Consult the wiki.

Megas don’t cause lag, though there are weirdnesses that can occur, such as camera strangeness inside a hollowed-out mega. They mess with physics sometimes and are more than a little undefined as to behavior when they hang off the edge of one sim and into another. This is why responsible users of very large megaprims tend to set them to be phantom.

The use of megas alone is not a violation of TOS or CS. They used to be returned if they were annoyances, but that’s true of any prim.

There’s currently a debate stirred up by Bettina Tizzy, over whether it’s a good idea to allow special edit privileges on private estates, letting builders set their prims to be larger than 10m on a side. Reading through comments there and on the associated JIRA gives you an idea how wide the range of opinions is on megaprims.

Left by SoloMornington on October 16th, 2009

A series of quotes from discussions about Megaprims.

“The original creator of Megaprims was a avatar by the name of Gene Replacement.”

“They aren’t made any more. There were two very brief periods when it was possible to exploit a bug in SL’s design tools and get around the 10m limits on prim size. During those periods, several enterprising builders produced a mountain of megaprims in a wide range of sizes. You can find them all over SL and on XStreetSL. Since then, LL has closed the loophole that made it possible to create megaprims. Although they tolerate the existence of megaprims, they do not allow new ones to be created now.”

“You can do all sorts of things to a megaprim too, like change its shape (cube to cylinder or sphere, for example), cut it, or put a hole in it. That means you can make loads of copies of a megaprim in your inventory and make them all look different. The one thing you cannot do is to change its size. If you try, it snaps back permanently to 10m maximum dimension.”

“The biggest caveat is that the more you have to use things like path cuts, dimples, and other tricks to alter the apparent size of a megaprim, the more likely it is to cause problems near the altered surface. And this is true with all sizes of prims, for that matter.

For example, I made a 70 x 70 x 10 megaprim as part of the 300+ that I created on the second oppourtunity to make them. (I had to use a slightly hacked client at the time, as well.) But it didn’t occur to me, before time ran out and LL closed that loophole in the server code, to make a 70 x 70 x 1 megaprim for a platform.

You can take that 70 x 70 x 10 megaprim, make it a sphere, dimple begin 90%, make it back into a box, and you’ll have what looks like a 70 x 70 x 1 Megaprim.

But there is a 70 x 70 x 9 area right under that platform that is still within the bounding box of the actual megaprim. When you try to do things inside the bounding box of a prim, things don’t always work as you would expect. On the top surface of that platform, you can walk normally, rez prims normally, and it acts prety much like any other prim. But turn it over so you are trying to walk or rez on the other face. You probably can’t rez a prim on its surface now, and if you can, it may appear 9M above the visible surface!

There are other issues, but they all relate to the fact that the game engine and physics engine that runs SL doesn’t anticipate such things as megaprims existing, and treats any cut, dimpled or otherwise “tortured” prims as odd exceptions to normal behavior.”

“No certain answers here. But I think I know where the myth/rumors come from. Some of this may have changed with havok 4, but I still see the problem.

Mega prims just sitting there do ok, phantom ones do even better. Where I start to see perfomance issues is when a gaggle(10 to 30) AVs are all standing on the same mega prim. Think sky store platform. This rarely happens though. But it might be worth cosidering normal prims for objects people walk on or bump into.”

“well, if you have 10 to 30 avatars standing on ANYTHING you will notice lag. It’s the avatars that cause the lag, not what they are standing on. I’ve used full sim size floors in our elevated store for quite some time (3 sims joined). the number of prims that saves is enormous, and well worth the use of megas.”

“True 10 to 30 avs will lag stuff. But is it realy the Avs or is it the collisions they are causing? Bouncing back and forth between top scripts and top collision looks to show its still collisions.”

“The only time collisions take place is if a prim has “physics” turned on. Physical megaprims will usually cause lag on the server side.

Non-physical megaprims in moderate sizes (up to about 128m) do not cause lag. If they are near sim borders, you may see some odd visuals from next-door sims (the megaprim may not always rez, because sims only usually load objects whose center is within 10m of the sim border) — but even that seems to be less of a problem than it used to be.

Huge megaprims, like those that Blaze uses, require a little care in use to ensure smooth sim crossings etc., but do not cause lag, in the hands of a good builder.

Be careful when using megaprims not to encroach on nearby parcels. Remember, when you cut them, their actual “size” is still the original prim size … and thus an invisible portion of your cut megaprim can actually be overhanging and messing up your neighbor’s build.”

“non physical megaprims still register collisions from physical objects (like avatars) based on their bounding box.”

“And for those that can’t figure this out on their own… megaprims are identical to regular prims on the server and client side, except for a few decimal places here and there. Physics calculations are done with the same accuracy, the server couldn’t assign more resources to megaprim physics even if it wanted to. Every time you replace 2 prims with 1 megaprim, you cut in half the amount of network data sent to the user for that object, and also cut in half the cost of rendering that object, and finally save yourself some prim usage.”

