joint, wheel joint" in yet - but might be coming later on... nothing on Rope Joint Going to PM Tomas to see if he might be able to chime in.

 

Physics 2D Extensions on the Asset Store has weld joints, mouse joints, and wheel joints (with pulleys and possibly gears coming soon):
http://u3d.as/5Sc

Disclaimer - I wrote this plugin - feel free to ask any questions about it on the forum thread here:
http://forum.unity3d.com/threads/210361-Physics-2D-Extensions

 

@Thinksquirrel
Thanks. That is an awesome demo in the Web Player LOL! Question... when will you finish implementing Pully Joints?

@Jesse
I've recently discovered that the Unity 2D DistanceJoint actually acts like a RopeJoint... you should try it

 

Does the distance joint have a min/max? I thought it was just a fixed distance?

 

A RopeJoint only has a max. So does the DistanceJoint. A WeldJoint has a min and max. Unity has not implemented this yet.

 

Er sorry I didnt mean min/max, I meant that the distance joint keeps things a fixed distance apart, (while the Rope Joint allows movement from 0 to max distance.)

http://docs.unity3d.com/Documentation/Components/class-DistanceJoint2D.html
"Distance Joint 2D
The Distance Joint 2D component allows two Sprite objects controlled by rigidbody physics to be attached together to keep them a fixed distance apart. Note that unlike the Spring Joint 2D, the Distance Joint is rigid and does not allow the distance between the objects to vary."

Are you saying this is not the case?

 

I'm saying this is not the case. Perhaps it's a bug, but it seems like the DistanceJoint is only enforcing a max distance. Try it out and tell me what you experience.

 

Well I'll be...

Giving it a quick test and it looks like you're right. I can't believe it but I guess I must not have tried this, just from looking at the docs, (tried the slider and hinge combination).

I'll need to sit down and implement this into my game and see the results ... if it is a bug, I hope if they "fix" it that they will keep this code and make it a Rope Joint.

Anyways, thank you!

 

The current Distance Joint 2D indeed works exactly as I need. Big ups to Kurius for pointing this out! - But I imagine something will change in the future as it doesn't match the description ( which says it's always a fixed length). I imagine a toggle on the joint or a completely new joint.

At some point I'll need a joint that does have a fixed length, I've let UT know about the issue, but for now, I can move forward.

 

Sweet - well guess I won't update until I'm sure the Rope Joint is in Thanks!

 

@MelvMAy.... PLEASE DO NOT release the fix for the DistanceJoint2D until you release the RopJoint2D
I game needs a rope joint and right now the distance joint is serving this purpose very well.

Honestly how hard would it be for you guys to just rename the current implementation of the DistanceJoint2D to instead be called RopeJoint2D?!

Thanks

 

I'm guessing the RopeJoint2D they are implementing will have a bit more Rope specific features than what the DistanceJoint2D has. If the RopeJoint doesn't come the same time as the fix, I'd say to just not update then, if things are already working how you need them to.

 

I know I can choose to simply not update, but I'm also hoping the new release will have fixed this major problem...
http://forum.unity3d.com/threads/216657-Flipping-Sprites-Colliders

 

Awesome! Thanks!

Now can you please let us know what the plan (time frame) is to fix this other issue?......
http://forum.unity3d.com/threads/216657-Flipping-Sprites-Colliders

 

Hey guys, one quick question on this topic. How did you manage to use Distance & Hinge joints to create an elastic rope? (cut the rope like). Just some hints would help. I tested distance + hinge joints but the results were really weird...

Also, do these ropes work propperly with different riggidbodies masses? Every rope system I used didn't work well with different masses...

Some help would be greatly appreciated! ^^

 

Are you making sure you use several nodes in your rope? As a starting point, make 5 small circle bodies. Let's call them A, B, C, D, E. Attach A to the ceiling or to some static fixture via a HingeJoint. Attach B to A via a HingeJoint and DistanceJoint. Attach C to B via a HingeJoint and DistanceJoint. Attach D to C via a HingeJoint and DistanceJoint. Attach E to D via a HingeJoint and DistanceJoint. Now test it out... its probably a pretty short rope, but its elastic because of all the bodies involved.

Remember, Unity 2D uses Box2D Physics, so all of the Box2D Physics examples on the internet should be easily adaptable to your Unity project.

 

Is the "near future release" here soon?

I can't wait!

 

Oh! Thanks! I'll try that out.

It would be cool if you added it as a normal component along with some nice documentation.