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<article class="doc">
<h1 class="page">Collision and Intersection</h1>
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>The term collision can be used to refer to <a href="../../physics/collision/physics_listeners.html" class="xref page">physical interactions</a> (where <a href="../../physics/physics.html" class="xref page">physical objects</a> collide, push and bump off one another), and also to non-physical <em>intersections</em> in 3D space. This article is about the non-physical (mathematical) collisions.</p>
</div>
<div class="paragraph">
<p>Non-physical collision detection is interesting because it uses less computing resources than physical collision detection. The non-physical calculations are faster because they do not have any side effects such as pushing other objects or bumping off of them. Tasks such as <a href="../input/mouse_picking.html" class="xref page">mouse picking</a> are easily implemented using mathematical techniques such as ray casting and intersections.  Experienced developers optimize their games by finding ways to simulate certain (otherwise expensive physical) interactions in a non-physical way.</p>
</div>
<div class="paragraph">
<p><strong>Example:</strong> One example for an optimization is a physical vehicle&#8217;s wheels. You could make the wheels fully physical disks, and have jME calculate every tiny force – sounds very accurate? It&#8217;s total overkill and too slow for a racing game. A more performant solution is to cast four invisible rays down from the vehicle and calculate the intersections with the floor. These non-physical wheels require (in the simplest case) only four calculations per tick to achieve an effect that players can hardly distinguish from the real thing.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="collidable"><a class="anchor" href="#collidable"></a>Collidable</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The interface com.jme3.collision.Collidable declares one method that returns how many collisions were found between two Collidables: <code>collideWith(Collidable other, CollisionResults results)</code>.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>A <code>com.jme3.collision.CollisionResults</code> object is an ArrayList of comparable <code>com.jme3.collision.CollisionResult</code> objects.</p>
</li>
<li>
<p>You can iterate over the CollisionResults to identify the other parties involved in the collision.<br>
Note that jME counts <em>all</em> collisions, this means a ray intersecting a box will be counted as two hits, one on the front where the ray enters, and one on the back where the ray exits.</p>
</li>
</ul>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">CollisionResults Method</th>
<th class="tableblock halign-left valign-top">Usage</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>size()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the number of CollisionResult objects.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getClosestCollision()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the CollisionResult with the lowest distance.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getFarthestCollision()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the CollisionResult with the farthest distance.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getCollision(i)</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the CollisionResult at index i.</p>
</div></div></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>A CollisionResult object contains information about the second party of the collision event.</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">CollisionResult Method</th>
<th class="tableblock halign-left valign-top">Usage</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getContactPoint()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the contact point coordinate on the second party, as Vector3f.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getContactNormal()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the Normal vector at the contact point, as Vector3f.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getDistance()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the distance between the Collidable and the second party, as float.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getGeometry()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the Geometry of the second party.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getTriangle(t)</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Binds t to the triangle t on the second party&#8217;s mesh that was hit.</p>
</div></div></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>getTriangleIndex()</p>
</div></div></td>
<td class="tableblock halign-left valign-top"><div class="content"><div class="paragraph">
<p>Returns the index of the triangle on the second party&#8217;s mesh that was hit.</p>
</div></div></td>
</tr>
</tbody>
</table>
<div class="sect2">
<h3 id="code-sample"><a class="anchor" href="#code-sample"></a>Code Sample</h3>
<div class="paragraph">
<p>Assume you have two collidables a and b and want to detect collisions between them. The collision parties can be Geometries, Nodes with Geometries attached (including the rootNode), Planes, Quads, Lines, or Rays. An important restriction is that you can only collide geometry vs bounding volumes or rays. (This means for example that a must be of Type Node or Geometry and b respectively of Type BoundingBox, BoundingSphere or Ray.)</p>
</div>
<div class="paragraph">
<p>The following code snippet can be triggered by listeners (e.g. after an input action such as a click), or timed in the update loop.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">  // Calculate detection results
  CollisionResults results = new CollisionResults();
  a.collideWith(b, results);
  System.out.println("Number of Collisions between" +
      a.getName()+ " and " + b.getName() + ": " + results.size());
  // Use the results
  if (results.size() &gt; 0) {
    // how to react when a collision was detected
    CollisionResult closest  = results.getClosestCollision();
    System.out.println("What was hit? " + closest.getGeometry().getName() );
    System.out.println("Where was it hit? " + closest.getContactPoint() );
    System.out.println("Distance? " + closest.getDistance() );
  } else {
    // how to react when no collision occurred
  }
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>You can also loop over all results and trigger different reactions depending on what was hit and where it was hit. In this example, we simply print info about them.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">  // Calculate Results
  CollisionResults results = new CollisionResults();
  a.collideWith(b, results);
  System.out.println("Number of Collisions between" + a.getName()+ " and "
   + b.getName() " : " + results.size());
  // Use the results
  for (int i = 0; i &lt; results.size(); i++) {
    // For each hit, we know distance, impact point, name of geometry.
