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Global illumination
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<h2 id="algorithms">Algorithms</h2> <p>Images rendered using global illumination algorithms often appear more <a href="/facts/Photorealism/HLAHwEvb">photorealistic</a> than those using only direct illumination algorithms. However, such images are computationally more expensive and consequently much slower to generate. One common approach is to compute the global illumination of a scene and store that information with the geometry (e.g., radiosity). The stored data can then be used to generate images from different viewpoints for generating walkthroughs of a scene without having to go through expensive lighting calculations repeatedly. </p><p><a href="/facts/Radiosity_(computer_graphics)/qN75qQgV">Radiosity</a>, <a href="/facts/Ray_tracing_(graphics)/n6qpap2q">ray tracing</a>, <a href="/facts/Beam_tracing/Cc4zjIQ5">beam tracing</a>, <a href="/facts/Cone_tracing/Na7nS04J">cone tracing</a>, <a href="/facts/Path_tracing/fxhaM84b">path tracing</a>, <a href="/facts/Volumetric_path_tracing/bh3v93Cj">volumetric path tracing</a>, <a href="/facts/Metropolis_light_transport/wRoPqI2f">Metropolis light transport</a>, <a href="/facts/Ambient_occlusion/oeEEyb2x">ambient occlusion</a>, <a href="/facts/Photon_mapping/t58p6V7w">photon mapping</a>, <a href="/facts/Signed_distance_function/lqgd3TAv">signed distance field</a> and <a href="/facts/Image-based_lighting/dlQ28BYA">image-based lighting</a> are all examples of algorithms used in global illumination, some of which may be used together to yield results that are not fast, but accurate. </p><p>These algorithms model <a href="/facts/Diffuse_inter-reflection/wG9w4tAX">diffuse inter-reflection</a> which is a very important part of global illumination; however most of these (excluding radiosity) also model <a href="/facts/Specular_reflection/dwVIqLVb">specular reflection</a>, which makes them more accurate algorithms to solve the lighting equation and provide a more realistically illuminated scene. The algorithms used to calculate the distribution of light energy between surfaces of a scene are closely related to <a href="/facts/Heat_transfer/1IV3WbFL">heat transfer</a> simulations performed using <a href="/facts/Finite_element_analysis/eUcfP1vn">finite-element</a> methods in engineering design. </p> <h2 id="photorealism">Photorealism</h2> <p>Achieving accurate computation of global illumination in real-time remains difficult.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> In real-time 3D graphics, the <a href="/facts/Diffuse_inter-reflection/wG9w4tAX">diffuse inter-reflection</a> component of global illumination is sometimes approximated by an "ambient" term in the lighting equation, which is also called "ambient lighting" or "ambient color" in 3D software packages. Though this method of approximation (also known as a "cheat" because it's not really a global illumination method) is easy to perform computationally, when used alone it does not provide an adequately realistic effect. Ambient lighting is known to "flatten" shadows in 3D scenes, making the overall visual effect more bland. However, used properly, ambient lighting can be an efficient way to make up for a lack of processing power. </p> <h2 id="procedure">Procedure</h2> <p>More and more specialized algorithms are used in 3D programs that can effectively simulate the global illumination. These algorithms are numerical approximations of the <a href="/facts/Rendering_equation/jQHVyQ18">rendering equation</a>. Well known algorithms for computing global illumination include <a href="/facts/Path_tracing/fxhaM84b">path tracing</a>, <a href="/facts/Photon_mapping/t58p6V7w">photon mapping</a> and <a href="/facts/Radiosity_(computer_graphics)/qN75qQgV">radiosity</a>. The following approaches can be distinguished here: </p> <ul><li>Inversion: L = ( 1 − T ) − 1 L e {\displaystyle L=(1-T)^{-1}L^{e}\,} <ul><li>is not applied in practice</li></ul></li> <li>Expansion: L = ∑ i = 0 ∞ T i L e {\displaystyle L=\sum _{i=0}^{\infty }T^{i}L^{e}} <ul><li>bi-directional approach: <a href="/facts/Photon_mapping/t58p6V7w">Photon mapping</a> + Distributed ray tracing, Bi-directional path tracing, <a href="/facts/Metropolis_light_transport/wRoPqI2f">Metropolis light transport</a></li></ul></li> <li>Iteration: L n t l e + = L ( n − 1 ) {\displaystyle L_{n}tl_{e}+=L^{(n-1)}} <ul><li><a href="/facts/Radiosity_(computer_graphics)/qN75qQgV">Radiosity</a></li></ul></li></ul> <p>In Light-path notation global lighting the paths of the type L (D | S) corresponds * E. </p><p>A full treatment can be found in <a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> <h2 id="image-based-lighting">Image-based lighting</h2> <p>Another way to simulate real global illumination is the use of <a href="/facts/High-dynamic-range_imaging/bC6NAbGS">high-dynamic-range images</a> (HDRIs), also known as environment maps, which encircle and illuminate the scene. This process is known as <a href="/facts/Image-based_lighting/dlQ28BYA">image-based lighting</a>. </p> <h2 id="list-of-methods">List of methods</h2> <table><tbody><tr><th>Method</th><th>Description/Notes</th></tr><tr><td><a href="/facts/Ray_tracing_(graphics)/n6qpap2q">Ray tracing</a></td><td>Several enhanced variants exist for solving problems related to sampling, aliasing, and soft shadows: <a href="/facts/Distributed_ray_tracing/FNv3FBSC">Distributed ray tracing</a>, <a href="/facts/Cone_tracing/Na7nS04J">cone tracing</a>, and <a href="/facts/Beam_tracing/Cc4zjIQ5">beam tracing</a>.