Please Help - Shifting Prims

Hello! I am a builder in SL and lately I have noticed that when linking my builds, the prims shift. For instance, if I link a floor to the ceiling, I notice one floor prim may be at 180.075 in height, another at 180.083, and so on. However, when I built it, I had them all set to 180.075 in height so that the floor was completely level. Why when I link is prims on the build shifting? Its causing me a lot of problems and going back having to re-enter height #s by hand to get things evend out again. This problem isnt limited to the floor prims, it happens to the side of the building as well. Its a real pain.

I haven't noticed that -- maybe it's a new "feature". I'll see if I can duplicate your issue, when I get a chance.

I asked this question a little while ago myself, but what I found was that I was actually moving and altering prims without realizing it. What I was doing was pushing the arrow keys in various directions to make my avatar move, but since the Edit window for a prim had focus, I was changing the dimension or position of a prim, but so slightly that I didn't notice it until later.

This was my thread:

/8/90/328214/1.html

Since I've been paying attention to that (and moving the camera instead of my avatar), I haven't had any prims move.

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Isn't "Prim Drift" over time a known issue (when you start looking VERY close?)

And of course as we all know "Height of build" plays a bit part also.

I always link my builds in sub links first ,, ie: floor sections ,,then walls , always making the root as close to center as possible and on a horizontal flat plane. Then i go back and link the whole structure ,putting the linksets together in the most economical way to have the least amount of linksets and not so far apart from each other.
Sometimes i notice a drift regardless and have to go back and make minor adjustments. I wonder if it`s just server load that causes this?Or maybe flooding the server with a large amount of information all at once instead of in little peices?
I was told long ago the most accurate place on the sim to build is closest to the X1 Y 1 vector position on the sim. Another wifes tale? Maybe , maybe not . I notice less errors on this corner of the sim than the oppisite corner.

One of the reasons objects can move is if they're scripted with a touch handler but without calling the LSL function to prevent grabbing. Or at least, that used to be the case; maybe it's fixed now. What would happen is that when you click on it, it could move a bit, somehow mistaking the click for a grab. I don't know the details, but remember it happening and that there's a scriptable cure.

But that only happens for an object (not just a prim in an object), and only if it's scripted (IIRC), so I suspect it's not your problem here. But mentioned it just in case.

There is a reason that this is true, but I doubt it would be noticeable. It would be due to the precision of floating point numbers (23 bit mantissa, not including the assumed top bit which is always set). The precision of a floating point number is the value divided by 4 to 8 million.

So, near <1,1,1>, your precision is better than .4 millionths of a meter, whereas at <100,100,100>, it's better than 40 millionths of a meter, or about .4 millimeter.

Another way to look at it is to not expect numbers to be accurate to more than 7 significant decimal digits. So, 100.001 is good, 100.0001 is on the edge, and 100.00001 is an unrealistic expectation.

This begs the question of whether that much accuracy is meaningful. And if you build it at <1,1,1> but move it to <100,100,100>, you lose the accuracy anyway (but maybe not as much as if you'd built it at <100,100,100>.

It is not what Seven said. My problem is really bad. I am building about 600m up in the air. I just unlinked the entire floor, and set every floor prim height to "603.0427" I then linked 6 of these pieces and rechecked their height. It was no longer at "603.0427" each prim had a different height number and you can see that the prims are no longer lined up when you look at the floor. I then checked the sides of the building, same problem. It all got unaligned when I linked it, and it doesnt seem to matter if I link many pieces or a few. Im so confused and very very irritated. Though the movement of the prims is small it is notieceable and I do not feel right marketing any of these buildings. I am wasting so much time with this, its impossible to link without my prims shifting.

Edit to say I think some shifting must be normal for even when i link only 5 prims they shift in number but it is not noticeable to the naked eye. It is when I start to link everything together the numbers keep getting further apart and the shift in the prims manifest themselves, you can see a little line like a miniature step where the floor has shifted downwards and each floor prim has a new number. I set the floor prims by hand to 603.0427. I grouped the floor into 3 objects with 9 prim linkset. The first 2 objects turn into a height of "603.0431" when linked, the last one stays at "603.0427".

This isnt normal is it? seems like a huge flaw in how linked objects work. What do I do? sell my building completely unlinked?

Here is a picture of it, that black looking line is where the prims are unlevel due to the shifting.

I am building over 600m in the air, could this be my problem?

I will try to move the build to the ground somewhere and relink to see if that works.

I can't remember what it was (but it was small, cheap and good) and NOT the brand leader, but I have got a thing that you use to auto snap prims edges together and I found it worked perfect with little fuss.

Have you tried one of these things?

I'll try and find it again to see what it was called.

To be honest, 600m is not a LOT.

I was able to get things almost perfect at 3500

There is a sort of bug, where if you type in the positions, they don't quite take if you are doing smaller adjustments. It can help to move the prim well over, then plug in the numbers you want. This one is not so much drift, as that the prims don't really settle where you thought they did in the first place.

Oh, just had a thought.

What viewer are you using?

There is a bug in Emerald with adjusting some numbers (Z values definatly) at height.

I found this on my 1st day with the viewer.

