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Code Editor : scenegraph.html

<!DOCTYPE html><html lang="en"><head>
    <meta charset="utf-8">
    <title>Scene Graph</title>
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<p></p>
<p>To think of it another way, you living on the Earth do not have to think
about the Earth's rotation on its axis nor its rotation around the
Sun. You just walk or drive or swim or run as though the Earth is
not moving or rotating at all. You walk, drive, swim, run, and live
in the Earth's "local space" even though relative to the sun you are
spinning around the earth at around 1000 miles per hour and around
the sun at around 67,000 miles per hour. Your position in the solar
system is similar to that of the moon above but you don't have to concern
yourself. You just worry about your position relative to the earth in its
"local space".</p>
<p>Let's take it one step at a time. Imagine we want to make
a diagram of the sun, earth, and moon. We'll start with the sun by
just making a sphere and putting it at the origin. Note: We're using
sun, earth, moon as a demonstration of how to use a scene graph. Of course
the real sun, earth, and moon use physics but for our purposes we'll
fake it with a scene graph.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// an array of objects whose rotation to update
const objects = [];

// use just one sphere for everything
const radius = 1;
const widthSegments = 6;
const heightSegments = 6;
const sphereGeometry = new THREE.SphereGeometry(
    radius, widthSegments, heightSegments);

<p></p>
<p>Now let's add in the earth.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const earthMaterial = new THREE.MeshPhongMaterial({color: 0x2233FF, emissive: 0x112244});
const earthMesh = new THREE.Mesh(sphereGeometry, earthMaterial);
earthMesh.position.x = 10;
scene.add(earthMesh);
objects.push(earthMesh);
</pre>
<p>We make a material that is blue but we gave it a small amount of <em>emissive</em> blue
so that it will show up against our black background.</p>
<p>We use the same <code class="notranslate" translate="no">sphereGeometry</code> with our new blue <code class="notranslate" translate="no">earthMaterial</code> to make
an <code class="notranslate" translate="no">earthMesh</code>. We position that 10 units to the left of the sun
and add it to the scene.  Since we added it to our <code class="notranslate" translate="no">objects</code> array it will
rotate too.</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>You can see both the sun and the earth are rotating but the earth is not
going around the sun. Let's make the earth a child of the sun</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">-scene.add(earthMesh);
+sunMesh.add(earthMesh);
</pre>
<p>and...</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth-orbit.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth-orbit.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>What happened? Why is the earth the same size as the sun and why is it so far away?
I actually had to move the camera from 50 units above to 150 units above to see the earth.</p>
<p>We made the <code class="notranslate" translate="no">earthMesh</code> a child of the <code class="notranslate" translate="no">sunMesh</code>. The <code class="notranslate" translate="no">sunMesh</code> has
its scale set to 5x with <code class="notranslate" translate="no">sunMesh.scale.set(5, 5, 5)</code>. That means the
<code class="notranslate" translate="no">sunMesh</code>s local space is 5 times as big. Anything put in that space
 will be multiplied by 5. That means the earth is now 5x larger and
 its distance from the sun (<code class="notranslate" translate="no">earthMesh.position.x = 10</code>) is also
 5x as well.</p>
<p> Our scene graph currently looks like this</p>
<p><img src="../resources/images/scenegraph-sun-earth.svg" align="center"></p>
<p>To fix it let's add an empty scene graph node. We'll parent both the sun and the earth
to that node.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">+const solarSystem = new THREE.Object3D();
+scene.add(solarSystem);
+objects.push(solarSystem);

const sunMaterial = new THREE.MeshPhongMaterial({emissive: 0xFFFF00});
const sunMesh = new THREE.Mesh(sphereGeometry, sunMaterial);
sunMesh.scale.set(5, 5, 5);
-scene.add(sunMesh);
+solarSystem.add(sunMesh);
objects.push(sunMesh);

