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== Light sources ==
Light sources allow for different ways to introduce light into graphics scenes.<ref name=":72">{{Cite web|url=https://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/LightingAndShading.html|title=Intro to Computer Graphics: Lighting and Shading|website=www.cs.uic.edu|access-date=2019-11-05}}</ref>.
=== Point ===
Point sources emit light from a single point in all directions, with the intensity of the light decreasing with distance.<ref name=":73">{{Cite web|url=https://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/LightingAndShading.html|title=Intro to Computer Graphics: Lighting and Shading|website=www.cs.uic.edu|access-date=2019-11-05}}</ref>. An example of a point source is a standalone light bulb.<ref name=":8">{{Cite web|url=http://www.bcchang.com/immersive/ygbasics/lighting.html|title=Lighting in 3D Graphics|website=www.bcchang.com|access-date=2019-11-05}}</ref>.
[[File:Real-time Raymarched Terrain.png|thumb|309x309px|A directional light source illuminating a terrain.]]
=== Directional ===
A directional source (or distant source) uniformly lights a scene from one direction.<ref name=":8" />. Unlike a point source, the intensity of light produced by a directional source does not change with distance, as the directional source is treated as though it is extremely far away from the scene.<ref name=":8" />. An example of a directional source is sunlight.<ref name=":9">{{Cite web|url=https://www.pluralsight.com/blog/film-games/understanding-different-light-types|title=Understanding Different Light Types|website=www.pluralsight.com|language=en|access-date=2019-11-05}}</ref>.
=== Spotlight ===
A spotlight produces a directed [[cone]] of light.<ref name=":73" />. The light becomes more intense closer to the spotlight source and to the center of the light cone.<ref name=":73" />. An example of a spotlight is a flashlight.<ref name=":9" />.
=== Ambient ===
Ambient light sources illuminate objects even when no other light source is present.<ref name=":73" />. The intensity of ambient light is independent of direction, distance, and other objects, meaning the effect is completely uniform throughout the scene.<ref name=":73" />. This source ensures that objects are visible even in complete darkness.<ref name=":9" />.
== Lighting interactions ==
In computer graphics, light usually consists of multiple components.<ref name=":82">{{Cite web|url=http://www.bcchang.com/immersive/ygbasics/lighting.html|title=Lighting in 3D Graphics|website=www.bcchang.com|access-date=2019-11-05}}</ref>. The overall effect of a light source on an object is determined by the combination of the object's interactions with these components.<ref name=":82" />. The three primary lighting components (and subsequent interaction types) are diffuse, ambient, and specular.<ref name=":82" />.
[[File:Phong components revised.png|thumb|544x544px|Decomposition of lighting interactions.]]
=== Diffuse ===
Diffuse lighting (or [[diffuse reflection]]) is the direct illumination of an object by an even amount of light interacting with a [[Scattering|light-scattering]] surface.<ref name=":83">{{Cite web|url=http://www.bcchang.com/immersive/ygbasics/lighting.html|title=Lighting in 3D Graphics|website=www.bcchang.com|access-date=2019-11-05}}</ref><ref name=":10">{{Cite web|url=http://graphics.cs.cmu.edu/nsp/course/15-462/Spring04/slides/07-lighting.pdf|title=Lighting and Shading|last=Pollard|first=Nancy|date=Spring 2004|website=|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. After light strikes an object, it is reflected as a function of the surface properties of the object as well as the angle of incoming light.<ref name=":10" />. This interaction is the primary contributor to the object's brightness and forms the basis for its color.<ref name=":83" />.
=== Ambient ===
As ambient light is directionless, it interacts uniformly across all surfaces, with its intensity determined by the strength of the ambient light sources and the properties of objects' surface materials, namely their ambient [[Reflection coefficient|reflection coefficients]].<ref name=":83" /><ref name=":10" />.
=== Specular ===
The [[Specular highlight|specular lighting]] component gives objects shine and highlights.<ref name=":83" />. This is distinct from mirror effects because other objects in the environment are not visible in these reflections.<ref name=":10" />. Instead, specular lighting creates bright spots on objects based on the intensity of the specular lighting component and the specular reflection coefficient of the surface.<ref name=":10" />.
