Physics for mesh builders

[Kwakkelde Kwak]

Trying to find some information about how the Havok physics engine works and how to minimise physical load on the servers, I stumbled across this document, on the SL servers of all places.

Artist Best Practices

I love the footer btw.

[Rahkis Andel]

I've seen this already, but kudos anyway for a good read.

[Codewarrior Congrejo]

yeah the footer stll makes me laugh. Also the demonstration with the hammer onto the poor example guy

[Ciaran Laval]

Read that before too and I'm, also amused by the footer.

However the document raises some good points and suggests using a mesh physics shape isn't ideal, did I read that right?

[Jenni Darkwatch]

I'd read it as "simplify physics shapes as much as possible" which makes sense... whether it's mesh or not is probably irrelevant.

[Pamela Galli]

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Jenni Darkwatch wrote:

I'd read it as "simplify physics shapes as much as possible" which makes sense... whether it's mesh or not is probably irrelevant.

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Thanks Jenni, wondered what that article meant. As a believer in keeping things as simple as possible, I use regular prims or sometimes planes for physics as needed, and mostly just for houses.

[Jenni Darkwatch]

Same here... matching physics shapes is sometimes just too much of a pain due to the "same-bounding-box" requirement

[Ciaran Laval]

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Jenni Darkwatch wrote:

I'd read it as "simplify physics shapes as much as possible" which makes sense... whether it's mesh or not is probably irrelevant.

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Well the document lists triangles/mesh as the most expensive physics type. I don't know though how inworld prims are composed, whether they are triangle based meshes too.

[Kwakkelde Kwak]

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Ciaran Laval wrote:

Well the document lists triangles/mesh as the most expensive physics type. I don't know though how inworld prims are composed, whether they are triangle based meshes too.

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It really depends on the prim. A box (no matter the proportions) is seen as a convex hull. That is until you use any modifiers. The same is the case for cylinders, except a different x and y value will turn it to triangle based as well. Spheres without modifiers and a 1:1:1 proportion are convex hull.

All other prims are triangle based going by the metadata in the develop menu.

This means you should be careful with using them. It does not mean you should always use prim boxes as physical shapes. Both server and download weight will increase by adding prims. Even if this doesn't affect LI, it does affect performance. Better to use the same box shapes in a seperate file and let the uploader "analyze" that. For floors that might be an issue, since you can't rezz on convex hull mesh items, at least as far as I've experienced. Floors might be the only objects where a prim could be preferred. Can't think of any other ones, maybe others can.

What I read in the article is that one should use convex hulls or physical primitives(NOT SL primitives!) where one can. But the article also says "a" triangle is cheap. So if you need a lot of convex hulls to replace a few triangles, that's probably not a good idea. It also depends on the situation. On a racetrack a wall is best done as convex hull from what I understand, since you can't get trapped in those. "Speed+collision=get trapped" in most cases when the shapes are triangle based. Walking into a wall is less likely to give you issues.