Mesh has been around for long enough for many of us to understand convex hull physics and the ability it offers to lower the land impact (LI) of the items we use, both prim and mesh, if we exercise a judicious approach to object linking. 

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There's nothing new in this - I first wrote about convex hull far back in January 2012. However, after a couple of recent IM conversations in which I talked some people through converting prims to convex hull and then linking them to mesh, I thought that perhaps a little write-up on them might be useful for those looking to reduce their "used" LI in their Bellisseria Linden Home.

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Pre-mesh we had what's called the "prim accounting" system. It measured everything we made in-world on the basis of the humble prim - a single cube, sphere, cylinder, torus, ring, sold or hollow, cut or not, and / or single sculpty - had a "land impact" of 1. If you then linked a number together, their "land impact" would increase to the total linked - so link 2, get a "land impact" of 2; link 12, get a "land impact" of 12, etc.

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While this made things very easy to understand, it also ignored certain things (like how hard the viewer has to work to render something like a hollow prim or a torus or a sculpty, for example). When mesh support was introduced, the Lab added a new "accounting" system called convex hull physics. Among other things, this more correctly accounts for the rendering complexity of objects (which is one of the reasons why some mesh objects can change their LI when resized). 

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There's a lot more to this, but as a simple explanation it will suffice, and it leaves us with two key points that are relevant here:

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1.    As shown in the image above (a very simple example) when working with simple prims, you can actually "lower" their land impact by using the "mesh accounting" convex hull shape (Edit / Builder floater > Features tab > Physics Shape type), such that 2 link prims have a combine LI of 1; 4 linked prims have an LI of 2, and so on. 

 

2.    As shown in the example below, it also means that relatively simple mesh objects (which already use the convex hull shape) can often be linked to lower their overall LI. 

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Thus, providing you take a little care with things, you can use a combination of linking objects and convex hull to lower the land impact of your Linden Home additions and decor.

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However, there are some important points to note:

•   Items you're linking will need to be Modify (and preferably Copy as well, to avoid upset if something gets "broken"). 
•   It's better to use this approach to relatively simple linksets: in the case of prims, cubes and cylinders, in the case of mesh things

    that are already relatively low LI / complexity.
•   When using prims:
    o   Avoid torii, rings, and the more advanced prim forms, and those that have been extensively cut, hollowed, tapered, twisted,

         etc., or combinations thereof.
           These are actually complex objects to render, and converting to convex hull can cause their LI to dramatically rise.
           Instead, if you do link them, keep them as Prim type. It won't reduce their combined LI, but it might make it easier should

              you wish to move them around.
    o   Similarly, do not convert sculpties to convex hull. Again, sculpties are very rendering intensive, and converting them to convex

         hull is liable to see their LI increase.  
•    When linking mesh objects to prims so that a prim is the root object, make sure the prims are set to convex hull, otherwise you

      might find LI increasing unexpectedly.
    o   This can happen if the mesh in fact has multiple parts, each of which is now being treated as an individual prim as a result of

          the mesh being convert to Prim physics.
    o   Use Edit / Builder floater > Features tab > Physics Shape type to convert the linkset back to convex hull. 
•   Unless you are particularly proficient with in-world building, avoid linking complex mesh objects / mesh linksets you have

     purchased, and you may encounter complications (such as an unexpected increase), and safely unlinking things might prove a

     little difficult. 
•   As a general rule of thumb, it's often best to avoid linking:
    o   Objects with animations - these can either push up LI or you could easily mess-up sit targets, etc., on furniture designed to

         interact with avatars. 
    o   No Copy items - doing so can cause a range of problems (and if you're intending to add your items to a rezzing system as per

         my article Adding a little space to your Linden Houseboat / Home with a vehicle rezzing system, you won't want to be using

         No Copy items as you will lose them.
•   Work in small groups of things. For example:
    o   Got a series of prim walls to divide your space up? Link and convert them first.
    o   Got a set of simple 1 LI mesh photo frames on those walls? link those together and see how it affects LI, then link those to

         your walls.
    o   Got rugs on the floor? Same approach. Link them, then link to your walls.
    o   This way, if anything drastic happens, you can go back a step (just unlink the last set of items from the linkset) and re-

          evaluate. 
•   Note that when linking multiple items together, the LI might not immediately come down. However, when they are in turn added

     to another linkset, the overall LI comes down.
    o   For example, you might link "mesh A" (1 LI) with "mesh B" (2 LI), and find that they have an LI of 3. However, when you then 

         add that linkset to "linkset C", which has an LI of 19, the total for that linkset might only go up to 20 LI, not 22 LI.
    o   This is because the way the mesh accounting system "rounds out" the LI for objects to the nearest whole number.
•   Don't try linking absolutely everything together. Keep things to reasonable groupings, and keep in mind the "don'ts" noted above. 

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Most of all, experiment first, and if you've not done this kind of thing before, take the time to understand what you're doing. For example, linked a 6 LI mesh item to a prim and found the LI has risen to 14? Check the Physics Shape type - you might find that as you were linking mesh-to-a-prim, the mesh was converted to Prim physics, and its LI increased because it has multiple parts, each of which is now being treated as an individual prim.

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As mesh is much more prevalent today than when I first wrote about using convex hull for prims far back in 2012, the dividends in fiddling like this might be limited to some extent, but with a little effort and care, "savings" can be made.