== Illumination models ==
Lighting models are used to replicate lighting effects in [[Rendering (computer graphics)|rendered]] environments where light is approximated based on the physics of light.<ref name=":1">{{Cite web|url=https://learnopengl.com/Lighting/Basic-Lighting|title=LearnOpenGL - Basic Lighting|website=learnopengl.com|access-date=2019-11-08}}</ref>. Without lighting models, replicating lighting effects as they occur in the natural world would require more processing power than is practical for computer graphics.<ref name=":1" />. This lighting, or illumination model's purpose is to compute the color of every pixel or the amount of light reflected for different surfaces in the scene.<ref>{{Cite web|url=https://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/LightingAndShading.html|title=Intro to Computer Graphics: Lighting and Shading|website=www.cs.uic.edu|access-date=2019-11-08}}</ref>. There are two main illumination models, object oriented lighting and global illumination.<ref name=":2">{{Cite web|url=https://www.cc.gatech.edu/classes/AY2003/cs4451a_fall/global-illumination.pdf|title=Global Illumination|last=|first=|date=2002|website=Georgia Tech Classes|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. They differ in that object oriented lighting considers each object individually, whereas global illumination maps how light interacts between objects.<ref name=":2" />. Currently, researchers are developing global illumination techniques to more accurately replicate how light interacts with its environment.<ref name=":2" />.
=== Object oriented lighting ===
Object oriented lighting, also known as local illumination, is defined by mapping a single light source to a single object.<ref name=":3">{{Cite web|url=http://www.cs.kent.edu/~farrell/cg01/lectures/color/illum_local.html|title=Local Illumination|last=Farrell|date=|website=Kent University|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. This technique is fast to compute, but often is an incomplete approximation of how light would behave in the scene in reality.<ref name=":3" />. It is often approximated by summing a combination of specular, diffuse, and ambient light of a specific object.<ref name=":1" />. The two predominant local illumination models are the Phong and the Blinn-Phong illumination models.<ref name=":4" />.
==== Phong illumination model ====
{{Main|Phong reflection model}}
One of the most common shading models is the Phong model.<ref name=":1" />. The Phong model assumes that the intensity of each [[pixel]] is the sum of the intensity due to diffuse, specular, and ambient lighting.<ref name=":3" />. This model takes into account the ___location of a viewer to determine specular light using the angle of light reflecting off an object.<ref name=":4" />. The [[Trigonometric functions|cosine]] of the angle is taken and raised to a power decided by the designer.<ref name=":3" />. With this, the designer can decide how wide a highlight they want on an object; because of this, the power is called the shininess value.<ref name=":4" />. The shininess value is determined by the roughness of the surface where a mirror would have a value of infinity and the roughest surface might have a value of one.<ref name=":3" />. This model creates a more realistic looking white highlight based on the perspective of the viewer.<ref name=":1" />.<br />
==== Blinn-Phong illumination model ====
{{Main|Blinn–Phong reflection model}}
The Blinn-Phong illumination model is similar to the Phong model as it uses specular light to create a highlight on an object based on its shininess.<ref name=":16">James F. Blinn (1977). "Models of light reflection for computer synthesized pictures". ''Proc. 4th annual conference on computer graphics and interactive techniques'': 192–198. [[CiteSeerX]] 10.1.1.131.7741. {{doi|10.1145/563858.563893}}<br /></ref>. The Blinn-Phong differs from the Phong illumination model, as the Blinn-Phong model uses the vector normal to the surface of the object and halfway between the light source and the viewer.<ref name=":1" />. This model is used in order to have accurate specular lighting and reduced computation time.<ref name=":1" />. The process takes less time because finding the reflected light vector's direction is a more involved computation than calculating the halfway [[Normal (geometry)|normal vector]] .<ref name=":16" />. While this is similar to the Phong model, it produces different visual results, and the specular reflection exponent or shininess might need modification in order to produce a similar specular reflection.<ref>Jacob's University, "[http://www.faculty.jacobs-university.de/llinsen/teaching/320322_Fall2010/lecture12.pdf Blinn-Phong Reflection Model]", 2010.</ref>.
