# [ODE] Basic newbie questions

Jon Watte hplus-ode at mindcontrol.org
Tue Apr 5 17:14:55 MST 2005

```> dMatrix3 is 4x3 - 12 reals. Is this column or row major

I have been known to transpose my memory of this matrix in
the past, but here's what I remember:

If you were to put translation into this matrix, you would
put translation in the unused values that are already there.
This means that to transform vector xyz by this matrix, the
output is:

x = x*R[0] + y*R[1] + z*R[2]
y = x*R[4] + y*R[5] + z*R[6]
z = x*R[8] + y*R[9] + z*R[10]

The reason I won't reply "column major" or "row major" is
that that, in turn, depends on whether you think of your
vectors as row vectors that you post-multiply the matrix on
to, or colum vectors that you pre-multiply the matrix on...

> dQuaternion - Is this x,y,z,w or w,x,y,z

I am 100% certain the order is w,x,y,z.

> In dMass I is about POR - What is this? Point of reference.
> Does this just mean specifiy it in body coordinates?

Yes, it's in body coordinates. The "translation" part of the
mass IS IGNORED by the body functions. To translate the center
of gravity, you translate the entire body, and then counter-
translate any Geoms that are attached (using a GeomTransform).

> Since is 4x3 which entries make the 3x3 matrix - back to first question.

Cheers,

/ h+

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