const earthMaterial = new THREE.MeshPhongMaterial({color: 0x2233FF, emissive: 0x112244});
const earthMesh = new THREE.Mesh(sphereGeometry, earthMaterial);
earthMesh.position.x = 10;
-sunMesh.add(earthMesh);
+solarSystem.add(earthMesh);
objects.push(earthMesh);
</pre>
<p>Here we made an <a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a>. Like a <a href="/docs/#api/en/objects/Mesh"><code class="notranslate" translate="no">Mesh</code></a> it is also a node in the scene graph
but unlike a <a href="/docs/#api/en/objects/Mesh"><code class="notranslate" translate="no">Mesh</code></a> it has no material or geometry. It just represents a local space.</p>
<p>Our new scene graph looks like this</p>
<p><img src="../resources/images/scenegraph-sun-earth-fixed.svg" align="center"></p>
<p>Both the <code class="notranslate" translate="no">sunMesh</code> and the <code class="notranslate" translate="no">earthMesh</code> are children of the <code class="notranslate" translate="no">solarSystem</code>. All 3
are being rotated and now because the <code class="notranslate" translate="no">earthMesh</code> is not a child of the <code class="notranslate" translate="no">sunMesh</code>
it is no longer scaled by 5x.</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth-orbit-fixed.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth-orbit-fixed.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>Much better. The earth is smaller than the sun and it's rotating around the sun
and rotating itself.</p>
<p>Continuing that same pattern let's add a moon.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">+const earthOrbit = new THREE.Object3D();
+earthOrbit.position.x = 10;
+solarSystem.add(earthOrbit);
+objects.push(earthOrbit);

const earthMaterial = new THREE.MeshPhongMaterial({color: 0x2233FF, emissive: 0x112244});
const earthMesh = new THREE.Mesh(sphereGeometry, earthMaterial);
-earthMesh.position.x = 10; // note that this offset is already set in its parent's THREE.Object3D object "earthOrbit"
-solarSystem.add(earthMesh);
+earthOrbit.add(earthMesh);
objects.push(earthMesh);

+const moonOrbit = new THREE.Object3D();
+moonOrbit.position.x = 2;
+earthOrbit.add(moonOrbit);

