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# What is convex hull?

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Basically it is a simplified envelope used by the physics engine to calculate collisions. Using it will often cut the land impact of an object in half. The downside is that hollowed prims will act solid. .. you may find you can't go through a door.

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Basically it is a simplified envelope used by the physics engine to calculate collisions. Using it will often cut the land impact of an object in half. The downside is that hollowed prims will act solid. .. you may find you can't go through a door.

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What it does is take a shape and make it so that the physics simulation doesn't think there are any holes or convex areas - it's like "shrink-wrapping" the shape, so the "convex hull" of a tube would be a cylinder. It makes it a lot easier for the simulation to calculate things that might collide iwth the object because it can ignore things going THROUGH the shape. To the eye it will look exactly the same. The only time it would be a problem is when you've got something like a wall with a door opening through it - the physics engine wouldn't let someone walk through the door if it;s set to "convex hull."

Since it makes it simpler to calculate things the "land impact" gives convex hulls an advantage and gives tubes and other hollow shapes a big penalty. A rule of thumb is that an object made up of simple shapes and no sculpts can have its prim usage cut in half by changing it to "convex hull.' There are more detailed discussions in the forums if you search for them.

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If you look in the Features tab of your Edit/Build tool, you'll see that you are offered two physics shapes (plus NONE) that you can apply to any prim.  The choice you make determines how SL treats the object when it determines what it looks like to the physics engine.  If you choose PRIM, the physics engine will see the object exactly the way it appears to you visually.  If you choose CONVEX HULL, it will see a somewhat simplified shape in which any indentations (dimples, grooves, holes, etc.) are smoothed out.  Imagine the difference between a waffle and a pancake, for example.  The Convex Hull shape is much easier for the physics engine to deal with when it has to decide whether something it trying to occupy the same space as the object, so you are rewarded by having a lower calculated Land Impact when you choose Convex Hull.

Generally speaking, then, choosing Convex Hull for any objects will lower L.I. and let you pack more objects into the prim allowance on your land.  On my parcel, for example, I have converted all of my trees from PRIM to CONVEX HULL and reduced their impact by about 1/3.  Mesh objects are uploaded as Convex Hull by default, so they typically have much lower L.I. than ordinary prim objects.

So why not simply use Convex Hull all the time?  For at least two reasons.  First, there are times when you want the indentations and holes to remain.  Doorways will only work if they have an opening to walk through, for example, so creators have to use the PRIM option for them.  Also, some objects actually increase their L.I. when you use Convex Hull.  Sculpties, for example, are not as simple as they appear.  From a toplogical point of view, they are more like origami objects than the solid items they appear to be.  When you convert a sculpty to Convex Hull, all of its topological folds add complications to the physics shape, driving the L.I. up instead of down.

Finally, there are times when you want to choose physics shape NONE.  If you do that, you're telling the physics engine that it doesn't need to worry about another obect or avatar occupying the same space.  NONE means "make this object Phantom".  You cannot choose physics shape NONE for the root prim in an object, but all of the child prims can be NONE if you wish.  Doing that can drive the L.I. down too.

You may want to read more at http://www.lokieliot.com/blog/?p=1193 and, if you want a more technical article, http://wiki.secondlife.com/wiki/Mesh/Mesh_physics .

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