❔ Land Impact Calculation Mystery ❔

[Count Burks]

A week ago I am contacted by a Resident who has the following question about his island.

"The Resident is building a boat using prims. His boat construction uses 43 prims. Upon linking the ship the Land Impact goes to 251 Prim Land Impact."

I contact Concierge Support regarding this issue and after more than a week I receive a reply I need to ask this in the forums. Meaning Concierge Support or "Concierge Support Specialists" as they like to name themselves is unable to answer this question to me. It seems like nobody at Linden Lab even knows this.

I am really curious to know why linking a prim based Object consisting out of 43 prims goes to 251 Land Impact usage when linked.

Can anyone explain why a non mesh purely based prim build goes up with an insane factor when linked?

I do have the Resident his permission to Modify his Objects. I did link and unlink the build myself and checked for usage of scripts or mesh but nothing could be found. 

I am posting this in this section of the forum as it might be a system related bug.

 

Count Burks

[Qie Niangao]

It's probably a case of Mesh Land Impact accounting being applied to prims that have had Materials applied to one or more faces. For example, a prim with a face set to Alpha Mode: Alpha Masked will force the entire linkset (when linked) to use Mesh accounting, which may matter a great deal if some prims in the linkset are, say, path-cut rings or anything that gets a stupidly high Physics weight when Mesh accounting is used.

The owner may get some hints by looking at "More Info" in the Build Tool, specifically to confirm that it's the Physics Weight that's blowing everything up.

[Henri Beauchamp]

It's a well know bug encountered when converting from the prim-based to the land-impact based accounting for objects. Old objects and objects made only out of ”Prim” type primitives (and without materials) are using the primitives count as the LI.

To convert (relatively) safely such an object to the land impact accounting, first convert its root primitive as a ”Convex hull” type (this should avoid seeing the sky-rocketing of the LI), then convert the other prims as needed. Be aware that each ”tortured prim” (any prim using Twist/Taper/Shear/Slice) in the link set will cost dozens of LI unless you convert them to the ”Convex hull” or ”None” types.

I recommend doing such conversions on a parcel with a lot of available primitives (in a sandbox sim for example) to avoid the auto-return it would potentially trigger on ”full parcel” event.

Edited January 18, 2022 by Henri Beauchamp

[Count Burks]

2 hours ago, Henri Beauchamp said:

It's a well know bug encountered when converting from the prim-based to the land-impact based accounting for objects. Old objects and objects made only out of ”Prim” type primitives (and without materials) are using the primitives count as the LI.

To convert (relatively) safely such an object to the land impact accounting, first convert its root primitive as a ”Convex hull” type (this should avoid seeing the sky-rocketing of the LI), then convert the other prims as needed. Be aware that each ”tortured prim” (any prim using Twist/Taper/Shear/Slice) in the link set will cost dozens of LI unless you convert them to the ”Convex hull” or ”None” types.

I recommend doing such conversions on a parcel with a lot of available primitives (in a sandbox sim for example) to avoid the auto-return it would potentially trigger on ”full parcel” event.

It seems like you are correct Henri. Since I do have permission to Modify the Objects of the Resident I did what you did mention using Convex Hull.

The boat which has recently been made by the Resident from scratch using prims is using 43 prims so the Land Impact should be 43.

When linked the Land Impact increases to 215 prim usage. Fantastic for people who want to do some building with prims on their island right?

When doing what you did mention putting the root prim to Convex Hull this did only make a small difference. However going further and setting the other prims also to Convex Hull one at the time kept reducing the Land Impact on the boat. Once I did all the prims now the Land Impact is 23 prims LI on a boat made with 43 prims.

A negative side effect is that I could not step into the boat anymore because of the Convex Hull shape. It was needed to switch back a couple of prims to prim type to enable this again.

In recent weeks I did receive a similar complaint from people who were building with prims on their land about the huge Land Impact.

@Linden Lab should really look into this as many people still enjoy building with prims. Such a system bug  (where the concierge support specialists not even have a clue about its existence) spoils people their building fun. 

 

Thank you for clearing this up Henri.

 

Count Burks

 

Edited January 19, 2022 by Count Burks

[Theresa Tennyson]

21 minutes ago, Count Burks said:

It seems like you are correct Henri. Since I do have permission to Modify the Objects of the Resident I did what you did mention using Convex Hull.

The boat which has recently been made by the Resident from scratch using prims is using 43 prims so the Land Impact should be 43.