+const moonMaterial = new THREE.MeshPhongMaterial({color: 0x888888, emissive: 0x222222});
+const moonMesh = new THREE.Mesh(sphereGeometry, moonMaterial);
+moonMesh.scale.set(.5, .5, .5);
+moonOrbit.add(moonMesh);
+objects.push(moonMesh);
</pre>
<p>Again we added more invisible scene graph nodes. The first, an <a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a> called <code class="notranslate" translate="no">earthOrbit</code>
and added both the <code class="notranslate" translate="no">earthMesh</code> and the <code class="notranslate" translate="no">moonOrbit</code> to it, also new. We then added the <code class="notranslate" translate="no">moonMesh</code>
to the <code class="notranslate" translate="no">moonOrbit</code>. The new scene graph looks like this.</p>
<p><img src="../resources/images/scenegraph-sun-earth-moon.svg" align="center"></p>
<p>and here's that</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth-moon.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth-moon.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>You can see the moon follows the spirograph pattern shown at the top
of this article but we didn't have to manually compute it. We just
setup our scene graph to do it for us.</p>
<p>It is often useful to draw something to visualize the nodes in the scene graph.
Three.js has some helpful ummmm, helpers to ummm, ... help with this.</p>
<p>One is called an <a href="/docs/#api/en/helpers/AxesHelper"><code class="notranslate" translate="no">AxesHelper</code></a>. It draws 3 lines representing the local
<span style="color:red">X</span>,
<span style="color:green">Y</span>, and
<span style="color:blue">Z</span> axes. Let's add one to every node we
created.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// add an AxesHelper to each node
objects.forEach((node) =&gt; {
  const axes = new THREE.AxesHelper();
  axes.material.depthTest = false;
  axes.renderOrder = 1;
  node.add(axes);
});
</pre>
<p>On our case we want the axes to appear even though they are inside the spheres.
To do this we set their material's <code class="notranslate" translate="no">depthTest</code> to false which means they will
not check to see if they are drawing behind something else. We also
set their <code class="notranslate" translate="no">renderOrder</code> to 1 (the default is 0) so that they get drawn after
all the spheres. Otherwise a sphere might draw over them and cover them up.</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth-moon-axes.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth-moon-axes.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>We can see the
<span style="color:red">x (red)</span> and
<span style="color:blue">z (blue)</span> axes. Since we are looking
straight down and each of our objects is only rotating around its
y axis we don't see much of the <span style="color:green">y (green)</span> axes.</p>
<p>It might be hard to see some of them as there are 2 pairs of overlapping axes. Both the <code class="notranslate" translate="no">sunMesh</code>
and the <code class="notranslate" translate="no">solarSystem</code> are at the same position. Similarly the <code class="notranslate" translate="no">earthMesh</code> and
<code class="notranslate" translate="no">earthOrbit</code> are at the same position. Let's add some simple controls to allow us
to turn them on/off for each node.
While we're at it let's also add another helper called the <a href="/docs/#api/en/helpers/GridHelper"><code class="notranslate" translate="no">GridHelper</code></a>. It
makes a 2D grid on the X,Z plane. By default the grid is 10x10 units.</p>
<p>We're also going to use <a href="https://github.com/georgealways/lil-gui">lil-gui</a> which is
a UI library that is very popular with three.js projects. lil-gui takes an
object and a property name on that object and based on the type of the property
automatically makes a UI to manipulate that property.</p>
<p>We want to make both a <a href="/docs/#api/en/helpers/GridHelper"><code class="notranslate" translate="no">GridHelper</code></a> and an <a href="/docs/#api/en/helpers/AxesHelper"><code class="notranslate" translate="no">AxesHelper</code></a> for each node. We need
a label for each node so we'll get rid of the old loop and switch to calling
some function to add the helpers for each node</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">-// add an AxesHelper to each node
-objects.forEach((node) =&gt; {
-  const axes = new THREE.AxesHelper();
-  axes.material.depthTest = false;
-  axes.renderOrder = 1;
-  node.add(axes);
-});

+function makeAxisGrid(node, label, units) {
+  const helper = new AxisGridHelper(node, units);
+  gui.add(helper, 'visible').name(label);
+}
+
+makeAxisGrid(solarSystem, 'solarSystem', 25);
+makeAxisGrid(sunMesh, 'sunMesh');
+makeAxisGrid(earthOrbit, 'earthOrbit');
+makeAxisGrid(earthMesh, 'earthMesh');
+makeAxisGrid(moonOrbit, 'moonOrbit');
+makeAxisGrid(moonMesh, 'moonMesh');
</pre>
<p><code class="notranslate" translate="no">makeAxisGrid</code> makes an <code class="notranslate" translate="no">AxisGridHelper</code> which is a class we'll create
to make lil-gui happy. Like it says above lil-gui
will automagically make a UI that manipulates the named property
of some object. It will create a different UI depending on the type
of property. We want it to create a checkbox so we need to specify
a <code class="notranslate" translate="no">bool</code> property. But, we want both the axes and the grid
to appear/disappear based on a single property so we'll make a class
that has a getter and setter for a property. That way we can let lil-gui
think it's manipulating a single property but internally we can set
the visible property of both the <a href="/docs/#api/en/helpers/AxesHelper"><code class="notranslate" translate="no">AxesHelper</code></a> and <a href="/docs/#api/en/helpers/GridHelper"><code class="notranslate" translate="no">GridHelper</code></a> for a node.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// Turns both axes and grid visible on/off
// lil-gui requires a property that returns a bool
// to decide to make a checkbox so we make a setter
// and getter for `visible` which we can tell lil-gui
// to look at.
class AxisGridHelper {
  constructor(node, units = 10) {
    const axes = new THREE.AxesHelper();
    axes.material.depthTest = false;
    axes.renderOrder = 2;  // after the grid
    node.add(axes);

