Contact and Collision

Collision Group and Mask

Collision groups and masks in RaiSim use bit operations, as in most other physics engines. The example below is self-explanatory.

raisim::World world;
auto sphere0 = world.addSphere(1, 1, "default", raisim::COLLISION(0), raisim::COLLISION(0) | raisim::COLLISION(1));
auto sphere1 = world.addSphere(1, 1, "default", raisim::COLLISION(1), raisim::COLLISION(0) | raisim::COLLISION(1));
auto sphere2 = world.addSphere(1, 1, "default", raisim::COLLISION(2), raisim::COLLISION(1));
auto sphere3 = world.addSphere(1, 1, "default", 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 3 and can collide with any object (-1 means all groups).

The collision group and mask use AND logic. For A and B to collide, A must be able to collide with B and vice versa.

sphere0 can collide with sphere1. sphere1 cannot collide with sphere2 (both conditions must be satisfied). sphere3 cannot collide with any objects (because sphere0, sphere1 and sphere2 cannot collide with the collision group 3).

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

Contacts

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();

The 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 arbitrarily. Here is a more detailed example:

/// 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 acts 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;
    /// the 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;
    if (contact.getPairContactIndexInPairObject() != raisim::BodyType::STATIC) {
      /// Static objects do not store contacts, so check whether the pair object is static.
      /// This saves computation in RaiSim.
      world.getObject(contact.getPairObjectIndex())->getContacts(); /// You can use the same methods on the pair object
    }
    std::cout<<"See Contact.hpp for the full list of methods"<<std::endl;
  }
}

Collision detection details

For a detailed breakdown of collision pairs, narrowphase algorithms, and per-pair contact counts, see the Collision Detection and Colliders section.

API

You can get a vector of collisions on an object using raisim::Object::getContacts. Each element in the vector has the following API:

class Contact

Public Functions

inline const Vec<3> &getPosition() const

the contact position

Returns:: the contact position

inline const Vec<3> &getNormal() const

the contact normal vector pointing to ObjectA

Returns:: frame

inline const Mat<3, 3> &getContactFrame() const

returns a TRANSPOSE of the frame that the impulse is expressed.

Returns:: contact frame

inline size_t getIndexContactProblem() const

returns the corresponding index in raisim::World::getContactProblem

Returns:: contact index

inline size_t getIndexInObjectContactList() const

returns the corresponding index in raisim::Object::getContacts

Returns:: contact index

inline size_t getPairObjectIndex() const

returns the contacting object index in raisim::World::getObjectList

Returns:: object index

inline size_t getPairContactIndexInPairObject() const

returns the contact index in the contacting (the paired) object in raisim::Object::getContacts This does not work if the pair body is STATIC because contacts on static bodies are not stored. First check the pair body type with getPairObjectBodyType

Returns:: contact index

inline const Vec<3> *getImpulsePtr() const

returns the impulse pointer. You have to divide this number by the time step to get the force

Returns:: impulse

inline const Vec<3> &getImpulse() const

returns the impulse. You have to divide this number by the time step to get the force

Returns:: impulse

inline bool isObjectA() const

returns if the object is objectA. When there is a contact, the two paired objects are assigned to be objectA and objectB arbitrarily. The contact frame is defined such that its z-axis is pointing towards the objectA. The contact impulse is defined as an external force that objectA experiences.

Returns:: impulse

inline BodyType getPairObjectBodyType() const

returns the body type of the paired object. Please read https://raisim.com/sections/Object.html#body-types to learn about it

Returns:: the body type

inline Mat<3, 3> &getInvInertia()

returns the inverse apparent inertia of the contacting point

Returns:: inertia

inline size_t getlocalBodyIndex() const

get local body index of this contact. The local index is assigned to each moving body in an object

Returns:: the body index.

inline double getDepth() const

The penetration depth of the contact

Returns:: the depth of the contact

inline bool isSelfCollision() const

returns if the contact is self collision

Returns:: if this is a self collision

inline bool skip() const

this is set true for one self-collision point so that you don’t count them twice

Returns:: if you can skip this contact point while iterating contact points

inline CollisionBodyHandlePtr getCollisionBodyA()

get the collision body handle of this object

Returns:: collision body

inline CollisionBodyHandlePtr getCollisionBodyB()

get the collision body handle of the pairing object

Returns:: collision body