[ODE] How to use the rotation matrix returned by dGeomGetRotation

[Jon Watte]

I think the ODE Wiki FAQ is wrong on matrix conversion. 
You do not need to invert or transpose matrices for use 
with ODE. If you feed ODE left-handed coordinates and 
matrices and forces, it will happily hand you back the 
same; the only difference to be aware of is winding order 
in trimesh triangle list data.

The rotation matrices returned by ODE consist of three 
rows of 4 elements, where the 4th element of each row is 
un-used. This is assuming row-major ordering, and putting 
vertices as row vectors on the left of the matrix. This 
is, in turn, equivalent to putting column vertices on the 
right of a column-major matrix, so you can view it that 
way too :-)

It's been a while since I did ODE/DirectX integration (I'm 
back to good-old OpenGL), but IIRC: To get a matrix ready 
for D3D OR OpenGL (same code!), all you have to do is:

  float matrix[16];
  dReal const * R = dBodyGetRotation( myBody );
  memcpy( matrix, R, sizeof(float)*12 );
  dReal const * P = dBodyGetPosition( myBody );
  memcpy( matrix, P, sizeof(float)*3 );
  matrix[15] = 1;
  m_p3dd_d3dDevice->SetTransform(D3DTS_WORLD, (D3DMATRIX*)matrix);

This assumes you're feeding ODE left-handed data just like 
you'd feed DirectX.

Cheers,

			/ h+


-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf Of
Duncan Frostick
Sent: Sunday, October 10, 2004 7:32 AM
To: 'ODE mailing list'
Subject: [ODE] How to use the rotation matrix returned by
dGeomGetRotation


Hi all,

I'm having quite some trouble working with the rotation matrix returned 
by dGeomGetRotation. I've got some trimeshes loaded into DX and ODE and 
I'm trying to get the position and rotation of the trimeshes from ODE to 
render them in the correct locations in DX.

The matrix I'm getting back from ODE on the first (and subsequent) 
worldstep is rubbish basically, all 16 elements are 0 except 2 elements 
which are 1.875. Even if I combine this with an identity matrix to try 
and make it render something visible, there's massive distortion of the 
mesh. All I'm doing is getting the rotation matrix from ODE then 
converting it using the method outlined on the ODE wiki FAQ area then 
using the resultant matrix as the rotation matrix combined with a 
translation to the location specified by ODE. That seems to me to be the 
most obvious thing to do. The position vector returned by ODE is fine, 
just the rotation matrix doesn't seem to make any sense.

What exactly needs to be done to make the rotation matrix returned by 
dGeomGetRotation useable? Could I be missing a step in initialising ODE 
or my Geom object which is confusing ODE and making it return a garbage 
matrix? I've been having trouble with getting trimeshes loaded anyway, 
could dodgy trimesh data be causing the rubbish matrix? The test code in 
test_moving_triwmesh seems to do this to get the transforms correct:

dGeomID g2 = dGeomTransformGetGeom (g);
const dReal *pos2 = dGeomGetPosition (g2);
const dReal *R2 = dGeomGetRotation (g2);
dVector3 actual_pos;
dMatrix3 actual_R;
dMULTIPLY0_331 (actual_pos,R,pos2); //6
actual_pos[0] += pos[0]; //7
actual_pos[1] += pos[1]; //8
actual_pos[2] += pos[2]; //9
dMULTIPLY0_333 (actual_R,R,R2); //10
drawGeom (g2,actual_pos,actual_R,0);

But I have no idea why and what lines 6, 7, 8, 9 and 10 do (the 
functions in 6 and 10 don't seem to be documented) so I can't glean much 
information from the test code.

Can anyone help?

Cheers, Duncan
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