    float     dist = results.getCollision(i).getDistance();
    Vector3f    pt = results.getCollision(i).getContactPoint();
    String   party = results.getCollision(i).getGeometry().getName();
    int        tri = results.getCollision(i).getTriangleIndex();
    Vector3f  norm = results.getCollision(i).getTriangle(new Triangle()).getNormal();
    System.out.println("Details of Collision #" + i + ":");
    System.out.println("  Party " + party + " was hit at " + pt + ", " + dist + " wu away.");
    System.out.println("  The hit triangle #" + tri + " has a normal vector of " + norm);
  }</code></pre>
</div>
</div>
<div class="paragraph">
<p>Knowing the distance of the collisions is useful for example when you intersect Lines and Rays with other objects.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="bounding-volumes"><a class="anchor" href="#bounding-volumes"></a>Bounding Volumes</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A <code>com.jme3.bounding.BoundingVolume</code> is an interface for dealing with containment of a collection of points. All BoundingVolumes are <code>Collidable</code> and are used as optimization to calculate non-physical collisions more quickly: It&#8217;s always faster to calculate an intersection between simple shapes like spheres and boxes than between complex shapes like models.</p>
</div>
<div class="paragraph">
<p>jME3 computes bounding volumes for all objects. These bounding volumes are later used for frustum culling, which is making sure only objects visible on-screen are actually sent for rendering.</p>
</div>
<div class="paragraph">
<p>All fast-paced action and shooter games use BoundingVolumes as an optimization. Wrap all complex models into simpler shapes – in the end, you get equally useful collision detection results, but faster. <a href="http://en.wikipedia.org/wiki/Bounding_volume">More about bounding volumes&#8230;&#8203;</a></p>
</div>
<div class="paragraph">
<p>Supported types:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Type.AABB = Axis-aligned bounding box, that means it doesn&#8217;t rotate, which makes it less precise. A <code>com.jme3.bounding.BoundingBox</code> is an axis-aligned cuboid used as a container for a group of vertices of a piece of geometry. A BoundingBox has a center and extents from that center along the x, y and z axis. This is the default bounding volume, since it is fairly fast to generate and gives better accuracy than the bounding sphere.</p>
</li>
<li>
<p>Type.Sphere: <code>com.jme3.bounding.BoundingSphere</code> is a sphere used as a container for a group of vertices of a piece of geometry. A BoundingSphere has a center and a radius.</p>
</li>
<li>
<p>Type.OBB = Oriented bounding box. This bounding box is more precise because it can rotate with its content, but is computationally more expensive. (Currently not supported.)</p>
</li>
<li>
<p>Type.Capsule = Cylinder with rounded ends, also called &#8220;swept&#8221; sphere. Typically used for mobile characters. (Currently not supported.)</p>
</li>
</ul>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>Note: If you are looking for bounding volumes for physical objects, use <a href="../../physics/physics.html" class="xref page">CollisionShapes</a>.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="usage"><a class="anchor" href="#usage"></a>Usage</h3>
<div class="paragraph">
<p>For example you can use Bounding Volumes on custom meshes, or complex non-physical shapes.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">mesh.setBound(new BoundingSphere());
mesh.updateBound();</code></pre>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="mesh-and-scene-graph-collision"><a class="anchor" href="#mesh-and-scene-graph-collision"></a>Mesh and Scene Graph Collision</h2>
<div class="sectionbody">
<div class="paragraph">
<p>One of the supported <code>Collidable</code>s are meshes and scene graph objects. To execute a collision detection query against a scene graph, use <code>Spatial.collideWith()</code>. This will traverse the scene graph and return any mesh collisions that were detected. Note that the first collision against a particular scene graph may take a long time, this is because a special data structure called <a href="http://en.wikipedia.org/wiki/Bounding_interval_hierarchy">Bounding Interval Hierarchy (BIH)</a> needs to be generated for the meshes. At a later point, the mesh could change and the BIH tree would become out of date, in that case, call <a href="https://javadoc.jmonkeyengine.org/v3.4.1-stable/com/jme3/scene/Mesh.html#createCollisionData--">Mesh.createCollisionData()</a> on the changed mesh to update the BIH tree.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="intersection"><a class="anchor" href="#intersection"></a>Intersection</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A <code>com.jme3.math.Ray</code> is an infinite line with a beginning, a direction, and no end; whereas a <code>com.jme3.math.Line</code> is an infinite line with only a direction (no beginning, no end).</p>
</div>
<div class="paragraph">
<p>Rays are used to perform line-of-sight calculations. This means you can detect what users were &#8220;aiming&#8221; at when they clicked or pressed a key. You can also use this to detect whether game characters can see something (or someone) or not.</p>
</div>
<div class="ulist">
<ul>
<li>
<p><strong>Click to select:</strong> You can determine what a user has clicked by casting a ray from the camera forward in the direction of the camera. Now identify the closest collision of the ray with the rootNode, and you have the clicked object.</p>
</li>
<li>
<p><strong>Line of sight:</strong> Cast a ray from a player in the direction of another player. Then you detect all collisions of this ray with other entities (walls versus foliage versus window panes) and use this to calculate how likely it is that one can see the other.</p>
</li>
</ul>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
<div class="paragraph">
<p>These simple but powerful ray-surface intersection tests are called Ray Casting. As opposed to the more advanced Ray Tracing technique, Ray Casting does not follow the ray&#8217;s reflection after the first hit – the ray just goes straight on.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Learn the details of how to implement <a href="../input/mouse_picking.html" class="xref page">Mouse Picking</a> here.</p>
</div>
<hr>
<div class="paragraph">
<p>TODO:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Bounding Interval Hierarchy (<code>com.jme3.collision.bih.BIHNode</code>)</p>
</li>
<li>
<p>com.jme3.scene.CollisionData</p>
</li>
</ul>
</div>
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