Create an Entity Interaction Plugin

An entity interaction plugin is used to apply constraints and physical interactions between entities in the simulation. For example, simple collisions between entities are implemented through the entity interaction plugin interface. Also, ground collisions for flying aircraft are implemented using this interface. Given a high-fidelity motion model, the Bullet entity interaction SCRIMMAGE plugin implements mesh-based collision detection between entities. To create an entity interaction plugin, enter the following at the terminal:

$ cd /path/to/scrimmage/scripts
$ ./generate-plugin.sh interaction MyInteraction ~/scrimmage/my-scrimmage-plugins

Build the entity interaction plugin:

$ cd ~/scrimmage/my-scrimmage-plugins/build
$ cmake ..
$ make

Examine Entity Interaction Plugin

The entity interaction plugin implements the init method like all plugins. It also implements the step_entity_interaction method, which takes a list of SCRIMMAGE entities as input. Let’s take a look at the “GroundCollision” entity interaction that ships with SCRIMMAGE. The GroundCollision plugin simply determines whether an entity is below a certain z-position threshold and calls the entity’s collision method if it is below that threshold.

bool GroundCollision::step_entity_interaction(std::list<sc::EntityPtr> &ents,
                                           double t, double dt)
{
    // Account for entities colliding with
    for (sc::EntityPtr ent : ents) {
        if (ent->state()->pos()(2) < ground_collision_z_) {
            ent->collision();

            auto msg = std::make_shared<sc::Message<sm::GroundCollision>>();
            msg->data.set_entity_id(ent->id().id());
            collision_pub_->publish(msg);
        }
    }
    return true;
}

The GroundCollision plugin also makes use of SCRIMMAGE’s internal publish-and-subscribe system. When the plugin determines that a ground collision exists, it creates a GroundCollision message and publishes it using the collision_pub_ variable. The collision_pub_ member variable was initialized in the init method with the advertise method, which takes a string network name and a string topic name as arguments.

collision_pub_ = advertise("GlobalNetwork", "GroundCollision");

The GroundCollision entity interaction plugin is used in a SCRIMMAGE mission by adding the following XML tag:

<entity_interaction>GroundCollision</entity_interaction>

Multiple entity interactions can be used in the same mission file. The order in which the entity interaction plugins are executed is based on their relative positions from the top of the XML file. Plugins that are listed first are executed first. It is common to use both the GroundCollision and SimpleCollision (collision between entities) in the same simulation. This would be accomplished with the following XML lines:

<entity_interaction>SimpleCollision</entity_interaction>
<entity_interaction>GroundCollision</entity_interaction>

Since the entity interaction plugin isn’t applied to a single entity, the entity_interaction tag should exist outside of the entity block.