Some "quick guesses from the unknowing":

- Have you tested what happens when you move all your objects to a hight of exactly 600 meters, then link them all together and shift the whole link set to its final hight ?

- Then have you tried to work on a level of 603.04 above ground instead of 603.0427? My weird thought here is: Since ordinary prims can be as thin as 1 cm, maybe they can be adjusted with absolute accuracy only on a 1 cm grid ? Maybe this effect is hight dependent ? So it wont occur (or be invisibly small) at ground level, while with raising hight it gets more apparent ?

- And finally i noticed that inaccuracies occur much more frequently, when the objects are slightly rotated. So have you tried to link with zero rotation and then rotate the whole linkset ?

- Ah, (silly question, but still possible) Have you checked, that your floor objects have all the same thickness ?

I doubt it, if the reason is the precision of 32-bit floating point numbers.

First, I was off by about one decimal place, above. We get about 10 times the accuracy I said (one more decimal place to the right).

600 / (2^23) = 0.00007, which is roughly 0.00001. If your errors are much more than this, it's not due to the floating point precision. (You'd have to be over 8000 meters before the floating point error would be bigger than the rounding to 3 digits done by most viewers.)

[Note to perfectionists: The correct factor might be 2^24, which would mean "twice as accurate", but that's not very significant when talking decimal digits, as I'm doing here. I always forget whether the mantissa's implicit high bit of 1 is significant in this calculation, and I'm too lazy at the moment to figure it out.]

Now, I assume that when linking and unlinking, coords of non-root prims would be converted from global (region-relative) to local (object-relative), and when unlinking, converted back again. That increases the error term, but less than 10 times the amounts I'm giving here.

Perhaps for some reason the numbers used internally are bigger than the coordinates, for some reason. If so, that would increase the error term proportionately. But, other than being able to see more than one sim at a time, I can't imagine why that would happen -- and I would assume that the current sim would be the one "at zero", with the coords for the neighboring ones bumped up by 256. Furthermore, that would only affect the x and y coords; not the z. Just thinking out loud, sorry. See my byline.

Nope. I've made tiny objects like jewelry at 600M, where most prims are 1cm or smaller, adjusted with high accuracy relative to each other. That's why I'm sure this problem isn't as simple as floating point precision and height above ground.

However, I've never inspected the absolute accuracy of large objects like houses and their parts. I'm not that fussy a builder, and if I were, I'd use prim alignment tools like ... arrgh, can't remember the name, but Skidz has one.

Eek, I am on Emerald!! I will try to sign on LL viewer and link.

This could be my problem, I just recently jumped aboard Emerald.

Sometimes, a prim can become corrupt... Couple things you can try...
1. Right click and edit it.. change the hollow, then change it back...
2. Shift drag copy will help sometimes as well..

But sometimes, you have to replace the prim...

I have a lot of customers that are very... picky when it comes to there prim alignment, and get IMs once about ever 2 months about this problem. No clue what causes it... Sometimes the size becomes corrupt, and others its the rotation or position..

Hmm I moved to LL viewer and the problem happens but in a different way. The shifting prims read with the correct #s and once that prim is unlinked it snaps back to the correct place. I did try hollowing it and such and that did not do the trick. Its terrible because witch such big builds you are bound to miss the shifting in areas and the product will look faulty. But atleast with the LL viewer once you find the shift its easy to fix via an unlink and just link it to something else. With emerald it literally messes up the coordinates and you have to type in new #.


@ Gaia, good one about making sure the prims were the same measurements, I did check that, thanks

Just another guess: You are not using Megaprims, no ?

I recently took some time and tried to reproduce your problem with standard prims in our SIM, but i couldn't find any severe errors. I was on level 600 meters and everything worked as expected. I only got some effects, when i worked within millimeter range. But these effects where some inaccuracies while aligning the prims horizontally.

I could create exact link sets when i created one single plane objects, went to local edit mode, duplicated the object and let it snap according to the local grid. In that way i always got perfect results.

And i always got more messy (but still reasonably accurate) results when the prims where rotated before aligning them.

Maybe that helps a bit to get hold on your problem...

Here are some problems/adjustments we have encountered in building:

a) once you start building it, DO NOT MOVE IT, do not take it to inventory, do not pick the whole thing up and move it about, just build it in place, only when the entire thing is complete and linked should you move the build...(take to inventory, rerez etc). On complex or even semi complex builds (over 100 prims), prims start becoming lazy, I call it lazy because you can see this occurring when you try to move too many prims around at once, things dont snap where they should when they should or start to rotate badly.

b) zero rotations: its so much easier to copy a prim and flip or rotate it into place, however this has also caused drifting and odd rotations especially on builds over 100 prims - floors walls ceilings all need their own 0 rotation, especially ones that have been copied.

c) copying parts of the build and moving them into place: this doesnt always work as expected and many times prims will become corrupted like skidz said and there is nothing you can do about it except delete it and make a new one. No idea why this happens, but most often it happens to me over time, meaning I have been building for a while and I have taken a prim and copied it for say the 4th or 5th time, suddenly it will no longer hold its location/rotation accurately.

So our advice is to stay away from doing those things as much as possible....and take lots of breaks.