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lecture07_tesselationShaders.html
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</head>
<body class="article">
<div id="header">
<h1>Tesselation Shaders</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_what_are_tesselation_shaders">What are Tesselation Shaders?</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Tessellation shaders allows us to subdivide a mesh into more a more detailed/dense mesh</p>
</li>
<li>
<p>Extra [OPTIONAL] stages in the pipeline</p>
</li>
<li>
<p>Advantages include:</p>
<div class="ulist">
<ul>
<li>
<p>simple/small representation in RAM can be amplified</p>
<div class="ulist">
<ul>
<li>
<p>GPU RAM in limited and is frequently a cause of delays</p>
</li>
<li>
<p>Generally, memory on modern hardware (CPU or GPU) is a restriction</p>
</li>
<li>
<p>both in terms of speed (bottleneck)</p>
</li>
<li>
<p>also in terms of size (limited RAM)</p>
</li>
</ul>
</div>
</li>
<li>
<p>amount of detail of a mesh can be changed dynamically</p>
<div class="ulist">
<ul>
<li>
<p>for example, a sphere far away could be represented by around 12 vertices, but close up 10,000 vertices</p>
</li>
<li>
<p>always using 10,000 vertices would be <strong>expensive</strong> and <strong>inefficent</strong></p>
</li>
</ul>
</div>
</li>
<li>
<p>amount of detail can be changed semi-continuously</p>
<div class="ulist">
<ul>
<li>
<p>reduced popping</p>
</li>
</ul>
</div>
</li>
<li>
<p>small representation in persistent storage (files/network)</p>
<div class="ulist">
<ul>
<li>
<p>smaller amounts of data sent from CPU→GPUs</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_shaders_examples">Tesselation Shaders - examples</h2>
<div class="sectionbody">
<div class="imageblock stretch">
<div class="content">
<a class="image" href="https://www.panda3d.org/blog/2014/10/"><img src="assets/tessellated-displacement.png" alt="tessellated displacement"></a>
</div>
</div>
<div class="imageblock stretch">
<div class="content">
<a class="image" href="https://www.panda3d.org/blog/2014/10/"><img src="assets/screenshot-Fri-Oct-03-17-35-49-2014-1914.jpg" alt="screenshot Fri Oct 03 17 35 49 2014 1914"></a>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_popping_one_motivation">Popping (one motivation)</h2>
<div class="sectionbody">
<div class="videoblock stretch">
<div class="title"><a href="https://www.youtube.com/watch?v=KfeFcZDjCRg">LOD Popping (VonSchpam)</a></div>
<div class="content">
<iframe width="900" height="600" src="https://www.youtube.com/embed/KfeFcZDjCRg?rel=0" frameborder="0" allowfullscreen></iframe>
</div>
</div>
<div class="videoblock stretch">
<div class="title"><a href="https://www.youtube.com/watch?v=lMV-IFxQY0c">Aliens: Colonial Marines - level of detail popping (Soylent Dave)</a></div>
<div class="content">
<iframe width="900" height="600" src="https://www.youtube.com/embed/lMV-IFxQY0c?rel=0" frameborder="0" allowfullscreen></iframe>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_shaders_in_opengl">Tesselation Shaders in OpenGL</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Tesselation shaders are a relatively new feature in core OpenGL</p>
<div class="ulist">
<ul>
<li>
<p>introduced to core in OpenGL 4.x</p>
</li>
<li>
<p>prior to 4.x available as <a href="https://www.opengl.org/registry/specs/ARB/tessellation_shader.txt">ARB extensions</a> to OpenGL 3.2</p>
</li>
</ul>
</div>
</li>
<li>
<p>We’ll need to get a higher version context</p>
<div class="ulist">
<ul>
<li>
<p>or use extensions (the harder way)</p>
</li>
</ul>
</div>
</li>
<li>
<p>Our hardware will have to support this version, or the extension</p>
</li>
<li>
