How to get mimic joint scaling and offset in C++ API?

[sea-bass]

Hello all! I was playing around with the new(ish) mimic joint functionality and after a lot of searching and trial and error, I found:

1. You have to set a boolean value to true when building the model from URDF to actually get mimic joints
2. In Python, you can do e.g.

model, _, _ = pin.buildModelsFromUrdf(
    urdf_path, package_dirs=package_paths, mimic=True,
)
model.joints[-2].extract().scaling

In C++, what would be the equivalent of this extract() function? In other words, how do I cast down a base joint to its mimic type so I can extract its offset and scaling parameters?

Thanks!

[jorisv]

Hello @sea-bass,

ModelTpl::joints store a vector of generic JointModelTpl that inherit of boost::variant that can hold all joint types.
The python extract method allow to access the specific joint type inside the boost::variant.

To do the same in C++ you have to play with the boost::variant API by:

• Using boost::apply_visitor
• Using boost::get

I will give you an untested snippet with boost::get:

auto* joint = boost::get<pinocchio::JointModelMimic>(&model.joints[model.joints().size() - 3]);
if (joint)
{
  joint->scaling;
}

[sea-bass]

Thank you @jorisv -- that worked for me! I was trying dynamic_cast to no avail, and I'd overlooked to check the Boost magic going on besides this.

I did find a bug in mimic joint parsing too, but I'll raise that one as an issue.