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lecture06_shadows.html
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lecture06_shadows.html
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<!DOCTYPE html>
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<title>Shadows</title>
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</head>
<body class="article">
<div id="header">
<h1>Shadows</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_what_are_shadows">What are shadows?</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>absence of light on a surface due to occlusion from other objects</p>
<div class="ulist">
<ul>
<li>
<p>not necessarily complete absence</p>
</li>
<li>
<p>could be other light sources</p>
</li>
<li>
<p>light sources are usually unfocused - give soft edges to shadows</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_shadows_examples">Shadows - examples</h2>
<div class="sectionbody">
<div class="imageblock stretch">
<div class="content">
<a class="image" href="http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping"><img src="assets/shadow_mapping_with_without.png" alt="shadow mapping with without"></a>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_shadows_in_opengl_directx">Shadows in OpenGL/DirectX</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Both OpenGL and DirectX only resolve the generic <strong>rendering equation</strong> in a quite limited way</p>
<div class="ulist">
<ul>
<li>
<p>light only in straight lines</p>
</li>
<li>
<p>light transfer only one step from surface to camera</p>
</li>
<li>
<p>light transfer only one step from a light to surface (local lighting)</p>
<div class="ulist">
<ul>
<li>
<p>discounting any occlusion</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
<li>
<p>In other words, our normal lighting is <strong>extremely</strong> incomplete</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_local_lighting">Local lighting</h3>
<div class="ulist">
<ul>
<li>
<p>The pipeline nature of OpenGL and DirectX mean that each vertex/triangle only has information about itself</p>
<div class="ulist">
<ul>
<li>
<p>they don’t know where any other triangles are</p>
</li>
</ul>
</div>
</li>
<li>
<p>This makes things <strong>fast</strong></p>
<div class="ulist">
<ul>
<li>
<p>but <strong>limited</strong></p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_global_lighting">Global lighting</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>There are a variety of global lighting models/techniques.</p>
</li>
<li>
<p>Some you could look into include:</p>
<div class="ulist">
<ul>
<li>
<p>ray-tracing</p>
</li>
<li>
<p>path-tracing</p>
</li>
<li>
<p>radiosity</p>
</li>
<li>
<p>photon mapping</p>
</li>
<li>
<p>metropolis light transport</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_global_lighting_2">Global lighting 2</h3>
<div class="ulist">
<ul>
<li>
<p>These take into account how light is transported between (and within) surfaces</p>
<div class="ulist">
<ul>
<li>
<p>but <strong>require</strong> information about the whole scene at all stages</p>
</li>
</ul>
</div>
</li>
<li>
<p>They tend to be <strong>extremely</strong> expensive relative to local-lighting models</p>
<div class="ulist">
<ul>
<li>
<p><strong>except</strong> that for good local-lighting models we have to hack in some global-lighting effects</p>
</li>
<li>
<p>shadows are one such effect</p>
<div class="ulist">
<ul>
<li>
<p>and are important sense of depth</p>
</li>
<li>
<p>and for for visual quality</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:Radiosity_-_RRV"><img src="assets/Radiosity_-_RRV,_step_79.png" alt="Radiosity RRV, step 79" width="_step_79.png"></a>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_light_maps">Light maps</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>As global lighting is expensive we could pre-calculate some/all of the lighting/shadows effects</p>
</li>
<li>
<p>This technique is called light mapping</p>
<div class="ulist">
<ul>
<li>
<p>on offline process (usually in a 3D modelling package) "bakes" the light in to the scene</p>
</li>
<li>
<p>"bakes" = makes a fixed/static version of something (can also do with physics)</p>
</li>
<li>
<p>gives an static output that we can use in our rendering</p>
</li>
</ul>
</div>
</li>
<li>
<p>Usually the brightness of a surface is calculated offline and stored in an image to use as a texture</p>
</li>
<li>
<p>The lighting for lighting mapping can be arbitrarily good</p>
<div class="ulist">
<ul>
<li>
<p>it’s offline, so depends only on how much offline time we want to throw at it</p>
</li>
</ul>
</div>
</li>
<li>
<p>Doesn’t (easily) account for dynamic scenes - where things change move</p>
<div class="ulist">
<ul>
<li>
<p>Can combine with dynamic lighting models</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_shadow_maps">Shadow maps</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Modern method for dynamic shadows (2016)</p>
</li>
<li>
<p>Requires rendering to off-screen buffers</p>
<div class="ulist">
<ul>
<li>
<p>AKA render-to-texture</p>
</li>
<li>
<p>Frame-Buffer Objects</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_shadow_maps_outline">Shadow maps - outline</h3>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Render from each light - depth-only</p>
<div class="ulist">
<ul>
<li>
<p>store in a texture/FBO</p>
</li>
<li>
<p>everything the light can’t "see" is in shadow</p>
</li>
<li>
<p>this gives us the closest point on a ray, to the light</p>
<div class="ulist">
<ul>
<li>
<p>i.e. the nearest occluder of the light in a specific direction</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
<li>
<p>Render as usual</p>
<div class="ulist">
<ul>
<li>
<p>with extra test to determine shadow status</p>
</li>
<li>
<p>using the FBO</p>
</li>
<li>
<p>for each fragment calculate the distance to each light</p>
</li>
<li>
<p>compare with the distance stored in the shadow map for that direction</p>
<div class="ulist">
<ul>
<li>
<p>we have to sample the texture/FBO appropriately</p>
</li>
<li>
<p>we have to be able to calculate where in the texture/FBO to lookup</p>
</li>
</ul>
</div>
</li>
<li>