<p>We’ll have to move our GLSL to the new version</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_outside_of_opengl">Tesselation outside of OpenGL</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Tesseleation has a longer history than as part of OpenGL</p>
</li>
<li>
<p>We’ve always been able to write code to take a small set of vertices or triangles, and create more detailed geometry</p>
</li>
<li>
<p>For example:</p>
<div class="olist loweralpha">
<ol class="loweralpha" type="a">
<li>
<p>take a tetrahedron</p>
</li>
<li>
<p>for each face (triangle), find the mid point of the 3 vertices</p>
</li>
<li>
<p>move it away from a fixed "centre", to be a fixed distance from that centre (definition of a sphere)</p>
</li>
<li>
<p>make 3 new (replacement triangles) from each pair of original vertices and the new one</p>
</li>
<li>
<p>repeat for all the new triangles, until some limit</p>
<div class="ulist">
<ul>
<li>
<p>you can do this from a triangle, to create a circle, also</p>
</li>
</ul>
</div>
</li>
</ol>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_shaders_in_the_opengl_pipeline">Tesselation Shaders in the OpenGL Pipeline</h2>
<div class="sectionbody">
<div class="literalblock">
<div class="content">
<pre>+------------------+
| Vertex Shader |
+--------+---------+
|
+--------v----------------------+
| Tesselation Control Shader | OPTIONAL (T)
+--------+----------------------+
|
+--------v---------+
| Tesselator | fixed-function OPTIONAL (T)
+--------+---------+
|
+--------v-----------------------+
| Tesselation Evaluation Shader | OPTIONAL (T)
+--------+-----------------------+
|
+--------v---------+
| Geometry Shader | OPTIONAL (G)
+--------+---------+
|
+--------v---------+
| Clipping |
+--------+---------+
|
+--------v---------+
| Rasterisation |
+--------+---------+
|
+--------v---------+
| Fragment Shader |
+--------+---------+
|
+--------v---------+ +--------v---------+
| Blending >--| Frame Buffer |
+------------------+ +------------------+</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_geompatches">GeomPatches</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Tesselation shaders take in a new kind of primitive</p>
<div class="ulist">
<ul>
<li>
<p>Geometry Patches - <code>GL_PATCHES</code></p>
</li>
<li>
<p>rather than triangles, lines, or points</p>
</li>
<li>
<p>e.g. <code>glDrawArrays(GL_PATCHES, firstVert, vertCount);</code></p>
</li>
</ul>
</div>
</li>
<li>
<p>We also have to tell OpenGL how many vertices belong to each patch</p>
<div class="ulist">
<ul>
<li>
<p>`glPatchParameteri(GL_PATCH_VERTICES, 16); `</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_control_shader">Tesselation Control Shader</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>OPTIONAL - we haven’t had one until now</p>
</li>
<li>
<p>Executes once per incoming vertex</p>
</li>
<li>
<p>Transforms just that vertex</p>
</li>
<li>
<p><strong>Programmable</strong></p>
</li>
<li>
<p>Controls <strong>which</strong> points to subdivide</p>
</li>
<li>
<p>Controls <strong>How many</strong> times to subdivide each part of the patch</p>
</li>
<li>
<p>i.e. Controls the next (fixed-function) stage - the Tesselator</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="__code_gl_tesslevelinner_code_and_code_gl_tesslevelouter_code"><code>gl_TessLevelInner</code> and <code>gl_TessLevelOuter</code></h3>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>gl_TessLevelInner</code></dt>
<dd>
<p>the number of “nested” primitives
the</p>
</dd>
<dt class="hdlist1"><code>gl_TessLevelOuter</code></dt>
<dd>
<p>number of times to subdivide each edge</p>
<div class="ulist">
<ul>
<li>
<p>For both, a value of 1 means no subdivision</p>
<div class="ulist">
<ul>
<li>
<p>useful for testing your setup</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</dd>
</dl>
</div>
</div>
<div class="sect2">