const grid = new THREE.GridHelper(units, units);
    grid.material.depthTest = false;
    grid.renderOrder = 1;
    node.add(grid);

this.grid = grid;
    this.axes = axes;
    this.visible = false;
  }
  get visible() {
    return this._visible;
  }
  set visible(v) {
    this._visible = v;
    this.grid.visible = v;
    this.axes.visible = v;
  }
}
</pre>
<p>One thing to notice is we set the <code class="notranslate" translate="no">renderOrder</code> of the <a href="/docs/#api/en/helpers/AxesHelper"><code class="notranslate" translate="no">AxesHelper</code></a>
to 2 and for the <a href="/docs/#api/en/helpers/GridHelper"><code class="notranslate" translate="no">GridHelper</code></a> to 1 so that the axes get drawn after the grid.
Otherwise the grid might overwrite the axes.</p>
<p></p><div translate="no" class="threejs_example_container notranslate">
  <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/scenegraph-sun-earth-moon-axes-grids.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/scenegraph-sun-earth-moon-axes-grids.html" target="_blank">click here to open in a separate window</a>
</div>

<p></p>
<p>Turn on the <code class="notranslate" translate="no">solarSystem</code> and you'll see how the earth is exactly 10
units out from the center just like we set above. You can see how the
earth is in the <em>local space</em> of the <code class="notranslate" translate="no">solarSystem</code>. Similarly if you
turn on the <code class="notranslate" translate="no">earthOrbit</code> you'll see how the moon is exactly 2 units
from the center of the <em>local space</em> of the <code class="notranslate" translate="no">earthOrbit</code>.</p>
<p>A few more examples of scene graphs. An automobile in a simple game world might have a scene graph like this</p>
<p><img src="../resources/images/scenegraph-car.svg" align="center"></p>
<p>If you move the car's body all the wheels will move with it. If you wanted the body
to bounce separate from the wheels you might parent the body and the wheels to a "frame" node
that represents the car's frame.</p>
<p>Another example is a human in a game world.</p>
<p><img src="../resources/images/scenegraph-human.svg" align="center"></p>
<p>You can see the scene graph gets pretty complex for a human. In fact
that scene graph above is simplified. For example you might extend it
to cover every finger (at least another 28 nodes) and every toe
(yet another 28 nodes) plus ones for the face and jaw, the eyes and maybe more.</p>
<p>Let's make one semi-complex scene graph. We'll make a tank. The tank will have
6 wheels and a turret. The tank will follow a path. There will be a sphere that
moves around and the tank will target the sphere.</p>
<p>Here's the scene graph. The meshes are colored in green, the <a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a>s in blue,
the lights in gold, and the cameras in purple. One camera has not been added
to the scene graph.</p>
<div class="threejs_center"><img src="../resources/images/scenegraph-tank.svg" style="width: 800px;"></div>

<p>Look in the code to see the setup of all of these nodes.</p>
<p>For the target, the thing the tank is aiming at, there is a <code class="notranslate" translate="no">targetOrbit</code>
(<a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a>) which just rotates similar to the <code class="notranslate" translate="no">earthOrbit</code> above. A
<code class="notranslate" translate="no">targetElevation</code> (<a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a>) which is a child of the <code class="notranslate" translate="no">targetOrbit</code> provides an
offset from the <code class="notranslate" translate="no">targetOrbit</code> and a base elevation. Childed to that is another
<a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a> called <code class="notranslate" translate="no">targetBob</code> which just bobs up and down relative to the
<code class="notranslate" translate="no">targetElevation</code>. Finally there's the <code class="notranslate" translate="no">targetMesh</code> which is just a cube we
rotate and change its colors</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// move target
targetOrbit.rotation.y = time * .27;
targetBob.position.y = Math.sin(time * 2) * 4;
targetMesh.rotation.x = time * 7;
targetMesh.rotation.y = time * 13;
targetMaterial.emissive.setHSL(time * 10 % 1, 1, .25);
targetMaterial.color.setHSL(time * 10 % 1, 1, .25);
</pre>
<p>For the tank there's an <a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a> called <code class="notranslate" translate="no">tank</code> which is used to move everything
below it around. The code uses a <a href="/docs/#api/en/extras/curves/SplineCurve"><code class="notranslate" translate="no">SplineCurve</code></a> which it can ask for positions
along that curve. 0.0 is the start of the curve. 1.0 is the end of the curve. It
asks for the current position where it puts the tank. It then asks for a
position slightly further down the curve and uses that to point the tank in that
direction using <a href="/docs/#api/en/core/Object3D.lookAt"><code class="notranslate" translate="no">Object3D.lookAt</code></a>.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const tankPosition = new THREE.Vector2();
const tankTarget = new THREE.Vector2();

