Linking objects in SL

[OtterDavis69]

Will linking unscripted decor objects affect sim performance or create lag? Is there any type of rule to follow when doing this? I do it to decrease the prim count. 

[Bree Giffen]

I’ve done it before and I didn’t notice any problems or lag. I also set the linked prims to convex hull to reduce the land impact. 

[Anna Salyx]

if it's purely decor items, things on shelves or objects you won't be bumping into or don't mind if if you can walk through them, set the linkset physics type to "none".  You will get a message that the root prim physics cannot be set to no shape (or some such wording), which you can ignore.  After that, the root will remain physical but the rest of the links will all be, "phantom like"*. -- you can walk through them, path finding things will ignore them, etc.  The simpler the shape for the root the better, and best if you use a simple box for the root. You'll get the reduced LI that Bree was commenting about above but this will also stop all those items from being processed by the regions physics engine and that in some cases can be a nice benefit. 

You don't have to do this, it's not a "rule", and there is a Wiki page about physics optimization you can look at for more information and decide for yourself  if it's something you think is worth your time doing.   I do it for shelf items so I don't bump my head when walking through my apartment.

 

 

 

*(it's not truly phantom, it's something different, and the difference between no physics shape and phantom is beyond the scope of this comment).

[Love Zhaoying]

I am confused, perhaps "today I learned?" - did not realize you can reduce "prim count"  (I assume actually "Land Impact" is meant) simply by linking objects.

Doesn't this only apply to "Mesh" objects?

Thanks,

Love

 

[Anna Salyx]

Land Impact (LI) calculation is a bit of voodoo math that I won't begin to try and understand much less explain.  but the very simple side of it is that some objects (mainly, (usually) mesh) have less than 1 LI.  i think the lowest an object can have is 0.5 LI.  But the system will round up to the nearest when it's rezed, so a 0.5 LI object will be 1 LI when it's all alone. However, when linking objects, the system will look at the total LI of all the separate pieces and the link-set will be the rounded sum of all of them.  So, if two 0.5 LI objects are linked, they become a 1.0 LI link set. Four 0.5 object will link to 2 LI. etc.  a 1.75 + a 1.75 + a 0.5 set will be 4 LI as a set instead of the 5 LI they'd take up individually.  

The only way to tell if and how much LI savings you might get is just by trial and error linking.

 

edit to add:

That all said, some things link (I think it's mainly when PRIM and Mesh are mixed with materials involved can suddenly become a very HUGE LI values that can be quite shocking when linked. The fix is to change the type from PRIM to Convex HULL (or None) before linking things if possible.  I"m sure someone with more knowledge on this part might chime in, but i thought it'd best be a good idea to edit and add this disclaimer.  linking normally is a good fun thing, but sometimes it can be terrifying if the LI blooms unexpected like.

Edited November 3, 2022 by Anna Salyx

[Emma Krokus]

It doesn't just apply to mesh.

Two simple cubes or other non-tortured prims, linked together, and set to Physics Shape CONVEX HULL or NONE will have 1 LI land impact. If they contain scripts, this may NOT apply.

In general, I have found it an art to link objects together. Quite a few times I've been amazed at what happens when linking objects in a different order. Usually, a simple prim as root is best - but I have also found a difference in using a cube as as root prim compared to a (non-tortured) sphere.

Just to complicate things, linking a high land impact mesh to other meshes can sometimes result in NO inscrease to the land impact.

In consequence, I experiment all the time with linking to get the best result.

[Atomic Infinity]

The land impact of a mesh is measured to one decimal place, but is rounded for the purposes of in-world 'prim count'.

So sometimes linking meshes together can give an overall lower LI, but can also go the other way.

If I have a mesh that is 1.6LI, it will show in-world as rounded up to 2LI.  If I have two such objects unlinked, they will be 2LI each = 4LI

If I link them together, their total is 3.2LI, which will round down to 3LI, so I have saved 1LI by linking them.

However if my objects were 1.4LI each, they would be in-world as 1LI each. If linked they would be 2.8LI total and round up to 3LI for the linked ones, thereby increasing the total instead.

When we make a un-tortured prim into a convex hull, its LI changes from 1LI to 0.5LI, which we do not see because of rounding, unless we link it to another similar object then we get the combined rounded benefit.

