Contact and Collision

Collision Group and Mask

Collision group and mask in RaiSim uses bit operations as most other physics engines. The below example is self-explanatory

raisim::World world;
auto sphere0 = world.addSphere(1, 1, raisim::COLLISION(0), raisim::COLLISION(0) | raisim::COLLISION(1));
auto sphere1 = world.addSphere(1, 1, raisim::COLLISION(1), raisim::COLLISION(0) | raisim::COLLISION(1));
auto sphere2 = world.addSphere(1, 1, raisim::COLLISION(2), raisim::COLLISION(1));
auto sphere3 = world.addSphere(1, 1, raisim::COLLISION(3), -1);

In the above example, sphere0 is in collision group 0 and can collide with collision group 0 and 1. sphere1 is in collision group 1 and can collide with collision group 0 and 1. sphere2 is in collision group 2 and can collide with collision group 1. sphere3 is in collision group 2 and can collide with any object (-1 means all groups).

sphere0 can collide with sphere1. sphere1 cannot collide with spehre2 (both conditions must be satisfied). sphere3 cannot collide with spehre0 and sphere1 but not with sphere2.

By default, movable object are in collision group 1 and can collide with anything (collision mask = -1). All static objects (e.g., ground and height map) are in collision group 63 and can collide with anything.


raisim::Object (and thus raisim::ArticulatedSystem and raisim:SingleBodyObject) have a method getContacts which returns the list of contacts. For example,

auto& contactsOnAnymal = anymal->getContacts();

Contact class is header-only and can be found at include/raisim/contact/Contact.hpp.

Note that all vector quantities are expressed in the respective Contact Frame. A contact frame is defined such that its z-axis is colinear with the contact normal and its origin is at the contact point. Its x- and y-axes are chosen arbitrary. Here is an extensive example

/// Let's check all contact impulses acting on "LF_SHANK"
auto footIndex = anymal->getBodyIdx("LF_SHANK");

/// for all contacts on the robot, check ...
for(auto& contact: anymal->getContacts()) {
  if (contact.skip()) continue; /// if the contact is internal, one contact point is set to 'skip'
  if ( footIndex == contact.getlocalBodyIndex() ) {
    std::cout<<"Contact impulse in the contact frame: "<<contact.getImpulse()->e()<<std::endl;
    /// the impulse is acting from objectB to objectA. You can check if this object is objectA or B by
    std::cout<<"is ObjectA: "<<contact.isObjectA()<<std::endl;
    std::cout<<"Contact frame: \n"<<contact.getContactFrame().e().transpose()<<std::endl;
    /// contact frame is transposed.
    std::cout<<"Contact impulse in the world frame: "<<contact.getContactFrame().e().transpose() * contact.getImpulse()->e()<<std::endl;
    std::cout<<"Contact Normal in the world frame: "<<contact.getNormal().e().transpose()<<std::endl;
    std::cout<<"Contact position in the world frame: "<<contact.getPosition().e().transpose()<<std::endl;
    std::cout<<"It collides with: "<<world.getObject(contact.getPairObjectIndex())<<std::endl;
    std::cout<<"please check Contact.hpp for the full list of the methods"<<std::endl;