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<title>Skeletal Animation</title>
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</head>
<body class="article">
<div id="header">
<h1>Skeletal Animation</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_what_is_skeletal_animation">What is Skeletal Animation?</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>???</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_what_is_skeletal_animation_2">What is Skeletal Animation? 2</h3>
<div class="ulist">
<ul>
<li>
<p>Giving the visual impression of an articulated object</p>
<div class="ulist">
<ul>
<li>
<p>that the surface move under the control of "bones"</p>
</li>
</ul>
</div>
</li>
<li>
<p>AKA:</p>
<div class="ulist">
<ul>
<li>
<p>rigging / skinning</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_skeletal_animation_example">Skeletal Animation - example</h2>
<div class="sectionbody">
<div class="videoblock stretch">
<div class="content">
<iframe src="https://www.youtube.com/embed/oLRpnS2HvTs?rel=0" frameborder="0" allowfullscreen></iframe>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_compared_to_moving_whole_entities_around">Compared to moving whole entities around</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>We’ve already been moving whole entities around</p>
<div class="ulist">
<ul>
<li>
<p>in our vertex shaders</p>
</li>
<li>
<p>applying the same transformation to <strong>all</strong> vertices in an mesh/entity</p>
</li>
</ul>
</div>
</li>
<li>
<p>We still need to be able to do this</p>
<div class="ulist">
<ul>
<li>
<p>to locate and oriented our entities in the scen</p>
</li>
</ul>
</div>
</li>
<li>
<p>For skeletal animation, we want to <strong>also</strong>, and <strong>first</strong> transform individual (or collections of) vertices with the mesh differently</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_bones">Bones</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>With skeletal animation, we transform (collections of) vertices associated with a "bone", or "bones"</p>
</li>
<li>
<p>Usually bones are arranged in a hierarchy, but they don’t <strong>have</strong> to be</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_what_information_to_we_need">What information to we need?</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>???</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_what_information_to_we_need_2">What information to we need?</h3>
<div class="ulist">
<ul>
<li>
<p>Mesh</p>
</li>
<li>
<p>A hierarchy of bones</p>
</li>
<li>
<p>A set of weightings between bones and vertices</p>
</li>
<li>
<p>Information about how to move bones over time</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_mesh">Mesh</h3>
<div class="ulist">
<ul>
<li>
<p>collection of vertices</p>
</li>
<li>
<p>collection of faces (triangles) connecting those vertices</p>
</li>
<li>
<p>in a default / static pose (bind-pose)</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_a_hierarchy_of_bones">A hierarchy of bones</h3>
<div class="ulist">
<ul>
<li>
<p>Bones are usually in a hierarchy</p>
<div class="ulist">
<ul>
<li>
<p>just like a skeleton</p>
</li>
<li>
<p>there isn’t a consistent/standard <strong>root</strong> of the hierarchy, some options include:</p>
<div class="ulist">
<ul>
<li>
<p>back (if no articulated spine)</p>
</li>
<li>
<p>some part of the spine</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="imageblock stretch">
<div class="content">
<a class="image" href=""https://commons.wikimedia.org/wiki/File:Gray_111_-_Vertebral_column-coloured.png"><img src="assets/174px-Gray_111_-_Vertebral_column-coloured.png" alt="Vertebral column"></a>
</div>
<div class="title">Figure 1. Vertebral column</div>
</div>
</div>
<div class="sect2">
<h3 id="_a_hierarchy_of_bones_2">A hierarchy of Bones 2</h3>
<div class="ulist">
<ul>
<li>
<p>each bone has a <strong>translation</strong> and <strong>rotation</strong> relative to its parent</p>
<div class="ulist">
<ul>
<li>
<p>bones only rotate</p>
</li>
<li>
<p>but rotate around a fixed point, represented by the offset</p>
</li>
</ul>
</div>
</li>
<li>
<p>usually combined into a <code>mat4</code></p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_a_set_of_weightings_between_bones_and_vertices">A set of weightings between bones and vertices</h3>
<div class="ulist">
<ul>
<li>
<p>each vertex <strong>can</strong> be influenced by more than one bone</p>
</li>
<li>
<p>which bones influence which vertex</p>
<div class="ulist">
<ul>
<li>
<p>perhaps all bones</p>
</li>
</ul>
</div>
</li>
<li>
<p>how much each vertex is influenced by each bones transformation</p>
<div class="ulist">
<ul>
<li>
<p>AKA blend factor</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_information_about_how_to_move_bones_over_time">Information about how to move bones over time</h3>
<div class="ulist">
<ul>
<li>
<p>Often a sequence to move the bones through</p>
<div class="ulist">
<ul>
<li>
<p>from Motion Capture, or an animator</p>
</li>
<li>
<p>usually loaded from a model or animation file</p>
</li>
<li>
<p>could be calculated dynamically, or hard-coded for testing/development/debugging</p>
</li>
</ul>
</div>
</li>
<li>
<p>to cause the animation</p>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_our_job_as_graphics_programmers">Our job as Graphics programmers</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Our job is to take existing animations and display them</p>