...

// move tank
const tankTime = time * .05;
curve.getPointAt(tankTime % 1, tankPosition);
curve.getPointAt((tankTime + 0.01) % 1, tankTarget);
tank.position.set(tankPosition.x, 0, tankPosition.y);
tank.lookAt(tankTarget.x, 0, tankTarget.y);
</pre>
<p>The turret on top of the tank is moved automatically by being a child
of the tank. To point it at the target we just ask for the target's world position
and then again use <a href="/docs/#api/en/core/Object3D.lookAt"><code class="notranslate" translate="no">Object3D.lookAt</code></a></p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const targetPosition = new THREE.Vector3();

...

// face turret at target
targetMesh.getWorldPosition(targetPosition);
turretPivot.lookAt(targetPosition);
</pre>
<p>There's a <code class="notranslate" translate="no">turretCamera</code> which is a child of the <code class="notranslate" translate="no">turretMesh</code> so
it will move up and down and rotate with the turret. We make that
aim at the target.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// make the turretCamera look at target
turretCamera.lookAt(targetPosition);
</pre>
<p>There is also a <code class="notranslate" translate="no">targetCameraPivot</code> which is a child of <code class="notranslate" translate="no">targetBob</code> so it floats
around with the target. We aim that back at the tank. Its purpose is to allow the
<code class="notranslate" translate="no">targetCamera</code> to be offset from the target itself. If we instead made the camera
a child of <code class="notranslate" translate="no">targetBob</code> and just aimed the camera itself it would be inside the
target.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">// make the targetCameraPivot look at the tank
tank.getWorldPosition(targetPosition);
targetCameraPivot.lookAt(targetPosition);
</pre>
<p>Finally we rotate all the wheels</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">wheelMeshes.forEach((obj) =&gt; {
  obj.rotation.x = time * 3;
});
</pre>
<p>For the cameras we setup an array of all 4 cameras at init time with descriptions.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const cameras = [
  { cam: camera, desc: 'detached camera', },
  { cam: turretCamera, desc: 'on turret looking at target', },
  { cam: targetCamera, desc: 'near target looking at tank', },
  { cam: tankCamera, desc: 'above back of tank', },
];

const infoElem = document.querySelector('#info');
</pre>
<p>and cycle through our cameras at render time.</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const camera = cameras[time * .25 % cameras.length | 0];
infoElem.textContent = camera.desc;
</pre>
<p></p><div translate="no" class="threejs_example_container notranslate">
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  <a class="threejs_center" href="/manual/examples/scenegraph-tank.html" target="_blank">click here to open in a separate window</a>
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<p></p>
<p>I hope this gives some idea of how scene graphs work and how you might use them.
Making <a href="/docs/#api/en/core/Object3D"><code class="notranslate" translate="no">Object3D</code></a> nodes and parenting things to them is an important step to using
a 3D engine like three.js well. Often it might seem like some complex math is necessary
to make something move and rotate the way you want. For example without a scene graph
computing the motion of the moon or where to put the wheels of the car relative to its
body would be very complicated but using a scene graph it becomes much easier.</p>
<p><a href="materials.html">Next up we'll go over materials</a>.</p>

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Code Editor : scenegraph.html