When linked the Land Impact increases to 215 prim usage. Fantastic for people who want to do some building with prims on their island right?

When doing what you did mention putting the root prim to Convex Hull this did only make a small difference. However going further and setting the other prims also to Convex Hull one at the time kept reducing the Land Impact on the boat. Once I did all the prims now the Land Impact is 23 prims LI on a boat made with 43 prims.

A negative side effect is that I could not step into the boat anymore because of the Convex Hull shape. It was needed to switch back a couple of prims to prim type to enable this again.

In recent weeks I did receive a similar complaint from people who were building with prims on their land about the huge Land Impact.

@Linden Lab should really look into this as many people still enjoy building with prims. Such a system bug  (where the concierge support specialists not even have a clue about its existence) spoils people their building fun. 

 

Thank you for clearing this up Henri.

 

Count Burks

 

It's not a "bug", it was an intentional change to the system over ten years ago to better reflect the actual server load of certain types of modified prims. As you now see, it's very possible to make a prim build with less land impact then it previously had. You just need to know how to do it.

[Henri Beauchamp]

7 minutes ago, Theresa Tennyson said:

It's not a ”bug”,

It *is* a bug, because if you do not change the root primitive first (but a child primitive instead), the object LI will most often inexplicably sky-rocket to several hundreds, while if you change the root first, it does not...

[Henri Beauchamp]

41 minutes ago, Count Burks said:

A negative side effect is that I could not step into the boat anymore because of the Convex Hull shape. It was needed to switch back a couple of prims to prim type to enable this again

You do not need to use the ”Prim” type any more: ”Convex hull” does have the physics shape (unless you make the object ”Phantom”, of course). Only ”None” removes entirely the physics from the primitive (making it cheaper), but is (for this very reason) not available for the root primitive.

Edited January 19, 2022 by Henri Beauchamp

[Count Burks]

1 hour ago, Henri Beauchamp said:

You do not need to use the ”Prim” type any more: ”Convex hull” does have the physics shape (unless you make the object ”Phantom”, of course). Only ”None” removes entirely the physics from the primitive (making it cheaper), but is (for this very reason) not available for the root primitive.

As I wrote above. After setting all the prims to Convex Hull starting with the Root prim first I ran into another issue with the boat. When trying to enter the boat I was walking on top of it because of the Convex Hull issue. Then I needed to switch some of the prims back to prim in order to be able to enter the boat again.

This makes prim building very cumbersome if this is a feature instead of a system bug. I also doubt more than a handful of people know about this. We also speak about a relative simple build, I do not even want to know what mess it would be to build a larger structure with prims with this supposed "feature". What a joy having to debug every single build you create for invisible walls and holes.

 

[Qie Niangao]

7 hours ago, Henri Beauchamp said:

It *is* a bug, because if you do not change the root primitive first (but a child primitive instead), the object LI will most often inexplicably sky-rocket to several hundreds, while if you change the root first, it does not...

7 hours ago, Henri Beauchamp said:

You do not need to use the ”Prim” type any more: ”Convex hull” does have the physics shape (unless you make the object ”Phantom”, of course). Only ”None” removes entirely the physics from the primitive (making it cheaper), but is (for this very reason) not available for the root primitive.

These are simply incorrect. Other than a very old bug causing momentary miscalculation (which mostly affected sculpts and which  I haven’t seen for years) there’s nothing inexplicable about any of this* and prim builders very definitely need to use Prim physics type for parts of their builds if they want to retain accurate physics shape of concave prims.

It can also be advantageous to stick with prim LI accounting (by removing Materials) of linksets that contain sculpts although their once disastrous LI effects were changed to a max of 2 per sculpt back in 2012).

______________

* although the details are obscure and disorganized in documentation. The Knowledge Base overview is okay as far as it goes, but the detailed specs for Download, Physics, and Server weights are scattered around the wiki and the release note cited above.

Edited January 19, 2022 by Qie Niangao
(link KB overview)

[Henri Beauchamp]

25 minutes ago, Qie Niangao said:

These are simply incorrect. Other than a very old bug causing momentary miscalculation (which mostly affected sculpts and which  I haven’t seen for years)

This miscalculation is the bug and unlike you, I encounter it very frequently when converting old builds to the LI system.

6 hours ago, Count Burks said:

As I wrote above. After setting all the prims to Convex Hull starting with the Root prim first I ran into another issue with the boat. When trying to enter the boat I was walking on top of it because of the Convex Hull issue. Then I needed to switch some of the prims back to prim in order to be able to enter the boat again.

