[ODE] Realistic friction based on material.

[Alen Ladavac]

> Is there any table with friction constants for most common
> materials? And havint that, how I set the correct resulting value
> for surface.mu? Multiply? Sum?

Even though, as other people mentioned, to be "perfectly realistic",
you'd need a full table, you may get away with pretty good effects with
a multiply. Take coefficients for A-A friction (e.g. steel-steel) from
the realistic tables, then take the square root of those and remember
as friction coefficients for that material. On contact, multiply the
coefficients for the two materials and use that. This is not strictly
realistic, but retains some basically palusible properties, i.e. you
get the "correct" friction for all A-A combinations, and in an A-B
combination the coefficient is between A-A and B-B (which may not
always be correct in the real world, but is plausible).

You can use the same "trick" for bounciness as well.

Also note that since both the friction model used in ODE (and most
real-time simulators today) and this "trick" are just approximations,
taking "real world values" from a table is only good for orientational
purposes. It is always best that you tweak the values a bit for
desired results.

HTH,
Alen