</td></tr><tr><td><a href="/facts/Path_tracing/fxhaM84b">Path tracing</a></td><td>Unbiased, variant: Bi-directional path tracing and energy redistribution path tracing<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a></td></tr><tr><td><a href="/facts/Photon_mapping/t58p6V7w">Photon mapping</a></td><td>Consistent, biased; enhanced variants: Progressive photon mapping, stochastic progressive photon mapping (<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>)</td></tr><tr><td>Lightcuts</td><td>Enhanced variants: Multidimensional lightcuts and bidirectional lightcuts<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></td></tr><tr><td>Point based global illumination</td><td>Extensively used in movie animations<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></td></tr><tr><td><a href="/facts/Radiosity_(computer_graphics)/qN75qQgV">Radiosity</a></td><td>Finite element method, very good for precomputations. Improved versions are instant radiosity<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> and bidirectional instant radiosity<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></td></tr><tr><td><a href="/facts/Metropolis_light_transport/wRoPqI2f">Metropolis light transport</a></td><td>Builds upon bi-directional path tracing, unbiased, and multiplexed<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a></td></tr><tr><td><a href="/facts/Spherical_harmonic_lighting/wBS1B8GE">Spherical harmonic lighting</a></td><td>Encodes global illumination results for <a href="/facts/Real-time_rendering/jrILnb0K">real-time rendering</a> of static scenes</td></tr><tr><td><a href="/facts/Ambient_occlusion/oeEEyb2x">Ambient occlusion</a></td><td>-</td></tr><tr><td>Voxel-based global illumination</td><td>Several variants exist, including voxel cone tracing global illumination,<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> sparse voxel octree global illumination, and voxel global illumination (VXGI)<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a></td></tr><tr><td>Light propagation volumes global illumination<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></td><td>Light propagation volumes is a technique to approximately achieve global illumination (GI) in real-time.<p>It uses lattices and spherical harmonics (SH) to represent the spatial and angular distribution of light in the scene. Variant cascaded light propagation volumes.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a></p></td></tr><tr><td>Deferred radiance transfer global illumination<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a></td><td></td></tr><tr><td>Deep G-buffer based global illumination<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a></td><td></td></tr><tr><td>Signed Distance Fields Dynamic Diffuse Global Illumination<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a></td><td></td></tr><tr><td>Global Illumination Based on Surfels<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></td><td></td></tr></tbody></table> <h2 id="see-also">See also</h2> <ul><li>Category:Global illumination software</li> <li><a href="/facts/Bias_of_an_estimator/oxIvEgmd">Bias of an estimator</a></li> <li><a href="/facts/Bidirectional_scattering_distribution_function/z7pcyw9T">Bidirectional scattering distribution function</a></li> <li><a href="/facts/Consistent_estimator/HAKDWYEM">Consistent estimator</a></li> <li><a href="/facts/Unbiased_rendering/t6uL3u9T">Unbiased rendering</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://archive.org/details/MarcC_AoI-Global_Illumination">Video demonstrating global illumination and the ambient color effect</a></li> <li><a href="http://realtimeradiosity.com/demos">Real-time GI demos</a> – survey of practical real-time GI techniques as a list of executable demos</li> <li><a href="http://www.cs.kuleuven.be/~phil/GI/">kuleuven</a> - This page contains the Global Illumination Compendium, an effort to bring together most of the useful formulas and equations for global illumination algorithms in computer graphics.</li> <li><a href="http://scratchapixel.com/lessons/3d-basic-rendering/global-illumination-path-tracing/introduction-global-illumination-path-tracing">Theory and practical implementation of Global Illumination using Monte Carlo Path Tracing.</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Realtime Global Illumination techniques collection | extremeistan". extremeistan.wordpress.com. 11 May 2014. Retrieved 2016-05-14. <a href="https://extremeistan.wordpress.com/2014/05/11/realtime-global-illumination-techniques-collection/" target="_blank">https://extremeistan.wordpress.com/2014/05/11/realtime-global-illumination-techniques-collection/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Kurachi, Noriko (2011). The Magic of Computer Graphics. CRC Press. p. 339. ISBN 9781439873571. Retrieved 24 September 2017. <a href="9781439873571" target="_blank">9781439873571</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Dutre, Philip; Bekaert, Philippe; Bala, Kavita (2006). Advanced Global Illumination (2nd ed.). ISBN 978-1568813073. <a href="978-1568813073" target="_blank">978-1568813073</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Cline, D.; Talbot, J.; Egbert, P. (2005). "Energy redistribution path tracing". 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ACM Transactions on Graphics. 33 (4): 1–10. doi:10.1145/2601097.2601138. S2CID 79980. Archived from the original (PDF) on 2015-09-23. <a href="https://web.archive.org/web/20150923060448/http://www.ci.i.u-tokyo.ac.jp/~hachisuka/mmlt.pdf" target="_blank">https://web.archive.org/web/20150923060448/http://www.ci.i.u-tokyo.ac.jp/~hachisuka/mmlt.pdf</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Cyril Crassin. "Voxel Cone Tracing and Sparse Voxel Octree for Real-time Global Illumination" (PDF). On-demand.gputechconf.com. Archived (PDF) from the original on 2013-09-03. 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