[ODE] Re: Contact Constraints: First Order Slip?

[Jonathan Langridge]

As far as I can tell from experimentation, "slip" as defined in the ODE 
interface is sort of fudge factor. Instead of trying to pull the relative 
velocities of touching surfaces towards zero by friction, the system will 
only pull them towards a relative velocity of "slip". In other words, a 
sliding box will be slowed down until it reaches slip velocity, then it will 
continue to slide at that velocity forever. I guess this helps simplify 
constraint calculations.
(note: somebody scream if this explanation is rubbish!)

On the topic of contacts, I've thought of a couple of things that would help 
a fair bit in the usual car game that everyone makes with ODE. Since I find 
the source code fairly impenetrable (Jacobian matrix??) I can't yet 
implement this stuff myself, but perhaps others will agree that:

+ it would be nice to have an accurate (conical) friction model
  available, even if this caused a big performance hit. When objects
  are near the friction force limit, dependance on friction axes can
  be quite noticeable.

+ some feedback from contact joints would be good. For example,
  the normal force that has been used to correct object velocities
  (or would normal impulse be more appropriate?) in order to choose
  appropriate impact sound effect volume. Also, an indication of
  how much friction force was used (two applications: switching to
  dynamic friction, and again sound effects depending on the amount
  of "scraping")

Maybe I'll eventually understand how ODE works and be able to code this for 
my lazy self ;) In the mean time however, ODE is a godsend, and great fun 
too! Many thanks to Russ & contributors for this masterpiece.

Colas,
Jon


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