<p>if shadow map is nearer then this fragment is in shadow</p>
</li>
</ul>
</div>
</li>
</ol>
</div>
<div class="imageblock stretch">
<div class="content">
<a class="image" href="http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping"><img src="assets/shadow_mapping_theory_spaces.png" alt="shadow mapping theory spaces"></a>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_shadow_maps_hints">Shadow maps - hints</h3>
<div class="ulist">
<ul>
<li>
<p>The resolution of the shadow map is up to you</p>
<div class="ulist">
<ul>
<li>
<p>higher resolution gives better results</p>
</li>
<li>
<p>costs more render time</p>
</li>
</ul>
</div>
</li>
<li>
<p>Simulate the viewpoint of a directional light (one with parallel rays) with an orthographic projection matrix</p>
</li>
<li>
<p>Your shadow map rendering stages should use separate shaders</p>
<div class="ulist">
<ul>
<li>
<p>they need to do much less, so only do what you need to</p>
</li>
<li>
<p>you <strong>could</strong> use if conditions in your shaders</p>
<div class="ulist">
<ul>
<li>
<p>but generally (in other situation) <strong>DON’T</strong></p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</li>
<li>
<p>The matrix used to transform vertices for the shadow map is exactly what we need for calculating the lookup into the shadow map</p>
<div class="ulist">
<ul>
<li>
<p>when rendering the player-viewpoint, we have to pass extra information (about the lookup into the shadow map) from the vertex shader to the fragment shader</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_if_conditions_in_shaders">IF conditions in shaders</h3>
<div class="ulist">
<ul>
<li>
<p>all cores execute together</p>
</li>
<li>
<p>if any core flows one side of an IF branch</p>
<div class="ulist">
<ul>
<li>
<p>then ALL cores execute that side, just may throw away the result</p>
</li>
</ul>
</div>
</li>
<li>
<p>OK if the condition is a uniform</p>
<div class="ulist">
<ul>
<li>
<p>the condition will be constant for the entire pass, so all cores only do one branch</p>
</li>
</ul>
</div>
</li>
<li>
<p><a href="http://stackoverflow.com/questions/4176247/efficiency-of-branching-in-shaders" class="bare">http://stackoverflow.com/questions/4176247/efficiency-of-branching-in-shaders</a></p>
</li>
<li>
<p><a href="http://www.gamedev.net/topic/641960-glsl-which-is-faster-lots-of-if-statements-or-lots-of-seperate-shaders/" class="bare">http://www.gamedev.net/topic/641960-glsl-which-is-faster-lots-of-if-statements-or-lots-of-seperate-shaders/</a></p>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_shadow_map_artifacts">Shadow map - artifacts</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Shadow maps are theoretically reasonable straightforwards</p>
</li>
<li>
<p>Practically, lots of issues to get right</p>
<div class="ulist">
<ul>
<li>
<p>lots of artifacts if we aren’t careful</p>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Shadow acne</p>
</li>
</ol>
</div>
</li>
<li>
<p>depth map has limited precision and resolution</p>
</li>
<li>
<p>similar to z-fighting</p>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Peter panning</p>
</li>
</ol>
</div>
</li>
<li>
<p>solving shadow acne may detach shadows from their objects</p>
</li>
<li>
<p>objects visually appear to float on surfaces</p>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Over sampling</p>
</li>
<li>
<p>Aliased edges</p>
</li>
</ol>
</div>
</li>
<li>
<p>depth map has limited resolution</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="imageblock stretch">
<div class="content">
<a class="image" href="http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping"><img src="assets/shadow_mapping_zoom.png" alt="shadow mapping zoom"></a>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_shadow_maps_costs">Shadow maps - costs</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>Good shadow maps are pretty expensive</p>
</li>
<li>
<p>Good shadow maps are pretty complex</p>
<div class="ulist">
<ul>
<li>
<p>in theory</p>
</li>
<li>
<p>in practice</p>
</li>
</ul>
</div>
</li>
<li>
<p>Perhaps at some point, real-time light transport methods will be used instead</p>
<div class="ulist">
<ul>
<li>
<p>i.e. no more local lighting.</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_workshop_activities">Workshop Activities</h2>
<div class="sectionbody">
</div>
</div>
<div class="sect1">
<h2 id="_recommended_reading">Recommended Reading</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p><a href="http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-16-shadow-mapping/" class="bare">http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-16-shadow-mapping/</a></p>
</li>
<li>
<p><a href="http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping" class="bare">http://learnopengl.com/#!Advanced-Lighting/Shadows/Shadow-Mapping</a></p>
</li>
<li>
<p><a href="http://www.swiftless.com/tutorials/opengl/basic_shadows.html" class="bare">http://www.swiftless.com/tutorials/opengl/basic_shadows.html</a></p>
</li>
<li>
<p><a href="http://www.mbsoftworks.sk/index.php?page=tutorials&series=1&tutorial=29" class="bare">http://www.mbsoftworks.sk/index.php?page=tutorials&series=1&tutorial=29</a></p>
</li>
<li>
<p><a href="http://learnopengl.com/#!Advanced-Lighting/Shadows/Point-Shadows" class="bare">http://learnopengl.com/#!Advanced-Lighting/Shadows/Point-Shadows</a></p>
</li>
<li>
<p><a href="http://learnopengl.com/#!Advanced-Lighting/Shadows/CSM" class="bare">http://learnopengl.com/#!Advanced-Lighting/Shadows/CSM</a></p>
</li>
<li>
<p><a href="http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/" class="bare">http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/</a></p>
</li>
<li>
<p><a href="https://www.opengl.org/wiki/Framebuffer_Object" class="bare">https://www.opengl.org/wiki/Framebuffer_Object</a></p>
</li>
<li>
<p><a href="http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-15-lightmaps/" class="bare">http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-15-lightmaps/</a></p>
</li>
</ul>
</div>
</div>
</div>
</div>
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Last updated 2016-11-11 09:07:27 GMT
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