This is an old subject people keep asking about. Drongle McMahon just answered the question, again. I consider it a rather succinct answer and wanted to add the information to my content on mesh. (Reference) I’ve added to it and changed wording in a place or two, but most of it is Drongle’s words.
First, there are three “weights” for each mesh, and the highest of the three becomes the LI (Land Impact). You can find all the details in the SL Wiki by searching for mesh and following the links there. Briefly, the “download weight” depends mostly on the “amount of geometry” and the size of the object.
The” amount of geometry” for any LOD (Level of Detail) is dominated by the vertex count you see in the uploader. Note that this may be much higher than the vertex count in your 3D program because vertices get duplicated (or more) when they lie on sharp edges, UV seams or the boundaries of materials. Keeping as many edges smooth shaded as possible will keep the weight down.
However, it’s not that simple because there are increasingly simplified LOD meshes. The effect of the geometry at each LOD on the download weight depends on the size of the object. For small objects (<<10m) the lowest LOD geometry is by far the most important.
As the object gets bigger, the influence shifts towards the higher LODs. For very large objects, only the high LOD matters. Details of this are described here.
For most mesh, the download weight should be bigger than the others and becomes the LI. This is because the physics shape mesh can nearly always be simplified until its weight is lower, without adversely affecting performance. It’s best to reduce it as far as possible anyway to minimize the workload for the physics engine on the server.
The server weight (of a linkset) is only 0.5 LI per object (1 if it’s scripted). So that will only become dominant if you have a model consisting of several meshes with simple geometry, which become a linkset after uploading. In that case, you can usually reduce the overall LI by joining the parts into fewer, more complex, meshes. The main exception would be where you need the separate parts to move relative to each other, either manually or in a scripted animation.
ETA. Note that textures (and material maps) don’t affect LI. This is a pity, because texture lag can be a real problem, but it means your strategy of testing LI before texturing is the sensible one. I never upload textures with the model anyway, for other reasons. However, you do need to do the UV mapping if you want an accurate LI, because the seams affect the download weight. You also need to check at the correct size (or sizes if it’s going to be variable).
There is nothing overly complex about what reduces Land Impact (LI) costs. But, it is very difficult to successfully predict the LI of an item you are building. I test on the Preview Grid (ADITI) to see what I can do to reduce LI.
Also, I used to try and design as low poly could object as possible for the high LoD. Now I design a high poly well detailed item. I use that high poly version to bake the normal map. I then use what would have designed for my ‘medium’ LoD for the highest LoD version. The addition of materials allows me to farther reduce my poly count.
Drongle’s comment that using materials does not drive up LI is correct for mesh items. Using materials on an object pushes it into the new accounting system for LI. Since mesh is already something that uses ONLY the new LI accounting, adding materials is basically free. Using materials on a prim does add an LI cost.
The size of items is a large factor in the LI cost calculation. For small items one works with a different strategy for reducing LI than they use for large items. This means there is a lot to learn when one builds a range of differently sized items.
I suspect many see this difference as some unfathomable inconsistency. If they are building near one of the edge conditions where an object changes from being seen as small, medium, or large.None of this is hard to understand. But, I find it hard to remember where the changes happen and at which distances LoD render changes take affect for the different sizes. While I can get the idea quickly, it is hard to implement well because it is hard for me to predict what is going to be most effective for my item. Experience is the knowledge giver in this case. So, trail and error is my primary teacher.
I think it is really hard to explain these aspects of 3D modeling to those new to it. I hope this helps.
How Many Polygons are too many?
I see this question in the forum from time to time. There is no simple all-purpose answer. But, there are some guides one can use.
First, the avatar uses about 7,000+ polygons. The part of an avatar from the waist to the knees has about 500 polys. My thinking is a pencil skirt can have way fewer than that. The size of the polygons in the avatar gives an idea of the size polys we can use for a smooth pencil skirt.
Anyone can select the parts of the avatar in Blender or any modeling program and see how many polys are used for the area your item will cover. My thinking is I should be able to get by with the same or less. But, it depends on what I am making. For a T-shirt or pencil skirt, definitely. For a top with bows, fancy Victorian collar… etc. probably not. I’ll likely be over.
Those are guides I use. Then I try to go as far below my guiding values as I can and get the result I want. I can add detail using material’s normal maps.
Buyers, in general, are not yet aware of the impact mesh clothes have on SL performance. Thus they don’t shop on that the basis of how well an item is made. I do. I get the demo