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Why Doesn't Convex Hull Work Always, Or Worse, Doesn't Work to Reduce Prims?


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No, switching from prim to convex hull definitely does not work all the time. Really.

No, you may be saying that it does because a) you never log into SL b) you never actually try to change and improve things all day.

The reality is, for anybody trying to reduce prims all day, convex hull does not work all the time.

Here's what happens:

1. Sometimes it just flat doesn't work, period, and never will.

2. Sometimes unlinking a complex item and convexing some of the parts might work, but usually not.

3. Sometimes you flip back between convex and prim several times, it finally kicks in.

4. Sometimes -- too often, really -- it makes the object FAR WORSE. it is not a cure-all. Why?

5. Sometimes, if you change to "no shape" to clear it, which the server hates and sends you scary messages about, you can then force it to go to a convex that lowers LI. It's too bad that happens -- if the shapelessness is really the horror the server thinks. But it's often what is required to get the convex hull to work, i.e. to reduce LI from prim shape.

6. With things like houses and cabins that have doors, convex hull more often than not BLOCKS the door. House creators who don't actually ever try to live in their houses are guilty of causing this some time, you have to then get them to adjust it. Why does it do that? I'd love to know. THAT it does that is obvious to anyone who tries to place mesh houses or reduce prim houses.

Apparently convex hull is a "thing" that isn't "supposed to" always make less LI because XYZ[ physics, math, Lindens].

I do wonder why, however.

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It won't really help with prims that have been hollowed or cut, in fact it can make the LI far worse. it also doesn't make much difference for semi-complicated things like sculpted stairs, or for awkward shapes like the toroids and cylinders (anything above PRIM_TYPE 4 is my guess).

My approach has been to split large buildings into two link-sets, one contains all the cut, hollowed or scripted children, and is just type prim., the other contains all the simple prims that can be set to convex hull.

Don't forget that what you are doing with a lower or lesser physics model is reducing the collision behaviour, so where you could walk into the hollow in a prim you would not be able to in a convex hull, because there has been no calculation to allow for an indent into the general shape.

I recall seeing posts suggesting you actually have to do something to force the new LI accounting to happen, but can't remember.

Edited by Profaitchikenz Haiku
cogito, ergo correcto
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my basic strategy is to link my build to a standard box prim as the root. Then set the whole lot to Physics Shape Type: None. Then work thru each linked piece, setting only the must-be pieces to Physics Shape Type: Convex Hull. When this increases the LI (or doesn't give the wanted outcome for things like door frames, etc) then try Physics Shape Type: Prim and go with the lower (or go with Prim when Convex Hull blocks passage)

this doesn't always go as well as I think it should. Some days it goes easy as. Other days the numbers are not as they should be, which drives be bananas

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Most of the information you'll need is here: http://wiki.secondlife.com/wiki/Making_Mesh_Physics

And from: http://wiki.secondlife.com/wiki/Physics_Optimization

"Tip: Cubes, spheres, and circular cylinders are the cheapest shapes to use. Up to 14 cubes equates to the same value as one cut hollowed cube. Other shapes, such as pyramids, with no cuts, twists, hollows, or holes, or any shape set to PRIM_PHYSICS_SHAPE_CONVEX_HULL for a potentially more accurate (albeit more expensive) r for its physics shape type are more expensive. Any shape that contains holes, cuts, twists, or hollows (such as a torus) fall into the most expensive category."

The same applies to mesh. Fully convex shapes (no inward curves) are the cheapest. Triangle density in the shape also affects physics cost. Convex Hull doesn't automatically mean "lower LI," it's just a different calculation.

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