|Prims twisted, hollowed and plain (top to bottom)|
Prims, or primitive objects, are the basic building blocks which every avatar in Second Life can create. A basic prim comes in a number of shapes from cubes, cylinders and spheres to more complex rings, and toruses. Using the build menu, basic shapes can be cut, hollowed, twisted and dimpled to make other shapes. When I first entered Second Life, everything in world was made of prims.
The physics shape of the object, that is the notional "solid" shape that the physics engine uses if your avatar walks over or sits on an object you have made with prims, is the same as the visual shape of the object.
Flexi prims are certain shapes of prim which can be set to have a flexible nature. This can be useful for moving flags, flowing fabric and hair.
Sculpties were the second type of object introduced to Second Life. They need to be made outside Second Life, although there are a couple of gadgets which can do this in world and export the resulting texture. Sculpties use a special texture to make the shape - the colour in the texture dictates the shape of the object. Sculpties are one prim each unless you link them to other prims. They can take some time to rez compared with prims and mesh, and few people create new objects as sculpties nowadays - most of those who used to create in sculpties now produce mesh instead. If you want to see if an object is a sculpty you need to edit it by right clicking, choosing edit, and then looking at the object page of the edit window. The cost of uploading a sculptie is L$10, like any other texture. They are textured with a specially mapped texture to fit with the shape of the object (which will also cost L$10 to upload).
The physics shape of a sculptie is not the same as the visual shape, it's a flattened torus with the hole filled in.
|Mesh plant under edit|
You can set the physics shape of the object as you upload it, from a detailed physics shape which reflects the visual shape of the object to a much simplified cube or similar and everything in between. How detailed you need the physics shape to be depends upon what it is going to be used for.
|Mesh being edited to highlight the mesh|
There are a lot of commercial and free and open source programs available to make meshes. The shape of the object can be a lot more detailed, and can be textured in detail too, by "unwrapping" the object. It can be hard to tell whether an object you want to buy is mesh, sculpty or prim. One the whole, very detailed objects which are also low prim count will be mesh or sculpty. It is hard to tell with a complex linked object, which can contain both mesh prims and ordinary prims, for example. On the whole, it is fairly easy to see mesh if you use your camera to "see" into the object, which looks like a lot of little triangles, as can be seen in the picture above.