Bang!

Well Geez! That was exciting but certainly undesirable.


Yesterday I was working on my Doom Fortress over at the Abbots Aerodrome . My friend Cubey Terra who owns the Abbots Aerodrome suggested I try using a Vehicle script to fly my fortress. I told him that I didn't think that the new vehicle scripts could lift superheavy objects, but it turned out I was wrong. They can!

However, when I put a vehicle script in my Doom Fortress, and flew it off the end of Cubey's runway, something went terribly wrong...

BANG! All of the object links in my doom fortress broke, sending prims flying everywhere! I had to call Char Linden to help me delete them, what a mess!



Now, I'm the mistress of lifting superheavy objects, but this was the most catastrophic failure I've ever seen!


Has anyone else ever had this problem? Do you know what to do about it?


I believe it happened when I tried to cross the sim border...



I really hope I can solve this problem. Using vehicle physics in my Doom Fortress would dramatically inprove it's capabilities and performance without causing the dangerous side effects of my other superheavy lifting engines.

Heh, I found this problem to be very disconcerting as well. A few months ago I tryed linking my superhuge lab in the sky, found a way to do it by cheating (shrinking it, then linking, then scaleing back up). I inserted a vehicle script into the craft, and it flew around the sim perfecly stably. After crossing the nearest sim border, however, it shattered into 30-some extremely large pieces, spewed all over slate (was working in the olive sandbox when this happened).

There must be some odd interpenitration issues with crossing the sim border that causes the sim to attempt to unlink the pieces when crossing.

Actually... I think that just might be the way sim crossing works. Perhaps when you cross a sim, everything is unlinked derezzed, rerezzed and relinked extremely quickly on the other side. Because I used this 'linking cheat' on the vehicle, the destination sim was unable to relink the pieces.

Any lindens wish to comment?

Wow. Wish I'd seen that. Sounds fun

(And thanks for highlighting Abbots Aerodrome! I should mention that Abbots Aerodrome is owned by Apotheus Silverman too.)

That would be me.
(I tend to be pretty low-key...)

Isn't "low-key" that orange guy with the horns?

He's not Orange he's Red .



I'll just have to continue working on my insanity drive. It works perfectly as long as you don't fly into anything.


I didn't use the Linking Cheat fyi. I did shrink it and link it and grow it but when I grew it, it was only as big as it could get while still being in linking distance.


On the other hand, when I put Cubey's Airplane Script on my Super Heavy Object TM it was able to cross sims just fine...

It's made out of four ten by ten by ten metal prims glued together.



Well, I'll just have to test more! MWU HU HA HA HA HA HA!!!!

OK, this is bizarre. Now it happened to me. I flew an airship from Cardova to Abbots and on the border it unlinked. Pieces everywhere!

I didn't shrink it or grow it or anything after I linked it, so it should have been able to relink it all after I crossed.

Argh.

Bwahahaha! My plot to rule the air with my giant flying one-prim block as the only means of reliable transport is nearly complete! I shall sell them all for.... One Hunnndred Linden Dollars! MWAHAHAHAHA!

*makes a note to drop her Giant Superheavy Object on Moleculor Satyr*

Lots of stuff has been acting very strange this past weekend... missing images on people, slow inventory response, telehubs sending people off world (or not working at all)...

I've been having good luck with a non-physical flying fire hydrant of doom (it's difficult to explain, I'm making a race of superior space dogs bent on universal domination...or at least cornering the market on chew toys), although it does seem to do a deep think on sim borders, apart from that it's being very stable in these wild and wooly SL pre 1.3 times.

Chris, thanks for the shrink-link-cheat idea, I'll have to try that out.

Lum

If you try to make a very large linked object it will fail because the system tries to prevent objects from getting too big -- to reduce the "no collisions across sim borders" problem as well as to help keep the physics engine stable.

Aparently there is a "cheat" that can circumvent the size limitation -- first build the object small and then stretch it out bigger than it would normally be allowed.

I wouldn't recommend relying on that bug to build stuff you really care about because reconstruction of the objects will fail on sim save/reload cycle, and upon crossing a boundary (and eventually I'll get around to plugging that hole anyway).

When an object crosses a sim boundary the receiving sim tries to reconstruct the object from a description of all of its pieces. Unfortunately, the receiving sim is not smart enough to realize that it should first shrink eveything, link it, then rescale it back out. Instead tries to build it using the correct rules... and fails.

