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I bought 8th land in Mainland. Is perfect size skybox ok or is slightly smaller best for servers.


Vlade Darkfold
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As long as you are sure none of your prims goes over your property line then you are ok.  Note that if any of them do, the owner of the property they enroach on can return them to you without warning, even if the root prim of a linked set or the center of your single prim is on your property.

The best way to do this is either build the floor and walls on the ground where you can see your property lines then link it and move it up to where you want it to be by using the Z measurement under position under the Object tab.  You can then finish it in the air.  Alternatively you can buy a laser system to set out in the air where you want to build.  It will indicate your property line using laser beams.

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Actually, though, if you're using only six prims for this, at least four of them must be old school megaprims, larger than 64m on a side, to be "perfect size" for an eighth sim.

If it's just you and maybe a visitor or two who will be bumping into and clomping around on those megaprims, then no worries. If, however this is going to be a busy store or event venue with many avatars colliding against those surfaces, then you'll be happier using current prims instead of the old megas. If you link them with Convex Hull physics type, four more prims register only two extra land impact -- a good investment, IMHO.

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megaprims are nasty, high server impact, lag monsters.

Just use regular prims, your neighbours and visitors will thank you. And if you're ever going to be using pathfinding pets (or other animated objects that use pathfinding) it's a lot easier to get them to work on regular prims than it is on megaprims.

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  • 2 weeks later...

Convex hull messes with the physics of objects I only use that trick on stuff I will make phantom.

What sort of lag do mega prims create? I use them alot for full sim platforms, screens and even roads. The only lag I have noticed is from textures and some mesh\sculpts when they have transparencies or made with lots of sides\vertices.

How bad actually are megaprims and how do you tell?

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Why do you think convex hull "messes with the physics of objects"? Or maybe I'm misunderstanding what you mean. What it does is to simplify the physical envelope, speeding collision computations (hence, better for the sim) which is usually what you want anyway, unless you need to preserve a hollow (concave) space.

The main problem with megaprims, server-side, is that they're often carved from much larger megaprims, and Havok apparently has to work harder to rule-out the possibility of something colliding with those carved-up solids (or so we were told, back in the day). So, it's not so much that megaprims are inherently nasty for the physics engine so much as the fact that, before carving, they were so enormous.

Also, Havok was said to check for collisions based on the largest dimension of the object, so something 512 x 512 x 0.5 m (before carving) would trigger detailed collision calculations for objects moving anywhere in a 512 x 512 x 512 m space. Or something like that.

To be fair, though, a lot of this dates from early days of Havok, when it really was a piece of garbage, and it's now much, much harder to crash a sim with physics than it was back then. Nonetheless, large-scale events with crowds of visitors (e.g., birthday celebrations) still restrict use of those old carved-up megaprims, and I'll bet they know some of those old considerations still apply.

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Qie Niangao wrote:

Why do you think convex hull "messes with the physics of objects"? Or maybe I'm misunderstanding what you mean. What it does is to simplify the physical envelope, speeding collision computations (hence,
better
for the sim) which is usually what you want anyway, unless you need to preserve a hollow (concave) space.

The main problem with megaprims, server-side, is that they're often carved from much larger megaprims, and Havok apparently has to work harder to rule-out the possibility of something colliding with those carved-up solids (or so we were told, back in the day). So, it's not so much that megaprims are inherently nasty for the physics engine so much as the fact that, before carving, they were so enormous.

Also, Havok was said to check for collisions based on the largest dimension of the object, so something 512 x 512 x
0.5
 m (before carving) would trigger detailed collision calculations for objects moving anywhere in a 512 x 512 x
512
 m space. Or something like that.

To be fair, though, a lot of this dates from early days of Havok, when it really was a piece of garbage, and it's now much, much harder to crash a sim with physics than it was back then. Nonetheless, large-scale events with crowds of visitors (e.g., birthday celebrations) still restrict use of those old carved-up megaprims, and I'll bet they know some of those old considerations still apply.

