How can I detect collisions?

[omichel]

I need to programmatically detect collisions between objects while the simulation is running.
How can I achieve this?

[omichel]

Webots does automatically detect collisions and apply the contact forces whenever necessary. The collision detection mechanism is based on the shapes specified in the boundingObject fields. Now if you want to programmatically detect collision, there are several methods:

1. In controller code: you can detect collision by using TouchSensors placed around your robot body or where the collision is expected. You can use TouchSensors of type "bumper" that return a boolean status 1 or 0, whether there is a collision or not. In fact a "bumper" TouchSensor will return 1 when its bounding object intersects another bounding object and 0 otherwise.
2. In supervisor code: you can detect collisions by tracking the position of robots using the wb_supervisor_field_get_* functions. Here is a naive example assuming that the robots are cylindrical and moving in the XZ-plane.

#define ROBOT_RADIUS ...
    ...
    int are_colliding(WbFieldRef trans1, WbFieldRef trans2) {
      const double *p1 = wb_supervisor_field_get_sf_vec3f(trans1);
      const double *p2 = wb_supervisor_field_get_sf_vec3f(trans2);
      double dx = p2[0] - p1[0];
      double dz = p2[2] - p1[2];
      double dz = p2[2] - p1[2];
      return sqrt(dx * dx + dz * dz) < 2.0 * ROBOT_RADIUS;
    }
      ...
      // do this once only, in the initialization
      WbNodeRef robot1 = wb_supervisor_node_get_from_def("MY_ROBOT1");
      WbNodeRef robot2 = wb_supervisor_node_get_from_def("MY_ROBOT2");
      WbFieldRef trans1 = wb_supervisor_node_get_field(robot1, "translation");
      WbFieldRef trans2 = wb_supervisor_node_get_field(robot2, "translation");
      ...
      // detect collision
      if (are_colliding(trans1, trans2)) {
        ...
      }

3. In the physics plugin: you can replace or extend Webots collision detection mechanism. This is an advanced technique that requires knowledge of the ODE (Open Dynamics Engine) API. Your collision detection mechanism must be implemented in the webots_physics_collide function. This function is described in the Physics Plugin chapter of the Reference Manual.