Ah, yes, indeed, for ”tortured prims”, the physics convex hull is extremely coarse (simplified to a cube or a sphere, apparently) and as its name implies, hardly suitable for a concave prim... In this case, it might be advantageous (for example for your boat floor) to add an invisible (simple/non tortured) primitive set as ”Convex hull” (that will count for 0.5 LI only) and keep the tortured prims as ”None”. The LI system most often (not always) brings a benefit, but it implies some modifications in old builds or in how you conceive new builds to get the most out of it.

Edited January 19, 2022 by Henri Beauchamp

[Theresa Tennyson]

7 hours ago, Count Burks said:

As I wrote above. After setting all the prims to Convex Hull starting with the Root prim first I ran into another issue with the boat. When trying to enter the boat I was walking on top of it because of the Convex Hull issue. Then I needed to switch some of the prims back to prim in order to be able to enter the boat again.

This makes prim building very cumbersome if this is a feature instead of a system bug. I also doubt more than a handful of people know about this. We also speak about a relative simple build, I do not even want to know what mess it would be to build a larger structure with prims with this supposed "feature". What a joy having to debug every single build you create for invisible walls and holes.

 

1) Set the entire linkset to convex hull.

2) Find prims you have to enter the "footprint" of (i.e. hollow or concave shapes that you walk into) and change those to "prim" using "edit linked."

3) If your land impact is now more than half of what it would have been with the old calculation, start changing the physics shape of strictly decorative prims to "none."

You can reduce most prim builds to half their original land impact this way, unless it has a lot of sculpts or the builder tried to "save prims" by elaborate cutting to do things like using one prim for three walls. Honestly, it's a lot simpler than pathcutting. And it's been around at least since materials have been introduced.

 

[Emma Krokus]

11 minutes ago, Theresa Tennyson said:

1) Set the entire linkset to convex hull.

2) Find prims you have to enter the "footprint" of (i.e. hollow or concave shapes that you walk into) and change those to "prim" using "edit linked."

3) If your land impact is now more than half of what it would have been with the old calculation, start changing the physics shape of strictly decorative prims to "none."

You can reduce most prim builds to half their original land impact this way, unless it has a lot of sculpts or the builder tried to "save prims" by elaborate cutting to do things like using one prim for three walls. Honestly, it's a lot simpler than pathcutting. And it's been around at least since materials have been introduced.

 

What Theresa said.

I experiment a lot (always aiming to keep the LI as well as possible), setting the root prim to PRIM shape sometimes can definitely help. Often I end up swapping what is the root prim. Even inside the linkset using PRIM can be advantageous.

CONVEX HULL setting rarely allows me to pass through the "imaginary" opening of an object. Sometimes setting that object to NONE and making a walk on with basic prims keeps the LI way down rather than using PRIM for the object.

Occasionally I find something silly like using a BOX instead of a SPHERE basic prim (e.g. for an invisible light source or snow poofer), actually allows me to reduce the LI.

In addition to materials, scripts inside objects can increase the LI.

Another way to lower LI, is to experiment with the size of a mesh object. Some appear to have a point where the LI "jumps" dramatically. In some cases for me that has been but a fraction of a reduction in size, but with a big gain in reduced LI.

It seems more like magic to me!

 

 

[Ardy Lay]

5 minutes ago, Theresa Tennyson said:

1) Set the entire linkset to convex hull.

2) Find prims you have to enter the "footprint" of (i.e. hollow or concave shapes that you walk into) and change those to "prim" using "edit linked."

3) If your land impact is now more than half of what it would have been with the old calculation, start changing the physics shape of strictly decorative prims to "none."

You can reduce most prim builds to half their original land impact this way, unless it has a lot of sculpts or the builder tried to "save prims" by elaborate cutting to do things like using one prim for three walls. Honestly, it's a lot simpler than pathcutting. And it's been around at least since materials have been introduced.

 

Yes.  It's more complicated than just "One primitive is equal to one Land Impact point", which annoys some people to no end.  There is more benefit to it than the expressed reward of being able to reduce the overall link-set LI to as low as half the old-accounting score.  The real benefit is what is indicated by the change in score:  PHYSICS SIMULATION LOAD REDUCTION, most commonly.

