Editing Global illumination

{{redirect|Realistic rendering|physically-based realistic rendering|Physically-based rendering}}
{{short description|Group of rendering algorithms used in 3D computer graphics}}
{{More citations needed|date=May 2013}}
{{3D computer graphics}}
{{Multiple image|direction=vertical|align=right|image1=Direct lighting.png|image2=Global illumination1.png|width=200|caption1=Rendering without global illumination. Areas that lie outside of the ceiling lamp's direct light lack definition. For example, the lamp's housing appears completely uniform. Without the ambient light added into the render, it would appear uniformly black.|caption2=Rendering with global illumination. Light is reflected by surfaces, and colored light transfers from one surface to another. Notice how color from the red wall and green wall (not visible) reflects onto other surfaces in the scene. Also notable is the [[Caustic (optics)|caustic]] projected onto the red wall from light passing through the glass sphere.}}

'''Global illumination'''<ref name="wordpress">{{cite web|url=https://extremeistan.wordpress.com/2014/05/11/realtime-global-illumination-techniques-collection/|title=Realtime Global Illumination techniques collection &#124; extremeistan|date=11 May 2014 |publisher=extremeistan.wordpress.com|access-date=2016-05-14}}</ref> ('''GI'''), or '''indirect illumination''', is a group of [[algorithm]]s used in [[3D computer graphics]] that are meant to add more realistic [[computer graphics lighting|lighting]] to 3D scenes. Such algorithms take into account not only the light that comes directly from a light source (''direct illumination''), but also subsequent cases in which light rays from the same source are reflected by other surfaces in the scene, whether reflective or not (''indirect illumination'').

Theoretically, [[reflection (computer graphics)|reflections]], refractions, and shadows are all examples of global illumination, because when simulating them, one object affects the rendering of another (as opposed to an object being affected only by a direct source of light). In practice, however, only the simulation of [[diffuse reflection#Interreflection|diffuse inter-reflection]] or [[caustic (optics)|caustics]] is called global illumination.

==Algorithms==
Images rendered using global illumination algorithms often appear more [[Photorealism|photorealistic]] 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.

[[Radiosity (computer graphics)|Radiosity]], [[Ray tracing (graphics)|ray tracing]], [[beam tracing]], [[cone tracing]], [[path tracing]], [[volumetric path tracing]], [[Metropolis light transport]], [[ambient occlusion]], [[photon mapping]], [[Signed_distance_function|signed distance field]] and [[image-based lighting]] 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.

These algorithms model [[diffuse inter-reflection]] which is a very important part of global illumination; however most of these (excluding radiosity) also model [[specular reflection]], 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 [[heat transfer]] simulations performed using [[Finite element analysis|finite-element]] methods in engineering design.

==Photorealism==
[[File:Alexexterior2.jpg|thumb|left|Exterior view of an architectural model]]
[[File:Show how 3D real time ambient occlusion works 2013-11-23 10-45.jpeg|thumb|upright|Example of an ambient occlusion layer]]
Achieving accurate computation of global illumination in real-time remains difficult.<ref name="tmocg">{{cite book |title=The Magic of Computer Graphics |last=Kurachi |first=Noriko |year=2011 |publisher=CRC Press |isbn=9781439873571 |page=339 |url=https://play.google.com/store/books/details?id=YjLOBQAAQBAJ |access-date=24 September 2017}}</ref>  In real-time 3D graphics, the [[diffuse inter-reflection]] 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.

==Procedure==
More and more specialized algorithms are used in 3D programs that can effectively simulate the global illumination. These algorithms are numerical approximations of the [[rendering equation]]. Well known algorithms for computing global illumination include [[path tracing]], [[photon mapping]] and [[Radiosity (computer graphics)|radiosity]].
The following approaches can be distinguished here:
* Inversion: <math>L = (1-T)^{-1} L^e\,</math>
** is not applied in practice
* Expansion: <math>L = \sum_{i=0}^\infty T^iL^e</math>
** bi-directional approach: [[Photon mapping]] + Distributed ray tracing, Bi-directional path tracing, [[Metropolis light transport]]
* Iteration: <math>L_n tl_ e + = L ^{(n-1)}</math>
** [[Radiosity (computer graphics)|Radiosity]]
In Light-path notation global lighting the paths of the type L (D | S) corresponds * E.

A full treatment can be found in <ref name="WikiMarkup">{{cite book |title=Advanced Global Illumination |edition=2nd |first1=Philip |last1=Dutre |first2=Philippe |last2=Bekaert |first3=Kavita |last3=Bala |isbn=978-1568813073 |date=2006 }}</ref>

==Image-based lighting==
Another way to simulate real global illumination is the use of [[high-dynamic-range imaging|high-dynamic-range images]] (HDRIs), also known as environment maps, which encircle and illuminate the scene. This process is known as [[image-based lighting]].

