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Anyone else having major bug issue with SL Torus prim?

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I wrote a notecard about it a while ago and have been distributing it inworld to everybody who asked. Maybe I should post the text here too:



Just a very short explanation to one of the problems most builders run into every now and then: Why does the land impact (or "LI" or "prim count") suddenly raise or drop when objects are linked together?

    1. The History of LI
    2. The Jumping LI Problem
    3. The Solutions
    4. About OPQ
    5. Copyright Notice


1. The History of LI

Originally the land impact of a linkset (that is a set of objects linked together) was calculated very simply: 1 LI for each prim.

That didn't work in the long run though. LI is supposed to indicate how much load an object puts on the servers and the network and different types of prims can be very different there. The problem became even more critical with the introduction of mesh - there simply is no sensible way to handle meshes with that old system.

The only solution was to introduce a brand new model for calculating LI, based on three of the four "weights" an object has. This modern calculation is far from exact but it gives a much closer estimate of the actual load the objects causes.

There was still a problem though: Quite a few older builds would break under that new calculation method, that is their LI would increase beyond the limit allowed.

The solution to that was to use both calculation methods in parallel: anything that could have been built before the new LI formula was introduced still has its LI calculated the old way, anything that includes features that didn't exist back then, is calculated the new way. Quite confusing and hardly an ideal solution but there really was no alternative.


2. The Jumping LI Problem

One problem this dual model causes, is that the LI of a linkset can suddenly jump up or down when objects are added or modified. It only takes a single object with a single modern feature to switch the whole linkset between the two formulas and the difference in LI can be huge. Usually the modern formula gives the best (that is lowest) result but there are exceptions and it's not that uncommon for LI to increase by several hundreds - or even thousands - if the modern LI formula is triggered.


3. The Solutions

a) To trigger modern LI calculation
This is what you usually want to do and the solution is simple: just introduce one modern feature. Usually what you do is "convex" the linkset, that is change the Physics Shape Type to convex hull. One minor warning though, some prims may act a little bit funny when convexed. If that is a problem, just keep the physics shape type of those prims as "Prim". You only need to convex a single prim in the linkset to trigger modern LI calculation.

b) To fix LI jumps
The reason why the old method of LI calculation is kept is that some older builds have very high actual LI - in extreme cases several hundred or even thousand times the number of prims they contain. There aren't that many of them but it can be a rather nasty surprise if the one you're working on is one and you do something that triggers modern LI calculation.

The simplest and most obvious solution is of course to revert the build back to its original state. But maybe you'd want to fix the problem instead?

As far as I know, huge LI jumps are always caused by physics weight. (In theory it can also be caused by download weight but I can't think of many realistic scenarios where that will actually happen.)
  So the first thing we should try do is to reduce the physics weight. No, the very first thing we should do is take a backup copy of the linkset into our inventory, *then* we take a look at the physics weight!
  An object in SL can have three different physics shape types:
    • Prim: more or less the same as the shape you see.
    • Convex Hull: A simple rectangular or triangular box around the object. Can give a much lower physical weight than prim.
    • None: No physical shape at all. More or less the same as phantom  - except it works for individual objects within a linkset. Removes physical weight completely.
  The physics shape type determines the object's interaction with an avatar. It's the shape you crash into or walk upon. Usually it has no other function than that.
  To minimize physics weight, keep all objects that actually need a detailed physics shape (the ones with walkable surfaces, hollow prims you're supposed to walk inside or through etc.) as "Prim", change all objects you're not supposed to interact physically with to "none" and change everything else to "Convex Hull".

Smaller LI jumps can be caused by download weight or physics weight. If it's physics weight, you can use the method above but usually there's no simple way to reduce download weight so if that is the problem, the only easy solution is to revert the build. That is, unless you're desperately short on LI, you can just leave it as it is. After all, the two different calculation methods don't affect the actual lag/load the object generates - the modern LI calculation is just a more accurate estimate of it. So unless you're running out of LI, the jump doesn't really have any practical significance.

c) When the LI count doesn't revert
Sometimes when you revert a linkset to use the old formula, the LI figure you get in the Build window stays high. It might be that there is some minor detail you missed when reverting the build but most likely it's just that the data isn't updated. If so, you can unlink and relink to force another recalculation but there's no real need to worry. The LI count you read in the Build window is calculated by your viewer and not the server and it shouldn't take long for it to be updated anyway.

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