[Pyode-user] Questions about inertia tensors

[Bill C. <we...@pr...>]

Hi :

I'm trying to build a rigid-body simulation using PyODE/PyOpenGL. Since 
I'm  just doing rigid-body,  it is my understanding that I have no need 
for joints; I just need to add masses to my body at the appropriate 
translations/rotations.

I have a number of cylindrical masses in my body.  These cylinders have 
fixed positions and rotations within the body, but are not necessarily 
aligned with the X, Y or Z axis.  PyODE does not bind the rotateMass() 
function from ODE; it only allows me to specify an X, Y or Z orientation 
for cylindrical mass (at creation time). This means that I am unable to 
rotate my cylindrical masses to the correct orientation.

My guess is that this would not be a problem if each of these masses had 
their own Body; I could just rotate the body after the mass was bound.  
Doing doing it that way might work, but it would mean connecting the 
cylinders to the main body via joints, and what little documentation 
there is seems to contraindicate the use of joints for rigid-body 
simulations.

My questions are as follows:

1) Am I correct in my assumption that I must rotate/translate the mass 
tensor to reflect the true location/orientation of my cylindrical masses 
within the containing body?  It seems (to me) to be intuitively obvious 
that I do, but I don't put much stock in my intuition regarding this 
subject.

2) If the answer to (1) is "yes", then can anybody point me a rotation 
implementation?  If "no", then would someone be willing to explain why not?

I have already spent a lot of time Googling about for this information, 
but I am not sufficiently grounded in the subject to really understand 
the answers I have been able to find on my own; not well enough to write 
a function to rotate inertial tensors, anyway.

Thanks in Advance,


Bill Call