Moment Of Inertia

i have a .mesh (no standard form like a box) but i would like ogrenewt to calculate the Moment Of Inertia,

i've tryed OgreNewt::MomentOfInertia::CalcBoxSolid(mass, size);
but that isn't realistic

second question: my object has a mass of 80kg,
should ik take mass=80.0 or is that incorrect?,
thx

If you want the mass to be 80kg, then that is how you would do it, just be consistent with your measurements everywhere.

Try this for a test, make a sphere that is 2 mm large (0.002f), and set it's mass to 2 billion (kg), it doesn't form a black hole, which with those measurements it should.

(sorry, little joke, but would be interesting if it followed physics that well. )

f? in 0.002f

'f' meaning float, as in a C float. Just a little joke showing that although newton is accurate, it is not that accurate. A black hole would form at roughly 1.8 billion units in mass if the size was 0.002 radius sphere.

use the new OgreNewt::ConvexCollision::calculateInertia (i believe that is the function name).

then take the resulting vector, and multiply by the mass of the object, and pass the result as the inertia for setMassMatrix().

I have the same question, is there a function to automatically calculate the moment of inertia given a convex hull ? i looked into OgreNewt::ConvexCollision for a function to do it, i only found OgreNewt::ConvexCollision::calculateInertialMatrix

which requires the inertia and offset. which means i need to provide an inertia value ....

guess what i am asking, is if theres a function to calculate the inertia based on teh size of the mesh (as convex hull) and its mass.

I have the same question, is there a function to automatically calculate the moment of inertia given a convex hull ? i looked into OgreNewt::ConvexCollision for a function to do it, i only found OgreNewt::ConvexCollision::calculateInertialMatrix

which requires the inertia and offset. which means i need to provide an inertia value ....

guess what i am asking, is if theres a function to calculate the inertia based on teh size of the mesh (as convex hull) and its mass.


no no, that function returns the inertia and offset. pass it 2 local variables, and it will fill them with the right values for you.

Hello ALL.
OgreNewt::ConvexCollision::calculateInertialMatrix method set the values of inertia and offset. But this code don't compile:


Vector3 inertia;
Vector3 offset;
conv->calculateInertialMatrix (&inertia,&offset );

Or this code:

Vector3 *inertia;
Vector3 *offset;
conv->calculateInertialMatrix (inertia,offset );

But this code compile without errors but vectors of inertia don't right.

Vector3 inertia;
Vector3 offset;
conv->calculateInertialMatrix (inertia,offset );

How this methon can set the right values, if arguments of this method is not pointers??? However VC 7.1 show that arguments is adresses on vectors.

I want to ask one more question. For what the vector of offset is necessary?

Sorry, thats all right, vectors changed, but inertia is not right. My app: the Ring fall on terrain(mesh), but FPS is very LOW and object have very bad physics. What I should to do for fix it? And answer for my two recent question of my up post please.


Vector3 inertia;
Vector3 offset;
conv->calculateInertialMatrix (inertia,offset );


after this, make sure you do this:


body->setMassMatrix( mass, inertia * mass );


oh, thanks, thats allright!

How to calculate inertia vector of compoind collision?

i thought it worked right away on a compound collision, but I haven't checked myself...

I'm currently trying to compute the correct moment of inertia for some arbitrary compound collision objects - currently I'm thinking I should be calculating the MoI for each convex collision in the compound and then somehow (How??) combine them.

Is this the right approach? If so, how would I combine inertial matrices?

calculateInertialMatrix is not directly applicable as it is a method on ConvexCollision which is not a parent class of CompoundCollision. (although of course I could call this on each member in the compound...)

To give a concrete example, here are three such compound collision shapes hitting each other:



(each one is composed of a randomly generated set of multicoloured hexagons)

Try this for a test, make a sphere that is 2 mm large (0.002f), and set it's mass to 2 billion (kg), it doesn't form a black hole, which with those measurements it should.

(sorry, little joke, but would be interesting if it followed physics that well. )

I have to say that in the age of Newton, human have no knowledge of black hole...
(little joke too )

when I get a chance I will review the officla way to use the calculateInertia function on compounds, and make sure OgreNewt can be friends with it... I'm sorry, but I don't have the time this week to look over it.

That would be great. I've found something that looks pretty useful on the Newton forums:

http://www.newtondynamics.com/forum/viewtopic.php?t=1199

I'm going to have a look at implementing this once i've fixed a few other issues - I'll post the results here if I get any worth posting


Vector3 inertia;
Vector3 offset;
conv->calculateInertialMatrix (inertia,offset );


after this, make sure you do this:


body->setMassMatrix( mass, inertia * mass );


ok, may sound like a silly question, but where does the offset factor into the calculations ? where do i provide the body with the offset (if its needed)

also, what type of object is conv ? and where did you get it/create it ?