[SDFormat->MJCF] Support `//collision/surface`

[azeey]

We should be able to convert a subset of //collision/surface, such as friction and restitution properties. We should also document which elements are not supported.

[scpeters]

from mujoco documentation:

Recall that contacts can have up to 5 friction coefficients: two tangential, one torsional, two rolling. Each contact in mjData.contact actually has all 5 of them, even if condim is less than 6 and not all coefficients are used. In contrast, geoms have only 3 friction coefficients: tangential (same for both axes), torsional, rolling (same for both axes). Each of these 3D vectors of friction coefficients is expanded into a 5D vector of friction coefficients by replicating the tangetial and rolling components.

from SDFormat spec:

//collision/surface/friction parameters

• //friction/ode/mu
• //friction/ode/mu2
• //friction/ode/fdir1
• //friction/ode/fdir1/@ignition:expressed_in
• //friction/ode/slip1
• //friction/ode/slip2
• //friction/torsional/coefficient

Most of the parameters in //friction are hidden in the //ode element and assume a friction pyramid with 2 friction directions. There is a long-standing issue to move the friction parameters out of that element: gazebosim/sdformat#31.

The torsional coefficient does have a reasonable specification, so it wouldn't need to change, and we could follow its pattern for rolling friction:

• //friction/rolling/coefficient

We would eventually want to allow for anisotropic parameters for rolling friction (for example when used with a pyramid friction model), but that is not urgent for purposes of this project because MuJoCo does not support specifying anisotropic friction parameters for geometric shapes.

Another reference is the ODE API and Dartsim API.

[scpeters]

So to summarize, in the short term we could do the following:

• use existing friction parameters:
  • //friction/ode/mu
  • //friction/torsional/coefficient
• add a new parameter:
  • //friction/rolling/coefficient

[scpeters]

I've committed scpeters/sdformat@deccd47 to a branch with two competing parameters for rolling friction (pick one to use):

• //surface/friction/rolling/deformation_length: units of length
• //surface/friction/rolling/coefficient: non-dimensional, scaled by the size of the object

[scpeters]

adding python bindings for sdf::Friction and sdf::ODE in gazebosim/sdformat#1036

[scpeters]

setting sliding friction in #70