All about joints

Here is a quick rundown of joints. I'm going to ommit the wheel-joint since we are probably going to remove soon, however I will describe how to make a "wheel" using a hinge joint instead.

To fully understand this quick tutorial you should already know all about linking. That is, you should know what the "root primitive" of a linked set is.

Making Joints

Each joint connects two objects. For this discussion I will call one of them the arm and the other the pivot.

To make a joint, select one of the objects, then hold down the shift and select the second objects. Go to the menu "Tools -> Make Joint" and select the type of joint you want.

The first object you selected will be the arm and the second object you selected will be the pivot.

For best results, disable "physics" for the two pieces before the joint is created. Then enable "physics" on the appropriate pieces.

Note that until one of the pieces has "physics" enabled, then selecting one will not necessarily select the entire jointed object. However when one of the pieces has "physics" enabled then selecting one should automatically select the other piece.

The Hinge

If you enable "physics" just for the arm, then the arm will rotate about the pivot's local Z-axis (i.e. the blue one) that passes through the center-of-mass of the pivot's root primitive.

If you enable "physics" just for the pivot, then the pivot will rotate about its own local Z-axis that passes through its center of mass.

This is how you make a "wheel-like" joint: secure the arm and use a cylindrical pivot that is free to rotate. Don't even bother trying to make a car using 4 wheels. Joints are not stable enough to make vechicles yet, but you can make doors, propellers (maybe), and some other stuff that doesn't suffer too many complicated collisions.

If you enable "physics" for both, well... you get both.

The Point-to-Point

If you enable "physics" for just the arm then the arm will swing freely from the center of mass of the pivot's root primitive. The arm will also be able to spin about the axis that connects it to the pivot.

If you enable "physics" for just the pivot then... uh... oops, that's bugged. The point of swing is not right... I'll have to fix it.

The wind-chimes were made using point-to-point hinges.

The Limited Point-to-Point

Don't use limited point-to-point joints for things you want to last for very long. The limited point-to-point joint has some problems surviving the save-load cycle which we haven't had time to fix yet. However, making a fresh one should work.

Basically, it is the same as the point-to-point except the center of the joint is the midpoint between the two centers of mass, and the amount of swing to the side is limited. Think of it like your shoulder (except with spin)... you can't swing your arm to arbitrary orientations behind you.

The Wheel

Don't use wheels anymore. We will probably remove wheels in the near future.

I think I know how to determine the root object, but want to make sure, since nobody has every actually told me...is it the last object you shift-select? I've noticed that when you shift-select a bunch of objects, and link them, they will all have a grey highlight except the last one, which will still have a yellow hightlight. Is that yellow highlighted object your root (and thus this gives you a means to determine the root of an object that you did not create/see linked)?

yup, last one seleted

I'm trying to figure out a rpe connecting two objects.

My thought is I could attach a thin cylinder to the first object with a joint, then use 3 or 4 more cylinders jointed together with the final one attached to the second object.

The problem I see is the joints are based on the center of each cylinder, so I can't string them end-to-end, right?

Any suggestions?

I don't think you can actually string joints. You can add multiple joints to the same parent, but linking a joint to a child gives you a warning error.

::blows dust off old thread::

This definately needs bumping!

-Chris

Whenever I want it, I just search for it!

awwwww, thought this was gunna teach me about joints and how to make em. dang, wrong type of joint.

So joints will be getting "more stable" so that vehicles can be made, at some point? Or maybe another type of joint will be created that somehow doesn't cause tons of complicated collisions?

Havok 2.0 is what will be making joints more stable.

will there be a way to link across joints on the near future?

What, one joint isnt enough for you hikaru?

Hey Andrew! I was wondering, being now, almost a year since posting this... how are joints coming along? Have there been any changes to the joint system that arent documented here?

Joints are basically broken for anything more complicated than a hinge between two single (unlinked) prims. Jointing linked objects is either difficult or impossible, depending on what you're trying to achieve. Taking a jointed contraption into inventory tends to fail and moving jointed objects across sim boundaries can also fail.

I was actually in the middle of fixing them a couple of weeks ago but got pulled away by higher priority stuff. I hope to get back on that project next week. My plan is to make them usable in the 1.3 release, and then provide some UI tools so that they are easy to use in 1.4.

Well the above says it. Do they work?

Joints are still broken. I made a few improvements but was unable to finish them to my satisfaction.

I'm sorry to say, but my advice for the timid is to stay away from joints for now, and if you do mess with them, stick with experimental stuff; don't make anything that you want to keep in one piece forever.

This is an incomplete list of some of the problems with joints that I'm aware of:

Limited point-to-point joints won't survive a single derez/rez cycle (including boundary crossings) without serious relative drift of the joint structures.

Point-to-point joints can experience significant drift during derez/rez cycles.

Jointing to child primitives will cause the joint motion to be just plain wrong, in general.

Finally, joints are very hard, if not impossible, to build and orient right.


Since joints need so much work they will not be fixed until after I'm done with the initial Havok-2 port; then I can do the re-organization necessary to fix them properly.

Does Havok 2 include new types of joints?
Will we ever get the wheel joint back? What about the spiral prim? I hear it was removed because it was hell on the physics engine, so when we get Havok 2, can we get the spiral prim back?

I know this is an old thread but I am intrested to see where it is up to now can I use joints now??

any help would be appreciated

Heh, no, not really. Amusingly, Havok 2 isn't even done yet, either. Fortunately, Havok 2 will be included in 1.7, due out... well, in a few weeks, judging by the time since the last release. We'll see what Andrew has managed to come up with joints-wise then.

I saw the title of this thread, and I thought we were talking about a Cheech & Chong movie. Oh well.

Anyway, I really hope they fix joints in 1.7 or else finally take them off the menu. I feel so teased seeing something there that I can't use.

They've been used for some occasionally brilliant builds, usually to create pendulum-like objects though; there's definitely nothing else quite like joints, and I hope the don't go away...

How might HAVOK2 affect joints?

HAVOK 2 is just a marketing gimmick to keep older players around ever hopeful to see vast improvements that will actually never come.. it is a conspiricy I tells ya!

Just kidding

Basically, "after Havok 2" has been a kind of catch-all response to any physics question or feature suggestion. Why is that? What will Havok 2 offer that Havok 1 does not? Well, very little, actually. From the Lindens' perspective, it'll mean that one major cause of sim crashes will no longer happen. From our perspective, we may (may!) eventually get more prims to use in every sim. Further, we should no longer be able to tunnel through objects, and the noticable gap between two "colliding" objects will be nowhere near as great as it is now. Sim performance may even improve.

So why have they said "after Havok 2" for two years? Simple: it's easier to do something once than to do it twice. There have been all kinds of features they (though more frequently, WE) wanted to add, but have wanted to wait until after Havok 2 was implemented so as to not have to simply redo the work a couple months down the road.

Havok 2's release has gone from "a couple months away" to over two years now due to problems at Havok's end, now resolved*, and the fact that it's just a long job for the Lindens and hasn't been in active development all this time. They've had other things to do.

Havok "2.0" couldn't have concave physical objects, which would have been kind of a drag in SL. Fortunately, Havok "2.1" does support them. Note: these are not the actual version numbers; only what has been popularly used on the forums.

take a rolling paper. lay it flat on the table in front of you.
role a piece of carton in a roll and put in on the paper on the right or left end.
empty a cigarette on top of the paper
put eehm.. grass on top of it
fold it once and then roll
lick lick
paste
light

sophos <==== dutch

bump coz i dont get any reply on my funny coment