Easily Change Prims to Convex Hulls to Lower Your Land ImpactDid you know you may be able to reduce the number of land impact units (aka: prims) used on your Second Life® property simply by linking objects together? That’s right, you may be able to reduce the number of land impacts used by up to 50%. Even better, it’s easy to do! Now to be realistic, you probably won’t see 50% savings. But even saving a few land impact units allows you to add more objects to your Second Life (SL) property.
The secret to reducing the land impacts is to link together objects that use “convex hulls.” This can be done in two simple steps. (1) Modify standard Second Life Prims to use convex hull physics. Mesh objects already use convex hulls so there is no need to modify them. (2) Link multiple objects that use convex hulls together. Once again, it only takes a few mouse clicks to do. When you link convex hull objects together it often reduces the combined land impact of the items. Warning: every once in a while the opposite occurs and the land impact goes up! For those of you who need to know the “why” and “how” see then end of this article for a brief explanation.
Sounds too Simple? Yes, there are some problems that can occur...There are a few important things to be aware of.
- You must link two or more objects together to lower the actual land impact. This means they become connected. So if you move one of the linked objects, the other will also move. Most of the time this isn’t a problem because you probably aren’t moving the objects around anyway. Fortunately it is very easy to unlink them if you want to change something. After you make the change, just re-link them again. Another way to do this is to check the "Edit linked" box and then select the part you want to move. You will be able to drag the selected part to a new position using the normal drag arrows.
- The objects must have “modify” permission, the same “owner”, and a no-copy object may not be linked to a no-transfer object. Yes, unfortunately that really puts limits on this.
- Some complex prim shapes will cause an INCREASE in land impacts if they are changed into a convex hull. I’ve done several experiments and the only time the land impact increased was when the prim was a torus shape. But one thing I have learned about Second Life is to never make assumptions and I haven’t tested every possible prim shape. The solution is simple, try converting to a convex hull and if the land impact increases, undo the change.
- When you link a prim to a mesh object the prim will be automatically turned into a convex hull. This can be good, or it can be really, really bad. If you link a complex prim, like a torus, to a mesh object it can cause a massive increase in the land impact. Before linking a prim to a mesh always test the prim. Convert it to a convex hull first, and see if the prim’s land impact increases.
- Broken scripts. In rare instances linking objects that contain scripts can break the script. So use care, make back ups, and do a test link if an item has a script in it.
- Does this work with sculpties? Not very well. Most, if not all, sculpties will increase land impact if they are changed to a convex hull. Sculpties may be linked to other objects, but there is no land impact reduction. Linking a sculpty to a mesh object will change the sculpty to a convex hull and increase land impact most of the time.
- How to identify a sculpty: right click on the object, select edit, then in the edit window select the “object” tab. You will see a multi-color “sculpt texture” displayed if it is a sculpty. A solid gray sculpt texture likely means you have a mesh object, not a sculpty.
- The minimum land impact of any object is 0.5. Even the tiniest, simplest object will add 0.5 to the land impact. To put it another way, the land impact will never be lower than half of the number of objects linked together. Therefore if you link 8 objects together, the resulting land impact will be at least 4. This is why the most you can reduce the land impact is 50% using this trick. (At least that’s how it worked when I wrote this in December 2013. Maybe this will change in the future.)
- OK, let’s get on with how to actually do this!
A. Change Standard Prims to Convex Hull:Start the process by changing standard prims to convex hulls as follows. (Mesh objects use the convex hull physics shape type by default so you don’t need to do this for mesh objects. Converting sculpties to convex hull will likely increase the land impact.) Doorways made from prims often do not work if converted to convex hulls! Avatars may no longer be able to walk through the doorway.
|Change Prim to use the “Convex Hull” Physics Shape Type|
- Right click on one of the prims and select edit, the edit window similar to that shown here will appear.
- Left click on the “Features” tab. Find the drop down box labeled Physics Shape Type. Left click in the box and a drop down list will appear, and left click on “Convex Hull” in the drop down list.
- Check the land impact count. If the land impact value is now the same or lower continue on to step 4. If the land impact increased, change the Physics Shape Type back to “Prim” and do not link this prim to other objects. An increased land impact means this prim has a complex shape and here will be no advantage to linking it to other objects.
- Close the edit window.
B. Link Objects:
|Link and Unlink Buttons in the Edit Window|
- Right click on one of the objects and select edit, the edit window will appear.
- Hold down the “shift” key and left click on the other prims to select them. The last object selected becomes the “root prim” and will be outlined in yellow when in editing mode.
- Left click on the button labeled “Link”. Now they are linked together.
- The number of land impacts should change. Usually it will go down. Sometimes it won’t change. If it does happen to go up then unlink the objects.
To Unlink Objects that you have Previously Linked:
- Right click on any one of the linked objects and select edit to open the edit window.
- Click on the button labeled “Unlink”. The prims will be unlinked. The land impacts will probably go up. Warning: if you don’t have enough land units available for all the unlinked prims you may get an error message.
Simplified Explanation of how this works:Second Life rounds UP the land impact value of each object to the next whole number value. So the actual land impact of a tiny, simple object might be only 0.5 but SL will round that value up to 1.0. However when you link multiple objects the linking process turns them all into a single object. So let’s say you have 2 objects on your Second Life property, both are simple shapes that use 0.5 land impacts. They will each be rounded up to 1.0, and together they will use up 2 of your land impacts. But if you link them they will become a single object and the land impacts of both will be combined. So that means 0.5 + 0.5 = 1.0 land impacts. So the combined land impact of both is now 1.0. In this example you will reduce the land impact of the two items by 50% simply by linking them together. To make it more confusing, SL sets the minimum land impact of any object at no less than 50% of the number of linked items in the object. So for example, 8 linked prims will become one object, but that object will never be less than 4 land impact units (8 x 0.5 = 4). I have no idea why they do this, I assume there are other factors about multiple linked objects that add to the server load making it necessary.
More Information:Note that my skill set is writing and explaining things, not 3d modeling! Much of the article above is based on my own experience, combined with tips I gleaned from various sources. Some of my explanations are likely to be technically incorrect! My aim is to make this topic understandable for the average Second Life user, not to create a technical manual for engineers.
If I’ve totally confused you, or you just want more technical details, here are some links to good articles on land impacts and linking :
- Calculating land impact by Jeremy Linden
- Prims, Prim Equivalent
, Land Impact… a too-long guide by Jenni Darkwatch
- Link Wiki