<h3 id="__code_gl_tesslevelinner_code_and_code_gl_tesslevelouter_code_table"><code>gl_TessLevelInner</code> and <code>gl_TessLevelOuter</code> table</h3>
<div class="imageblock stretch">
<div class="content">
<a class="image" href="https://commons.wikimedia.org/wiki/File%3ATessellation_Level_Table.png"><img src="assets/Tessellation_Level_Table.png" alt="Tessellation Level Table"></a>
</div>
</div>
<div class="imageblock stretch">
<div class="content">
<a class="image" href="https://commons.wikimedia.org/wiki/File%3AIcosahedron_to_Sphere.png"><img src="assets/Icosahedron_to_Sphere.png" alt="Icosahedron to Sphere"></a>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselator">Tesselator</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Must have a Tesselation Evaluation Shader in our current (GLSL) program for this to run</p>
</li>
<li>
<p>Fixed-function</p>
<div class="ulist">
<ul>
<li>
<p>we can <strong>control</strong> it from the TCS and the TES</p>
</li>
<li>
<p>we can’t directly <strong>program</strong> it</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_tesselation_evaluation_shader">Tesselation Evaluation Shader</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>What to do with the tessellated vertices</p>
</li>
<li>
<p>Receives more vertices than we had (in RAM, or in the Tesselation Control Shader)</p>
</li>
<li>
<p>Computes the interpolated positions and other per-vertex data from them</p>
</li>
<li>
<p><strong>Programmable</strong></p>
</li>
</ul>
</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/Tessellation" class="bare">https://www.opengl.org/wiki/Tessellation</a></p>
</li>
<li>
<p><a href="https://www.opengl.org/wiki/Tessellation_Control_Shader" class="bare">https://www.opengl.org/wiki/Tessellation_Control_Shader</a></p>
</li>
<li>
<p><a href="https://www.opengl.org/wiki/Tessellation_Evaluation_Shader" class="bare">https://www.opengl.org/wiki/Tessellation_Evaluation_Shader</a></p>
</li>
<li>
<p><a href="http://ogldev.atspace.co.uk/www/tutorial30/tutorial30.html" class="bare">http://ogldev.atspace.co.uk/www/tutorial30/tutorial30.html</a></p>
</li>
<li>
<p><a href="http://prideout.net/blog/?p=48" class="bare">http://prideout.net/blog/?p=48</a></p>
</li>
<li>
<p><a href="http://web.engr.oregonstate.edu/~mjb/cs519/Handouts/tessellation.1pp.pdf" class="bare">http://web.engr.oregonstate.edu/~mjb/cs519/Handouts/tessellation.1pp.pdf</a></p>
</li>
<li>
<p><a href="http://www.lighthouse3d.com/tutorials/glsl-tutorial/tessellation/" class="bare">http://www.lighthouse3d.com/tutorials/glsl-tutorial/tessellation/</a></p>
</li>
<li>
<p><a href="http://stackoverflow.com/questions/24083656/tessellation-shader-opengl" class="bare">http://stackoverflow.com/questions/24083656/tessellation-shader-opengl</a></p>
</li>
<li>
<p><a href="https://www.panda3d.org/blog/2014/10/" class="bare">https://www.panda3d.org/blog/2014/10/</a></p>
</li>
<li>
<p><a href="http://voxels.blogspot.co.uk/2011/09/tesselation-shader-tutorial-with-source.html" class="bare">http://voxels.blogspot.co.uk/2011/09/tesselation-shader-tutorial-with-source.html</a></p>
</li>
<li>
<p><a href="http://computer-graphics.se/TSBK07-files/pdf/PDF09/10%20LOD.pdf" class="bare">http://computer-graphics.se/TSBK07-files/pdf/PDF09/10%20LOD.pdf</a></p>
</li>
<li>
<p><a href="https://computeranimations.wordpress.com/2015/03/16/rasterization-of-parametric-curves-using-tessellation-shaders-in-glsl/" class="bare">https://computeranimations.wordpress.com/2015/03/16/rasterization-of-parametric-curves-using-tessellation-shaders-in-glsl/</a></p>
</li>
</ul>
</div>
</div>
</div>
</div>
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Last updated 2016-11-11 11:57:40 GMT
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