Shortly after the "new accounting" went live, I went around and inspected objects in a community build that has been around for years.  I discovered some link-sets that, when inspected, have huge physics loads.  Using the simple method brought forward here produced astounding results in simulator performance improvement with the reward of freeing up almost half of Land Impact resulting in the community decorators having lots of fun and freedom adding more objects to the area.  I occasionally review newly added objects for performance hogs and adjust them where possible.  Some items I find are so bad and "no mod" that I simply return them to their owners.  One item I returned was so important to the new seasonal theme that I decided to be awful and craft a replacement that the owner was pleased to use instead.

Yes, this is a complexity that was brought about by ignoring a severe problem for so long that it became part of the landscape.  Fixing it causes astonishment and annoyance, as the solution requires change, and that change brings complexity as one has to work with mathematical reality instead of pretending that Second Life is the real world instead of a simulation.

[Qie Niangao]

1 hour ago, Emma Krokus said:

Another way to lower LI, is to experiment with the size of a mesh object.

Yes. For mesh objects especially there also can be big wins in "gaming the system" of LI accounting by finding and linking objects with complementary weight imbalances, shown in the More Info window from the Build Tool. For example, some creators link together lots of very simple elements, driving up the Server weight despite the Download weight being very low. Others use lots of visual complexity in an object with one or very few links, so their Download weight is high and their Server weight is low. The Mesh accounting Land Impact of an individual object is the maximum of their weights, so linking these complementary maximums together can cut the combined Land Impact nearly in half.

This has no actual benefit for the viewer and nearly none for the simulator, but we have to pay for that Land Impact and rules are rules so may as well get our money's worth.

(Linking objects requires some care, especially if either of them is scripted. Also they must be within some reasonable proximity—55m IIRC—and have compatible permissions.)

[Emma Krokus]

2 minutes ago, Qie Niangao said:

Yes. For mesh objects especially there also can be big wins in "gaming the system" of LI accounting by finding and linking objects with complementary weight imbalances, shown in the More Info window from the Build Tool. For example, some creators link together lots of very simple elements, driving up the Server weight despite the Download weight being very low. Others use lots of visual complexity in an object with one or very few links, so their Download weight is high and their Server weight is low. The Mesh accounting Land Impact of an individual object is the maximum of their weights, so linking these complementary maximums together can cut the combined Land Impact nearly in half.

This has no actual benefit for the viewer and nearly none for the simulator, but we have to pay for that Land Impact and rules are rules so may as well get our money's worth.

(Linking objects requires some care, especially if either of them is scripted. Also they must be within some reasonable proximity—55m IIRC—and have compatible permissions.)

For clarity, by "size" I was referring to the actual dimensions of the mesh object, rather than number of objects within a linkset.

[Emma Krokus]

1 hour ago, Ardy Lay said:

Yes.  It's more complicated than just "One primitive is equal to one Land Impact point", which annoys some people to no end.  There is more benefit to it than the expressed reward of being able to reduce the overall link-set LI to as low as half the old-accounting score.  The real benefit is what is indicated by the change in score:  PHYSICS SIMULATION LOAD REDUCTION, most commonly.

Shortly after the "new accounting" went live, I went around and inspected objects in a community build that has been around for years.  I discovered some link-sets that, when inspected, have huge physics loads.  Using the simple method brought forward here produced astounding results in simulator performance improvement with the reward of freeing up almost half of Land Impact resulting in the community decorators having lots of fun and freedom adding more objects to the area.  I occasionally review newly added objects for performance hogs and adjust them where possible.  Some items I find are so bad and "no mod" that I simply return them to their owners.  One item I returned was so important to the new seasonal theme that I decided to be awful and craft a replacement that the owner was pleased to use instead.

Yes, this is a complexity that was brought about by ignoring a severe problem for so long that it became part of the landscape.  Fixing it causes astonishment and annoyance, as the solution requires change, and that change brings complexity as one has to work with mathematical reality instead of pretending that Second Life is the real world instead of a simulation.

Oh the curse of no mod objects!!! 

Not being able to link, relink or change feature properties. 😭

[Qie Niangao]

1 minute ago, Emma Krokus said:

For clarity, by "size" I was referring to the actual dimensions of the mesh object, rather than number of objects within a linkset.

Oh, I realize, I was off on a different tangent, but we're both talking (mostly) about Mesh objects. I'm a big fan of tweaking down object size to find LI savings; I usually use that More Info window to find objects that are close to half-way between integer Download weights, maybe 1.5 Download with a total Land Impact of 2, and see if I can bring it down to 1.4 or a "smaller 1.5" that rounds down to 1 instead of 2.

Plus furniture is always too damned big anyway! 😛