[ODE] dGeom offsets without bodies?

[Geoff Carlton]

Hi,
Geoms with bodyless offsets seem a nice idea, particularly since there 
is a spare offset_posr pointer that could be used for this purpose.  
However there are a few reasons I didn't go down that road. 

If you look at the code, you see the transforms are contained in a 
dxPosR structure (a position and rotation).  Geoms have two posr 
pointers, final_posr and offset_posr.  An offset geom currently gets the 
body's posr for its base transform, but if we wanted to support offsets 
without bodies, you'd need to add an extra "base_posr" field in geom, 
with a memory cost for all users.  Either that, or provide the transform 
manually on demand, which means the geom can no longer recalculate 
itself properly without outside help.

Besides, with or without a base_posr, it wouldn't work out as nicely as 
it seems.  When bodies are moved, they signal that fact to all attached 
geoms, and those geoms then know to recalculate their position.  How 
would we signal the group of bodyless geoms that the base transform has 
moved?  If the answer is to do it externally, in addition to the geoms 
not having their own base_posr, then really the client code has to do 
most the work anyway.

Instead, a nice feature might be to extract out the bits of transform 
math to make it easy for clients to do this sort of stuff themselves.  
That way the clients can have all sorts of transform hierarchies, with 
nested transforms, etc, that get flattened down to a single transform in 
ode.  Something like:
  void dGetFinalPosition(const dxPosR& base, const dxPosR& offset, 
dxPosR& final)
  void dGetOffsetPosition(const dxPosR& base, const dxPosR& final, 
dxPosR& offset)
  void dGetBasePosition(const dxPosR& offset, const dxPosR& final, 
dxPosR& base)

The only complication is that dxPosR isn't exposed to the client, as it 
is a implementation class only.

Geoff