Using Convex Hull To Lower Land Impact

Due to recent advances it’s quite possible to create builds, or modify your old builds and drastically reduce your land impact (prim count) score. This doesn’t require mesh, but it does require you change the physics shape of your prims from prim to Convex Hull and be warned, there are some gotchas with this, if you’re not using cube prims, then your land impact score could actually go up, so tread carefully. There’s also the issue of scripts, but we’ll come to that later.

So we start with my work in progress build:

Work In Progress Build

This weighs in at 19 prims currently, I’ll lower that during this process. The build is 28 x 28 top floor and 26 x 26 ground floor. Already I’m able to make savings because Linden Lab now allow us to create larger prims natively in the viewer, all of this can be done inworld with the inworld tools.

In old money, this build would already have a prim count of 63 prims, so the fact that I can create it with just 19 is already a whopping saving, that 10 meter prim limit really did add to the prim counts, now that’s gone, whopping savings can be made right away, however as I said, I’ll lower the count.

So to start with I have my build at 19 prims with a physics shape of prim:

Prim Linkset

Now if I change the physics shape type in that dropdown, the linkset will be counted in the same fashion as mesh and as cubes are low cost, when they’re linked they basically half in land impact score, so for example two cube prims linked and set to convex hull will only have a land impact score of 1, rather than 2 … so I’ll demonstrate that right now:

Convex Hull linkset with script

And as you can instead of a land impact score of 19 I now have a land impact score of 19 …. wait wait wait! What’s all this about? Well the problem is the door, yes just one door script means you can’t make prim savings by changing your build to convex hull, but there is an answer, unlink the door from the linkset:

Trublesome Script

To do this you merely right click the linkset, tick the edit linked parts box, right click the door and make sure that’s what you’re clicking and then click unlink. Now everything is linked except for the door and now I do make savings on land impact:

Convex Hull

So now my total build comes in at a land impact score of 10, 9 for the main build and 1 for the door, we can’t forget about the door. That means if you want to move it, you’ll have to select the main linkset and the door, otherwise the door will be left behind! However that’s really not that troublesome and if you want to sell your builds then you can utilise a utility such as CrystalShard Foo’s excellent rez-Foo which is designed for builds that have large linksets really but I’ve used it many times when I’ve added sculpted stairs to my builds. I tested this work in progress with Rez-Foo to see if there were any issues with the new physics shapes and Rez-Foo and it worked a treat.

So from 63 prims, down to 19 thanks to Linden Lab allowing us to use larger prims, and then down to 10, pretty good savings I’d say! As I said, there are some gotchas, this won’t work in all cases and could indeed increase the land impact (prim count) score if you’re using sculpts or curved/twisted prims, you have to be aware of the script limitations and convex hull isn’t suitable for hollowed out prims that you want people to be able to walk into, however if a hollowed out prim is part of your linkset, then set just that prim to physics shape prim and leave the rest as convex hull, you can make savings like that.

This is worth a try, but do tread carefully.

17 Replies to “Using Convex Hull To Lower Land Impact”

  1. I haven’t had time to mess with this but I’m curious how that works when you add the rezfoo script to each linkset. Wouldn’t adding it make it go back up to 18 prims? Does it go back to 9 when they are removed after the build is set in place?

    1. Fair point, what happens when you add a Rez-Foo script is as you suspect, the script penalty applies, which is the same if you have one script or many in a linkset and then when the scripts are removed when you save the rez-Foo package, the land impact score drops back down again.

      Which would mean you’d have to have enough prims spare in the first place to be able to rez the build at the higher level to take advantage of the savings that would come once it was all set in place.

    1. Yeah that’s what I’d do myself, but when you add a few more interior doors etc. it will get cumbersome for people to move, although far from impossible.

  2. WOW!!!

    Is this an accidental boost, or by intent?

    This change has basically just drastically altered how many “prims” we can rez on a lot, if I read you right – if we’re conscious of how we link them all together.

    1. This is deliberate, the larger prim sizes were implemented quite a few months ago now and the new physics shapes are designed with Mesh in mind, the idea really is that you may be using more prims than you were previously allowed, but the load on the servers is lower than 1 prim per object for certain shapes, they just never counted them this way before, so the new calculations is supposedly a more accurate reflection of the load you’re placing on the servers.

      That’s the theory, if you use the new physics shapes the land impact can actually go up rather than down, it all depends on the shape, from what I’ve seen curved shapes will increase land impact, flat prims seem to lower it.

      If you leave the physics shape as prim, the old counting method still applies.

    1. That’s exactly what happens Shug, I tested it out, the script penalty applied when I first rezzed the build (so if you were looking to sell this way you’d have to point out the number of prims (land impact) required to rez and then once it’s saved and in position, the scripts get removed and the land impact score drops down.

  3. ” if a hollowed out prim is part of your linkset, then set just that prim to physics shape prim and leave the rest as convex hull, you can make savings like that.”

    I tried this with the house i built for myself. It is made completely out of box prims, although some have been hollowed to form archways. I started with 81 prims. No doors, no scripts. I left the arches as prim physics, and set all the uncut box prims to convex hull. Suddenly the build went to 156 land impact!!! There are no sculpted prims, nothing but various sizes and shapes of uncut box prims set to convex hull. I expected a prim savings of 30 or more, but the dang thing DOUBLED??? Any ideas what happened here?

    1. Hi, the problem is likely to be your arches, unfortunately, cube shaped prims are fine but curves can greatly increase the land impact, if you can unlink the arches from your linkset as a test.

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