<div class="ulist">
<ul>
<li>
<p>in general, we would expect the animations, mesh, etc to be created by other parties</p>
</li>
<li>
<p>during testing, developing, debug, we may use <strong>programmer-generated</strong> animations</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_calculating_the_skeletal_transformation">Calculating the skeletal transformation</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>The final position of each vertex (before transformation of the whole entity) is calculated from by summing the results of:</p>
<div class="ulist">
<ul>
<li>
<p>each bone-transformation multiplied by the weighting for that bone for the present vertex</p>
</li>
</ul>
</div>
</li>
<li>
<p>We can (<strong>should</strong>/<strong>must</strong>) do this on the GPU</p>
<div class="ulist">
<ul>
<li>
<p>in the vertex shader</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_calculating_the_bone_transformations">Calculating the bone-transformations</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>For each bone we’re interested in applying its rotation to vertices</p>
<div class="ulist">
<ul>
<li>
<p>to create transformed vertices</p>
</li>
<li>
<p>but the rotation needs to take place around the <strong>towards-root</strong> end of the bone</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="imageblock">
<div class="content">
<a class="image" href="http://what-when-how.com/advanced-methods-in-computer-graphics/skeletal-animation-advanced-methods-in-computer-graphics-part-2/"><img src="assets/boneSpace.png" alt="bone space"></a>
</div>
<div class="title">Figure 2. bone space</div>
</div>
<div class="sect2">
<h3 id="_calculating_the_bone_transformations_2">Calculating the bone-transformations 2</h3>
<div class="ulist">
<ul>
<li>
<p>Recall that to rotate around a point we first have to translate to that point, then apply rotation, then undo the translation</p>
<div class="ulist">
<ul>
<li>
<p>the translation we need to do is exactly the translation of each bone</p>
</li>
</ul>
</div>
</li>
<li>
<p>Depending on our model (loaded from a file), the transformations stored in the file may already be appropriate to use directly</p>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_workshop_activities">Workshop Activities</h2>
<div class="sectionbody">
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Create (hard-coded) a model made up of multiple sections</p>
<div class="ulist">
<ul>
<li>
<p>start with just 2 sections</p>
</li>
<li>
<p>such as a snake</p>
</li>
<li>
<p>could even have a rectangular cross-section (i.e. a square snake)
<span class="image"><a class="image" href="https://www.blend4web.com/en/forums/topic/271/"><img src="assets/blenderSnake.jpg" alt="blenderSnake" title="blender snake"></a></span></p>
</li>
</ul>
</div>
</li>
<li>
<p>Make a program that renders this model</p>
</li>
<li>
<p>Imagine that you have a single bone, that influences as follows:</p>
<div class="ulist">
<ul>
<li>
<p>fully (100%) controls one end of your snake (head)</p>
</li>
<li>
<p>partially (50%) controls the centre of your snake</p>
</li>
<li>
<p>doesn’t influence the other end of your snake (tail)</p>
</li>
</ul>
</div>
</li>
<li>
<p>Simulate the rotation of this bone so it changes over time</p>
<div class="ulist">
<ul>
<li>
<p>like a pendulum</p>
</li>
</ul>
</div>
</li>
<li>
<p>Use the bone rotation and translation to make your snake head move</p>
</li>
<li>
<p>Use the bone rotation and translation to make your whole snake move appropriately</p>
<div class="ulist">
<ul>
<li>
<p>the tail shouldn’t move</p>
</li>
<li>
<p>the middle should move less than the head</p>
</li>
</ul>
</div>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_recommended_reading">Recommended Reading</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="https://www.opengl.org/wiki/Skeletal_Animation" class="bare">https://www.opengl.org/wiki/Skeletal_Animation</a></p>
</li>
<li>
<p><a href="http://ogldev.atspace.co.uk/www/tutorial38/tutorial38.html" class="bare">http://ogldev.atspace.co.uk/www/tutorial38/tutorial38.html</a></p>
</li>
<li>
<p><a href="http://ephenationopengl.blogspot.co.uk/2012/06/doing-animations-in-opengl.html" class="bare">http://ephenationopengl.blogspot.co.uk/2012/06/doing-animations-in-opengl.html</a></p>
</li>
<li>
<p><a href="https://research.ncl.ac.uk/game/mastersdegree/graphicsforgames/skeletalanimation/Tutorial%209%20-%20Skeletal%20Animation.pdf" class="bare">https://research.ncl.ac.uk/game/mastersdegree/graphicsforgames/skeletalanimation/Tutorial%209%20-%20Skeletal%20Animation.pdf</a></p>
</li>
<li>
<p><a href="http://what-when-how.com/advanced-methods-in-computer-graphics/skeletal-animation-advanced-methods-in-computer-graphics-part-1/" class="bare">http://what-when-how.com/advanced-methods-in-computer-graphics/skeletal-animation-advanced-methods-in-computer-graphics-part-1/</a></p>
</li>
<li>
<p><a href="http://what-when-how.com/advanced-methods-in-computer-graphics/skeletal-animation-advanced-methods-in-computer-graphics-part-2/" class="bare">http://what-when-how.com/advanced-methods-in-computer-graphics/skeletal-animation-advanced-methods-in-computer-graphics-part-2/</a></p>
</li>
</ul>
</div>
</div>
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Last updated 2016-10-28 13:01:54 BST
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