“If you can use megaprims for a personal project, you should. It reduces server resources, saves on prims, and reduces the amount of polygons your computer needs to render.

They do not “create lag”.”

“Avoid using the old ones made by Gene replacement if you can as they were made by prim torture and can get a little funky at times, no offense to Gene BTW, were very handy prims in their day but they have been superseded except for some larger sizes. The size limit is basically what you can fit on rour land, if you got 16 sims then sure rez a 1024×1024 in the middle of them.

The new ones can also have their shape changed, and be cut or hollowed, so you can change a cube into a cylinder etc, no problems.”

“Gene Replacement actually only made about a half a dozen sizes of megaprims. The sets that have 200 or more different sizes of his megaprims are ones made with prim torture – lots of cuts and dimples and other tricks to reduce one if his few sizes to some other usable, smaller size. The few that he made that have a bounding box exactly equal to the prim dimensions, like his 256 M cube, are just fine. But most of the megaprims that have him as Creator have lots of cuts, dimples and other alterations, and like any other tortured prim, they can cause issues because their bounding box and visible size don’t match. The worst case with Gene’s prims are a few large flat ones that the center of the actual prim is on one corner, and the prim was thinned down using dimples on both flat faces. They hang over way past the edges of the parcel if they are visibly aligned with parcel edges, and neither flat face behaves well for rezzing on.

When a second chance appeared (over a year ago) to make more megaprims, a lot of builders, including myself, pounced on the chance, and we made as many different sizes as we thought might be useful. We only had a few days to work in, but each of us made between 200 and 2000 new megaprims, and virtually all of us made those available to others for free.

Since the “New” megaprims were designed with a wide range of szes, there is virtually no reason to use prim torture to get the size you need. So, for example, a 50 x 50 x 1 M megaprim from my set has a bounding box that is exactly that size, and there is no problem rezzing things on its surface. But if you took my 100 M cube and dimpled it top and bottom to make a 100 x 100 x 0.5 M panel, that prim would have just as many issues as any prim-tortured Gene Replacement Megaprim.

All megaprims can be cut, dimpled, textured, and treated exactly like any other prim. The only thing you can NOT do is alter their actual dimensions on X, Y or Z, because if you do, they get chopped to a max of 10 M in the dimensions that exceed 10 M.

Megaprims are subject to the same bounding box limits as any other prims. So it isn’t possible to link the really big ones (over 40 M or so in any dimension) to other prims But you could, for example, link a 20 x 20 x 0.1 Megaprim anywhere that four 10 x 10 x 0.1 prims could have been used to make the same wall.

Be aware that unlinked megaprims will always show the original creator’s name. I get a LOT of queries each month from people looking to buy a dance floor, or a tree, or a large statue, that was made from my megaprims. Customers will tend to assume the prim’s creator is the maker of the item. And they often click on the largest prims to see who made something. If you make one of your own prims the root, and can link the megaprims to that root in the linkset, this confusion will not happen.

The Lindens evaluated Megaprims after Havok 4 was released. Many builders came up with valid reasons for their continued existance. At first the discussion was to allow tem up to 100 M in size. But others argied a solid enough case for even the very large ones, such as using them as skydomes over isolated private island sims, that the Lindens eventually said the rule would be that they would only act ro remove megaprims on the mainland if they seemed to be causing trouble (for example a cut megaprim was overlapping a neighboring parcel, causing rezzing issues for the neighbor), and that private sim owners vould use them or not, as they choose.

While it is possible on non-SL grids to create Megaprims, those megaprims can not be imported into SL. They will snap to the smaller size limits if you try, even if you use a third-party client that allows importing and exporting prim builds.”

“A general rule is that something else is more than likely to cause more lag on a sim than a megaprim does, avatars cause the greatest amounts of lag in a sim, so before you ban megaprims, better to just ban avatars”

Left by Dekka Raymaker on October 16th, 2009

Hi Jay, so happy the podcasts are back

There’s a good discussion on megaprims currently on the NPIRL blog:

http://npirl.blogspot.com/2009/10/giant-prims-2nd-anniversary-of-big-prim.html

and a JIRA:

http://jira.secondlife.com/browse/MISC-3450

Megaprims are not a violation of TOS.

They cause server lag if they have physics collisions.

If they’re phantom they don’t cause any more lag than normal prims, and perhaps less, if one replaces many.

This is my understanding anyway. Hope it helps.

Left by Mab MacMoragh on October 16th, 2009

Great posts … I think it has been clear for a while that lag is not the issue with megas, sim/parcel encroachment is really the primary issue.

I wonder about megaprim sculpties? I seems to me (but I don’t know this so don’t quote me) that there could be considerable client load as it figures out how to translate the sculpt texture into a nice mesh, but after that, is anything else happening that might be laggy?

Left by oberon.onmura on October 16th, 2009