==List of methods==
{| class="wikitable"
|-
! Method !! Description/Notes
|-
| [[Ray tracing (graphics)|Ray tracing]] || Several enhanced variants exist for solving problems related to sampling, aliasing, and soft shadows: [[Distributed ray tracing]], [[cone tracing]], and [[beam tracing]].
|-
| [[Path tracing]] || Unbiased, variant: Bi-directional path tracing and energy redistribution path tracing<ref name="psu">{{cite journal |last1=Cline |first1=D. |last2=Talbot |first2=J. |last3=Egbert |first3=P. |title=Energy redistribution path tracing |journal=ACM Transactions on Graphics  |volume=24 |issue=3 |pages=1186–95 |date=2005 |doi=10.1145/1073204.1073330 }}</ref> 
|-
| [[Photon mapping]] || Consistent, biased; enhanced variants: Progressive photon mapping, stochastic progressive photon mapping (<ref name="u-tokyo">{{cite web|url=http://www.ci.i.u-tokyo.ac.jp/~hachisuka/|title=Toshiya Hachisuka at UTokyo|publisher=ci.i.u-tokyo.ac.jp|access-date=2016-05-14}}</ref>)
|-
| [[Lightcuts]] || Enhanced variants: Multidimensional lightcuts and bidirectional lightcuts<ref name="Walter2005">{{cite journal |last1=Walter |first1=Bruce |last2=Fernandez |first2=Sebastian |last3=Arbree |first3=Adam |last4=Bala |first4=Kavita |last5=Donikian |first5=Michael |last6=Greenberg |first6=Donald P. |title=Lightcuts |journal=ACM Transactions on Graphics |date=1 July 2005 |volume=24 |issue=3 |pages=1098–1107 |doi=10.1145/1073204.1073318}}</ref>
|-
| [[Point based global illumination]] || Extensively used in movie animations<ref>{{cite web|url=http://graphics.pixar.com/library/PointBasedGlobalIlluminationForMovieProduction/paper.pdf |archive-url=https://web.archive.org/web/20110817145747/http://graphics.pixar.com/library/PointBasedGlobalIlluminationForMovieProduction/paper.pdf |archive-date=2011-08-17 |url-status=live |title=coursenote.dvi |website=Graphics.pixar.com |access-date=2016-12-02}}</ref><ref>{{cite web |first=Karsten |last=Daemen |title=Point Based Global Illumination An introduction [Christensen, 2010] |date=November 14, 2012 |publisher=KU Leuven |url=http://www.karstendaemen.com/thesis/files/intro_pbgi.pdf|archive-url=https://web.archive.org/web/20141222074728/http://www.karstendaemen.com/thesis/files/intro_pbgi.pdf |archive-date=2014-12-22 }}</ref>
|-
| [[Radiosity (computer graphics)|Radiosity]] || Finite element method, very good for precomputations. Improved versions are instant radiosity<ref>{{cite web|url=http://www.cs.cornell.edu/courses/cs6630/2012sp/slides/Boyadzhiev-Matzen-InstantRadiosity.pdf |archive-url=https://web.archive.org/web/20120618074123/http://www.cs.cornell.edu/Courses/cs6630/2012sp/slides/Boyadzhiev-Matzen-InstantRadiosity.pdf |archive-date=2012-06-18 |url-status=live |title=Instant Radiosity: Keller (SIGGRAPH 1997) |website=Cs.cornell.edu |access-date=2016-12-02}}</ref> and bidirectional instant radiosity<ref>{{cite book |last1=Segovia |first1=B. |last2=Iehl |first2=J.C. |last3=Mitanchey |first3=R. |last4=Péroche |first4=B. |chapter=Bidirectional instant radiosity |title=Rendering Techniques |publisher=Eurographics Association |date=2006 |isbn= |pages=389–397 |chapter-url=http://artis.imag.fr/Projets/Cyber-II/Publications/SIMP06a.pdf |archive-url=https://web.archive.org/web/20160130212610/http://artis.imag.fr/Projets/Cyber-II/Publications/SIMP06a.pdf |archive-date=2016-01-30  }}</ref>
|-
| [[Metropolis light transport]] || Builds upon bi-directional path tracing, unbiased, and multiplexed<ref>{{cite journal |last1=Hachisuka |first1=T. |last2=Kaplanyan |first2=A.S. |last3=Dachsbacher |first3=C. |title=Multiplexed metropolis light transport |journal=ACM Transactions on Graphics  |volume=33 |issue=4 |pages=1–10 |date=2014 |doi=10.1145/2601097.2601138 |s2cid=79980 |url=http://www.ci.i.u-tokyo.ac.jp/~hachisuka/mmlt.pdf |archive-url=https://web.archive.org/web/20150923060448/http://www.ci.i.u-tokyo.ac.jp/~hachisuka/mmlt.pdf |archive-date=2015-09-23 }}</ref>
|-
| [[Spherical harmonic lighting]] || Encodes global illumination results for [[real-time rendering]] of static scenes
|-
| [[Ambient occlusion]] || -
|-