Well, for screens I hollow out cubes or spheres. For roads I hollow and pathcut tubes and cubes, for platforms I often pathcut or holliow to allow access through the surface. So for each of my main uses of megaprims, using convex hull messes up their physics to make them unusable.

Incidentally, I look in top colliders for one of the sims I manage and the only thing listed is the surfing wave (scoring (67.24) , This is out of a sim full of megaprims and surfaces for walking\riding on running up from ground level to over 2000m of continuous road system. If megaprims really were a problem wouldn't they be listed in region top colliders?

On the neighbouring sim I can see some roaming Koi score 0.36 on the same scale and a handful of other items (one a bike) One is a megaprim coming in at 0.96. But it is the only megaprim of dozens listing,

To be honest the numbers don't actually mean anything to me but they all seem a lot lower than the unproblematic surfing wave.

I have read threads on lag before. And there is a lot of opinion but not very much that is actually substantive or useful to people trying to make their sims lag free and function the way they want them too..... Aside from avoiding 1024 textures (where they aren't otherwise important).

In the most extreme case simply removing some alpha grass on a sculpt in the middle of my sim reduced lag for me and a lot of other people who were complaining. Aside from that one element in a FelixvonKotwitz build every other change I have made to reduce lag has been imperceptible or involved so many elements being sure what was the problem isn't possible.

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Aethelwine wrote:

Incidentally, I look in top colliders for one of the sims I manage and the only thing listed is the surfing wave (scoring (67.24) , This is out of a sim full of megaprims and surfaces for walking\riding on running up from ground level to over 2000m of continuous road system. If megaprims really were a problem wouldn't they be listed in region top colliders?

That's a good question, and impossible for anybody but a Linden to answer, particularly because it not only depends on the internals of Havok as used by SL, but also the specifics of how they're collecting that "top colliders" metric. I mean, it could be that it only takes into account processing of actual collisions, whereas a major problem with megaprims is supposed to be screening-out of potential collisions. So: who knows?

And hence I quite agree that knowing what actually causes server-side lag is much more difficult (and prone to superstition) than is viewer-side lag, where the viewer's built-in detailed rendering statistics, texture console, etc., give us a lot more information. (What's a bit disappointing, though, is that TPVs haven't used this information to provide more user-accessible advice about what specific objects in the environment are causing lag, and why. Seems an opportunity missed, of more value than yet another way to slice and dice chat windows. But I digress.)

For servers, really, it's mostly a matter of watching the individual sim statistics while tinkering with objects in the sim, and that's especially tricky because mostly it's a question of how they interact with avatars -- and sometimes only with numerous avatars. It seems your sim has no server-side lag problems, so that's great, but it's very difficult to know how stable that situation is.  Let me explain:

Back in maybe 2008 -- anyway, while SL was still on Havok 1 -- there was a forums post (now buried in generations of archives) about a particular sim that would reliably go into deep time-dilation every time somebody chose a particular teleport destination. I visited it myself, and the effect was pretty scary, like the sim was just "stuck" for like a minute. Sure enough, the problem was that a teleporting avatar would just barely collide with a non-phantom megaprim; moving the teleport destination upward maybe 0.1m cleared-up the problem completely.

Now, that's not necessarily applicable today. Could be that all our fretting about megaprims is anachronistic. I certainly don't have a current example of such a problem, nor any quantitative data showing sim lag when lots of avatars clomp around on carved-up megaprims. But even if it's merely superstition today, it's not mere fantasy: it very definitely was a huge problem, at least at one point, and it was why it took so long for LL to expand the maximum prim dimension from 10m to 64m.

[ETA: You mentioned the surfing wave. I haven't yet stopped in to look, but I think those are actual physics-enabled objects so they can push around surfers, and if that's correct, it's not surprising it would have a high collisions score, inasmuch as it would be colliding with every non-phantom object it encountered on its path, probably including the terrain mesh. I suspect you'd find that relatively few such waves, operating simultaneously, would start to raise the physics time in the sim. I don't know this for a fact, though; I actually thought about getting one of those systems for a Mainland shoreline, then got paranoid about the potential lag effect, so never bought it.]

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