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Rough formula for collision inertia matrix?


mindboggles

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I'm just wondering if anyone has a rough formula for calculating a collision bodies inertia matrix based off it's mass? For standard sized creatures I can get it pretty much right first time with values the same / similar to the bodies mass itself. This changes when I step away from "normal" sized creatures and find myself having to do a lot more play testing before I get it right.

 

The Niftools documentation says to get Blender to calculate it when exporting, which is a little difficult when you're using 3DS. I also cut and paste in Nifskope when building a skeleton's collision and find it to be the fastest method, only issue being the inertia matrix.

 

So I'm hoping someone has a rough formula (or Blenders scripted formula) and I can calculate it on the run.

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Maybe a bit premature posting this thread? Got the poos with it and with the help of a glass of wine, calculator and some old textbooks think I've worked out what the damn thing actually does represent (other than the misleading Nifskope description). It appears to be nothing other than the moment of inertia as defined in Engineering / Physics for rotational inertia of a body about an axis, and can be calculated using standard formulas.

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Inertia is the amount of force needed to move an object along any given axis. The higher the Inertia Value, the slower object will move. Inertia Matrix also depends on the shape of the object.

 

m11 = X axis
m22 = Y axis
m33 = Z axis

 

I found two Inertia/mass ratio suggestion on the net:

m11, m22 = 50 times the mass m33 = 10 times

or

m 11, 22, 33 = 3-4 times the mass. Which is quite different.

 

I'm in process of making tables with values of different sized objects, to compare them and see if I find a pattern. Here are some notes about collision havok settings:

http://www.mediafire.com/download/jit145aizdce0yd/Collision_and_Havok.pdf

Edited by Guest
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It appears to not actually be the inertia but the moment of inertia, so instead of linear resistance to motion along an axis it's the rotation resistance to turning motion around an axis.

 

This has a fairly good description of it http://www.engineeringtoolbox.com/moment-inertia-torque-d_913.html

 

So for a solid sphere m11 = m22 = m33 = 2/5 x mass x r2

 

If you do a few image searches on the web for "moment if inertia" you can find some good tables relating the shape to the right formula.

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