| Voxel-based global illumination || Several variants exist, including voxel cone tracing global illumination,<ref>{{cite web|url=http://on-demand.gputechconf.com/gtc/2012/presentations/SB134-Voxel-Cone-Tracing-Octree-Real-Time-Illumination.pdf |archive-url=https://web.archive.org/web/20130903101803/http://on-demand.gputechconf.com/gtc/2012/presentations/SB134-Voxel-Cone-Tracing-Octree-Real-Time-Illumination.pdf |archive-date=2013-09-03 |url-status=live |title=Voxel Cone Tracing and Sparse Voxel Octree for Real-time Global Illumination |author=Cyril Crassin |website=On-demand.gputechconf.com |access-date=2016-12-02}}</ref> sparse voxel octree global illumination, and voxel global illumination (VXGI)<ref name="geforce">{{cite web|url=http://www.geforce.com/hardware/technology/vxgi|title=VXGI &#124; GeForce|date=8 April 2015 |publisher=geforce.com|access-date=2016-05-14}}</ref>
|-
| Light propagation volumes global illumination<ref name="unrealengine">{{cite web|url=https://wiki.unrealengine.com/Light_Propagation_Volumes_GI|title=Light Propagation Volumes GI - Epic Wiki|publisher=wiki.unrealengine.com|access-date=2016-05-14}}</ref> || Light propagation volumes is a technique to approximately achieve global illumination (GI) in real-time.
It uses lattices and spherical harmonics (SH) to represent the spatial and angular distribution of light in the scene. Variant cascaded light propagation volumes.<ref>{{cite book |last1=Engelhardt |first1=T. |last2=Dachsbacher |first2=C. |chapter=Granular visibility queries on the GPU |chapter-url=http://www.vis.uni-stuttgart.de/~dachsbcn/download/lpv.pdf |title=Proceedings of the 2009 symposium on Interactive 3D graphics and games |publisher= |location= |date=2009 |isbn=978-1-60558-429-4 |pages=161–7 |url= |doi=10.1145/1507149.1507176|s2cid=14841843 |archive-url=https://web.archive.org/web/20160118035359/http://www.vis.uni-stuttgart.de/~dachsbcn/download/lpv.pdf |archive-date=2016-01-18 }}</ref>
|-
| Deferred radiance transfer global illumination<ref>{{cite web|url=http://twvideo01.ubm-us.net/o1/vault/gdc2012/slides/Programming%20Track/Stefanov_Nikolay_DeferredRadianceTransfer.pdf |archive-url=https://web.archive.org/web/20140906141942/http://twvideo01.ubm-us.net/o1/vault/gdc2012/slides/Programming%20Track/Stefanov_Nikolay_DeferredRadianceTransfer.pdf |archive-date=2014-09-06 |url-status=live |title=Deferred Radiance Transfer Volumes: Global Illumination in Far Cry 3 |website=Twvideo01.ubm-us.net |access-date=2016-12-02}}</ref> ||
|-
| Deep G-buffer based global illumination<ref name="williams">{{cite web|url=http://graphics.cs.williams.edu/papers/DeepGBuffer14/|title=Fast Global Illumination Approximations on Deep G-Buffers|publisher=graphics.cs.williams.edu|access-date=2016-05-14|archive-url=https://web.archive.org/web/20160221013732/http://graphics.cs.williams.edu/papers/DeepGBuffer14/|archive-date=2016-02-21}}</ref> ||
|-
| Signed Distance Fields Dynamic Diffuse Global Illumination<ref name="Hu">{{cite arXiv
| title=Signed Distance Fields Dynamic Diffuse Global Illumination
| date       = 2020
| last1      = Hu
| first1     = Jinkai
| last2      = K. Yip
| first2     = Milo
| last3      = Elias Alonso
| first3     = Guillermo
| last4      = Shi-hao
| first4     = Gu
| last5      = Tang
| first5     = Xiangjun
| last6      = Xiaogang
| first6     = Jin
| class = cs.GR
| eprint = 2007.14394
}}</ref> ||
|-
|Global Illumination Based on Surfels<ref name="Halen">{{cite web|url=http://advances.realtimerendering.com/s2021/index.html|title=Global Illumination Based on Surfels|publisher=SIGGRAPH|access-date=2021-12-02}}</ref> ||
|}

==See also==

*[[:Category:Global illumination software]]
*[[Bias of an estimator]]
*[[Bidirectional scattering distribution function]]
*[[Consistent estimator]]
*[[Unbiased rendering]]

==References==
{{Reflist}}

==External links==
*[https://archive.org/details/MarcC_AoI-Global_Illumination Video demonstrating global illumination and the ambient color effect]
*[http://realtimeradiosity.com/demos Real-time GI demos] – survey of practical real-time GI techniques as a list of executable demos
*[http://www.cs.kuleuven.be/~phil/GI/ kuleuven] - 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.
*[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.]

{{DEFAULTSORT:Global Illumination}}
{{Computer graphics}}
[[Category:Global illumination algorithms]]