[ODE] Updating ODE documentation (was: hello world)

[Russ Smith]

> I don't quite understand which friction parameter in the contact 
> struct does what.  I would like to make a distinction between static 
> and dynamic friction (ie supply two different coeffs).  It seems to 
> me that in ODE, surface.mu is used for both.

correct. there are not separate coefficients for static and dynamic
friction because that would greatly complicate the friction model,
making the LCP problem harder to solve. the single mu value allows
a simply structured symmetric PD matrix to be used (faster and easier
to work with).

> Do I just change this when I detect that the
> surface is slipping, or what?

you can if you want. from the ODE user's point of view, detecting when
a surface is slipping or not must currently be done with velocity
comparisons - but the info is directly available in the constraint
solver, and could be easily exported through the APIs. if you need
this mechanism, can you add it to ODE?

> Also, what is surface.slip1,2?

'slip' is a coefficient that allows force applied in the frictional
direction to generate a resulting velocity. slip=0 is the normal
situation where the contact doesn't move. slip is useful for, e.g. 
rotating tyres, which will move perpendicular to their direction of 
rotation if a transverse force is applied. slip1/slip2 refer to the 
different friction directions.

russ.

--
Russell Smith
http://www.q12.org