 

edit : you can get the true LI of an object by editing it, and click the 'more info' link thingy in the edit window.

Edited November 3, 2022 by Atomic Infinity

[Anna Salyx]

3 minutes ago, Atomic Infinity said:

edit : you can get the true LI of an object by editing it, and click the 'more info' link thingy in the edit window.

oh!  I should have known that but i didn't.  thanks

[Atomic Infinity]

14 minutes ago, Anna Salyx said:

That all said, some things link (I think it's mainly when PRIM and Mesh are mixed with materials involved can suddenly become a very HUGE LI ...

lol yes this catches me out also - when prims and mesh are linked together, if the prim is complex or tortured and not set to 'convex' or 'none' in the physics type shape, the LI can get scary. Like a thousand LI and *poof*  "your object has been returned due to exceeding the parcel prim count" or whatever that error says

Also it gets messy if you link a sculpt to mesh - the sculpts LI will be unlikely to ever get better by linking to mesh - it almost always goes up and often doubles.

[Emma Krokus]

Another way to find out the exact LI of an item is to set it to Edit linked prims in the BUILD window. You can also see the LI of each link by cycling through the prims holding the CTRL key (OPT for Mac users... I think) and clicking the > key to go to the next prim.

[Quistess Alpha]

While some of it may be due to rounding, the main reason this works is that LI is a worst of 3 factors test. You reduce Li by linking objects that have high Li, but in different categories.

The best score a single mesh/prim can have is 0 Physics/0 Download/0.5 Server. (or so) Theoretically if you link 2 things that are the same amount of Li,  but in different categories, you will half the LI compared to them being separate objects (Ex. 14/0/0.5(14 Li) + 0/0/13.5(14 Li) = 14/0/14(14 Li) )

 

[Love Zhaoying]

21 minutes ago, Quistess Alpha said:

While some of it may be due to rounding, the main reason this works is that LI is a worst of 3 factors test. You reduce Li by linking objects that have high Li, but in different categories.

The best score a single mesh/prim can have is 0 Physics/0 Download/0.5 Server. (or so) Theoretically if you link 2 things that are the same amount of Li,  but in different categories, you will half the LI compared to them being separate objects (Ex. 14/0/0.5(14 Li) + 0/0/13.5(14 Li) = 14/0/14(14 Li) )

 

I noticed that the "new calculation" only applies in some situations, I bolded a bit below copied from the article.

Quote:

When is an object's land impact calculated using the new algorithm?

Legacy prim objects have a land impact rating equal to the number of prims they contain.  However, any object's land impact is calculated using download, server, and physics weights if it meets any of the following conditions:

• The object is an uploaded mesh.
• The object is linked to an uploaded mesh.
• The object, or any part of the object, has a physics shape type other than Prim.  You can change this on the Features tab of the Build Tools window.
• The object has a normal or specular map applied to it.

 

 

[Atomic Infinity]

With the OP's decor, the server rating is most likely to be the one determining the object in-world LI.  I doubt many content creators would upload a complex physics model with a decorative item as it would make the object unnecessarily high LI for no benefit, so our chances of halved land impacts by linking decor are alas tragically small 

Edited November 3, 2022 by Atomic Infinity

[ChinRey]

5 hours ago, Love Zhaoying said:

I noticed that the "new calculation" only applies in some situations, I bolded a bit below copied from the article.

Quote:

When is an object's land impact calculated using the new algorithm?

Legacy prim objects have a land impact rating equal to the number of prims they contain.  However, any object's land impact is calculated using download, server, and physics weights if it meets any of the following conditions:

• The object is an uploaded mesh.
• The object is linked to an uploaded mesh.
• The object, or any part of the object, has a physics shape type other than Prim.  You can change this on the Features tab of the Build Tools window.
• The object has a normal or specular map applied to it.

 

 

That's not the whole story.

"New" land impact calculation is used for any object/linkset that could not have been made before it was introduced. So any feature that was introduced later than that will trigger new LI calculation. In addition to the ones listed in the knowledge base article there are alpha modes other than blending, some script functions and possibly a few other things I can't remember right now.

Here's a more practical explanation (if I may so myself). And here's one for those who want all the gory details.

Edited November 3, 2022 by ChinRey