Selecting Rotation Axis Points

Imagine 2 cylinders in the same vertical plane, and a sphere with a slightly larger diameter between them.

These are the 3 parts that will make a crude "arm" (upper arm, joint and lower arm).

I was under the impression that, when using the object editor, I could select the upper arm part first, then, while pressing the Shift key, I could select the joint part last, so that when I link these 2 to make one object, the rotaional axis point would be at the center of the joint.

This is not the case...... (unless I'm screwing up)

Maybe someone can point out the error of my ways.....

There doesn't appear to be any way to change the rotaion axis points in an object via the object editor or am I missing something? A workaround?

Thanks in advance!

If you are trying to change the angle via typing in numbers in the edit box and the prims are UNLINKED, then it will not work.It will just spin the last prim selected.

If the 3 prims are LINKED, and you use the same "type in number" tecnique, then it will work.

If you have them all highlighted and are trying to change the angle by dragging the ball handles that appear when you hold down the CTRL button, then that should also work.


*Edit*

Just re-read your post. The center axis point of a linked set will always be on the center of the last prim selected.

>Just re-read your post. The center axis point of a linked set will always be on the center >of the last prim selected.

That's what I thought. But I'm in-world right now doing exactly that and when doing this with a cylinder and a sphere, no matter what order they are selected in, the rotaional center is always at the center of the *cylinder*.....very strange....

The Parent prim is always the last selected. In a linkset it will be highlighted yellow, with chhild prims highlighted blue. Make sure your sphere is the yellow object in the linkset.

Also, make sure you don't have "edit linked parts" checked in the edit window.

Other than that, I can't think why this should be happening.


*Another Edit*

The cylinder doesn't happen to be cut does it? Because it will still use the center point of the original size of the prim.Example, if your prim is 10x2, but you have made a cut so the cylinder looks to be 5x2. It will STILL use the center point of the 10x2 prim axis.

I did change the dimensions of the cylinders via numerical entry after I'd rezzed them but they're not what you would call "cut", I don't think.....

I'm building at the Braunworth sandbox if you want to take a look.....

I would offer to come take a peek, but i'm stuck at work right now and can't get online for a few hours. Hope you get it figured out Nyles ,sorry I can't be of much help to you.

: )

Hey, I do appreciate your help.

From the sound of things, I'm willing to wager the problem is some silly, obvious thing that I'm overlooking.

It wouldn't be the first time!

...and since it's quarter past 4 AM.....I should probably think about going to sleep.....lol

Thanks again!

the rotational axis of a linked object is it's center of mass, if your cylinders are of the same length, aligned perfectly, with the sphere placed exactly between them, then and only then will your sphere be at the rotational center.

another fun thing is the local rotation of the whole set, normally this would be the same as the last selected prim, unless you are one of the few and the proud for whom it's the SECOND_TO_LAST prim selected in a linkset (https://jira.secondlife.com/browse/SVC-998 Object Local Rotation Incorretly Aligned to Child Prim). if you have the bug, it doesn't go away, and even sticks with the build after transfer, and occurs when other people link with prims you created....

Rotational behavior is pretty simple. When you type numbers into the editor, the pivot point is the the center the parent prim. When you rotate by hand via the on-screen manipulator handles, the pivot point is the geometric center of the entire linkset.

So, if you want to rotate your "arm" from the "shoulder", put a sphere at the top of the upper arm to represent the shoulder joint, and make that that sphere is the parent of the linkset. Then type numbers into the editor, and the whole arm will rotate around the shoulder. If you try to use the manipulator, the arm will rotate around its geometric center, which is not what you want.



The bug Void mentioned is really annoying if you have it, but don't panic if you do. It won't change the locations of the pivot points, or any of the global rotational values. It only changes the apparent local rotation. If you do have it, and you want your local rotations to appear correct, make sure your second to last selected prim is identically rotated to the lastly selected one before you link.

Void Singer wrote:
>another fun thing is the local rotation of the whole set, normally this would be the same as >the last selected prim, unless you are one of the few and the proud for whom it's the >SECOND_TO_LAST prim selected in a linkset (https://jira.secondlife.com/browse/SVC->998? Object Local Rotation Incorretly Aligned to Child Prim). if you have the bug, it >doesn't go away, and even sticks with the build after transfer, and occurs when other >people link with prims you created....

Nice. Wouldn't this "feature" render this software effectively useless?

Chosen Few wrote:


Ah- HAH! Well, ultimately, the rotational parameters will be decided via a script, so I suppose (and hope) the behavior of the former will apply.....

Add to this conundrum the dynamic imposed by the Euler Representation vs Quaternions, and it all serves to make me come to the realization that.....

I should have paid attention in Geometry class!


Thanks everybody - and prepare yourselves for still more questions - hopefully not in the style / vein of The Spanish Inquisition.....

One more thing.....


Is it my imagination or (speaking of The Spanish Inquisition), is this forum software stuck in the Dark Ages?

I *see* VB code tags in posts, yet VB code is supposedly enabled.....


???????

There's a sticky at the top of the Resident Answers forum about that. They turned off bbCode a few months ago, citing security concerns. Supposedly, updating vBulletin to a newer version would maybe fix the problem, but they have yet to do that. The forums are not a high priority for LL, unfourtunately.



Nah, as I said, it's just an annoyance, a big annoyance, definitely, but not a show stopper. All it affects is the appearance of local rotation. The actual global rotational values remain intact.


I'm pretty sure it will.

I'm guessing you're looking to create some sort of animated character, controlled via script. That sort of thing has been done before in SL, so it's not like you're venturing into completely uncharted waters. How effectively people have been able to do it is another story, but it certainly can be done.

One other tactic you might want to explore is this. Use sculpt prims to create the various keyframed body positions. You can then "animate" the body by swapping the sculpt map textures. No rotation is necessary that way, so it saves you the math.

There are benefits and drawbacks to all methods, of course, so experiment liberally.

Chosen Few wrote:


Now *that* is interesting....

Sounds like it would use up more sim resources...


I was also wondering if certain types of "motion" could be done via playing with the alpha channel on multiple textures.....