Oh yeah, as to why the fortress was breaking...

The algorithm that determines whether a set of primitives can be linked pays attention to the size of the pieces ==> two 0.1-meter cubes will fail to link if they are 10 meters apart, but two 10 meter cubes will link at that distance.

Although I thought I had finally made the algorithm's success independent of the order of the primitives it is possible that its success still depends on which primitives are measured first, and that the order in which you first linked it is not the same as the order that the receiving sim gets.

So, if the scale/rescale cheat was NOT being employed to build your fortress then it is probably a real bug. If so, then it wold be great if you could drop a copy of your fortress on my in-world profile (with a descriptive title like "Relee's Fortress" instead of "Object" and I'll try to reproduce the link inconsistencies. Having an easy way to reproduce the bug in a controlled environment will greatly ease solve the problem.

If you think it is a bug. Then I have a few ideas that might suffice as a temporary workaround:

(1) If possible, eliminate the smallest pieces of your fortress. This should especially help if you have two small pieces on opposite ends of the object.

(2) Make the largest primitive the root.

(3) Make the second largest primitive the second-to-last object selected before linking. Especially if it is centrally located in the object.


The easiest way to do (2) and (3) is to drag-select the whole collection, then while holding SHIFT (or CTL) click first on the second-to-largest primitive to deselect it, then reselect it (this will remove that primitive from the list, then re-add it to the beginning of the list such that it would be root). Do the same thing to the largest primitive to make it root.

Well I wasn't able to link the original fort that used 10meter cube sized pyramids, so I shrank it to use eight meter cube sized ones. That linked just fine and I noticed I could increase the size afterwards, so I increased it to max, however all of the objects were still within linking distance, it wasn't as big as the old one was.


Is it possible that it's trying to re-link the objects in a different place than they were in? Only the smallest change would cause the fortress to leave linking distance.


I'll try shrinking it some later, maybe that will help. Though to be honest I'm thinking of totally redesigning my doom fortress. Having the largest physically flying vehicle in the world is kind of moot when anyone can pop a vehicle script in thier own.

Hey, Relee? I'll pay good L$ to watch your doom fortress explode.

Well it takes a while to pick up the pieces so I'd need at least $500 to do it. Maybe you and some friends of yours could pool your money?

Ten people fifty bucks, not so bad.

lol throw in some animated flames.. some particle fire and brimstone... mebbe a few delicately sampled audio moments from one of the die hard movies or the crash scene from castaway.. and you got yerself an *event* relee hehe

It happens too fast to be much of an event but it wouldn't be terrifically hard to lower the force of gravity on a large object and unlink the prims all at once, along with explosion effects.



What sort of thing would you like to see blow up?

That's probobly what's happening. Remember the linked objects crossing sim borders bug?

It caused the linked pieces to shift position *ever so slightly*. However, enough to notice after a few hundred sim border crossings.

Keep the engine stable? A big object crossing a border causes the physics engine to destabalize? Oooooh

Andrew, what would occur if this were to happen? Would the sim simply crash, or would odd Matrix-esque anomolies gradually crop up?

No collisions across sim borders 'problem'? I thought it was the intention of the devs that the object to be phantom if its center wasn't in the sim its in. It makes sence, since the simulator's physics implimentations are independent of one another (if I lag a sim's physics engine, it doesnt effect the adjacent sims).

How would the large physical object adversely effect the destination sim's physics engine?

I was able to create a relatively large craft that went partially phantom over a border, and still kept me on the 'floor', by using some *cough* clever llPushObject() tricks.

==Curious Chris

By "stable" I mean that objects don't achieve arbitrarily high velocities. Interactions between a very large object with very small object might lead to instabilities... the small object may oscillate to infinity.

Basically it boils down to this... whenever you try to solve large matricies with wildly divergent eigenvalues then floating point error becomes a major problem. If you aren't careful floating point errors can cause NaN's and infinities in the solution.

If we were to let objects be arbitrarily large and small then the physics engine could generate such large almost-singular matricies and would try to solve them.

Someday objects will collide across sim boundaries. That's the ultimate design, but it is a tricky puzzle to solve right. We think we know how to solve it, but until we do it is a "problem". Limiting the max size of objects reduces the problem and is hence another reason why the size limit exists.

Since the cross-sim collisions are not working then whenever a large object crosses into another sim it would be much more likely to cause "suprise" interpenetrations than would a small object. Did I ever mention that the current physics engine can't handle interpenetrations well?