Examples of the broken viewer land impact calculations of OS 0.9

 With Opensim 0.9, we got a change in physics engine to ubODE. This has made a number of noticeable improvements (linksets not moving around slightly when you link them anymore is so nice!), but it also broke a lot of mesh content that was made prior to the change.

Most noticeably, mesh with a hole through it (such as doorways and arches) got broken, making them impossible to move past unless you unlink them and make the offending pieces phantom, or re-upload them. 

But arguably the worst change was the total breaking of land impact values shown in the build menu. It seems that now the land impact value shown is inflated by around 3-5 times. The physically larger the mesh becomes, the wider the discrepancy. 

For example, this piece of terrain mesh in Second life is 7 PE/LI. As you can see from me standing on it, it is rather large, so 7PE with full physics is reasonable. 

But in Opensim, it now shows as an eye-watering value of 31. An over-estimate of nearly 4.5 times (7x4.5=31.5)!

As you can see, the LOD settings are exactly the same. The scale is the same. You'll have to take my word that the physics model is too as that info is not shown in the build menu.

But that is not all. Although the number shown is wildly inflated, it does not actually impact on the number of prims used on your sim. Your real land impact will still only count each physical part in a linkset or group of objects. Just as previously.

So......understandably, people are confused. This is also not great for attracting merchants to the space for obvious reasons.

I hope this can be addressed in future OS releases but from what I hear, it wont be. So.....

I think the following example best sums up how well you can trust the land impact calculations as shown in the build menu.....

Here we have a humble mesh plane, subdivided once. 1 LI. And next to it its twin brother, also 1 LI....

So....together, that would be two LI, right?

Dont be foolish! That would make too much sense. Its three, you idiot. (Bear in mind that in SL, each plane would be 0.5 PE = 1 PE total when linked)

Except when it IS two, like those other physically larger, more complex planes that you subdivided THREE times right behind the smaller ones.....

And the problem seems to get more acute the bigger the mesh is. This example shows a dune from my new "world in a box" kit. According to Opensim, its a whopping 239 LI! 

And here it is in SL, where we can see the ACTUAL impact is 20. Which I think is fine for such  huge mesh object with physics. Look how tiny I am on top! :)

So yeah. My new way of explaining Land Impact to people is to look at the data under the "Mesh information" in the "object" tab. 

1 sculpted prim = 256 tris. I think that gives most people at least a vague calculus to judge if a mesh is "worth it" or not.