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<p><font size="6"><i><b>How to implement an Effector</b></i></font></p>
<p><font size="4">In this HowTo we will focus on creating an effector. An Effector
allows an agent to somehow modify/interact with the virtual world. Implementing
an Effector is similar to <a href="howto_perceptor.html">implementing a Perceptor</a>.
All effectors derive from a common base class, kerosin::Effector, and must implement
the Perform() function. It's signature looks like this:</font></p>
<pre><span class="key">virtual </span><span class="typ">bool </span>Perform(boost::shared_ptr<BaseNode> &base, <span class="typ">float </span>deltaTime);<span class="dir"></span>
</pre>
<p><font size="4">The first parameter of this call is the node, which the effector
should act upon and the second parameter is the amount of time, which is to
be simulated (in miliseconds). The function returns true, if the operation was
successful and false if not. That's all there really is to it. A small question
yet remains: How can one pass parameters to an Effector? This is usually done
by other external functions. Let's look at the ForceEffector (lib/kerosin/agentaspect/forceeffector.h/cpp).
It applies an accumulated force vector to a Body object in its Perform() function.
The amount of force to apply is accumulated via the AddForce()-function. This
modifies the internal state of the ForceEffector instance. Its Perform() function
looks like this:</font></p>
<pre><span class="typ">bool </span>ForceEffector::Perform(boost::shared_ptr<BaseNode> &base, <span class="typ">float </span>deltaTime)
{
<span class="key">if </span>(!base) <span class="key">return false</span>;
<span class="com">// base should be a transform, or some other node, which has a Body-child
</span> shared_ptr<Body> body = shared_static_cast<Body>(base->GetChildOfClass(<span class="str">"Body"</span>));
<span class="key">if </span>(!body) <span class="key">return false</span>;
<span class="key">if </span>(mForce.Length() > mMaxForce) mForce = mMaxForce/mForce.Length()*mForce;
dBodyAddForce(body->GetODEBody(), mForce.x(), mForce.y(), mForce.z());
mForce.Set(<span class="num">0</span>,<span class="num">0</span>,<span class="num">0</span>);
<span class="key">return true</span>;
}
<span class="typ">void </span>ForceEffector::AddForce(<span class="typ">const </span>salt::Vector3f& force)
{
mForce += force;
}
</pre>
<p><font size="4">First, it checks if base is valid. Then it retrieves the Body
object belonging to the base object and then applies the accumulated force via
an ODE function.</font></p>
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