=== Global illumination ===
{{Main articles|Global illumination}}
Global illumination differs from local illumination because it calculates light as it would travel throughout the entire scene .<ref name=":2" />. This lighting is based more heavily in physics and optics, with light rays scattering, reflecting, and indefinitely bouncing throughout the scene.<ref name=":13" />. There is still active research being done on global illumination as it requires more computational power than local illumination.<ref name=":18">Li, Hao (Fall 2018). "Global Illumination" (PDF).</ref>.
==== Ray tracing ====
{{Main articles|Ray tracing (graphics)}}
[[File:Ray-traced steel balls.jpg|thumb|Image rendered using ray tracing]]
Light sources emit rays that interact with various surfaces through absorption, reflection, or refraction.<ref name=":72" />. An observer of the scene would see any light source that reaches their eyes; a ray that does not reach the observer goes unnoticed.<ref>{{Cite web|url=https://developer.nvidia.com/rtx/raytracing|title=Introducing the NVIDIA RTX Ray Tracing Platform|date=2018-03-06|website=NVIDIA Developer|language=en|access-date=2019-11-08}}</ref>. It is possible to simulate this by having all of the light sources emit rays and then compute how of each of them interact with all of the objects in the scene.<ref name=":17">Reif, J. H. (1994). "[https://users.cs.duke.edu/~reif/paper/tygar/raytracing.pdf Computability and Complexity of Ray Tracing]"(PDF). ''Discrete and Computational Geometry''. </ref>. However, this process is inefficient as most of the light rays would not reach the observer and would waste processing time.<ref name=":21">Wallace, John R.; Cohen, Michael F.; Greenberg, Donald P. (1987). "A Two-pass Solution to the Rendering Equation: A Synthesis of Ray Tracing and Radiosity Methods". ''Proceedings of the 14th Annual Conference on Computer Graphics and Interactive Techniques''. SIGGRAPH '87. New York, NY, USA: ACM: 311–320. {{doi|10.1145/37401.37438}}. {{ISBN|9780897912273}}.</ref>. Ray tracing solves this problem by reversing the process, instead sending view rays from the observer and calculating how they interact until they reach a light source.<ref name=":17" />. Although this way more effectively uses processing time and produces a light simulation closely imitating natural lighting, ray tracing still has high computation costs due to the high amounts of light that reach viewer's eyes.<ref name=":0">{{Cite journal|last=Greenberg|first=Donald P.|date=1989-04-14|title=Light Reflection Models for Computer Graphics|journal=Science|language=en|volume=244|issue=4901|pages=166–173|doi=10.1126/science.244.4901.166|issn=0036-8075|pmid=17835348}}</ref>.
==== Radiosity ====
{{Main articles|Radiosity (computer graphics)}}
Radiosity takes into account the energy given off by surrounding objects and the light source.<ref name=":2" />. Unlike ray tracing, which is dependent on the position and orientation of the observer, radiosity lighting is independent of view position.<ref name=":21" />. Radiosity requires more computational power than ray tracing, but can be more useful for scenes with static lighting because it would only have to be computed once.<ref>Cindy Goral, Kenneth E. Torrance, Donald P. Greenberg and B. Battaile,"[http://www.cs.rpi.edu/~cutler/classes/advancedgraphics/S07/lectures/goral.pdf Modeling the interaction of light between diffuse surfaces]", ''[[Computer Graphics (Publication)|Computer Graphics]]'', Vol. 18, No. 3. ([[PDF]])</ref>. The surfaces of a scene can be divided into a large amount of patches; each patch radiates some light and affects the other patches, then a large set of equations needs to be solved simultaneously in order to get the final radiosity of each patch.<ref name=":0" />.
==== Photon mapping ====
{{Main|Photon mapping}}
[[Photon]] mapping was created as a two-pass global illumination algorithm that is more efficient than raytracing.<ref name=":19">Wann Jensen, Henrik (1996). "[http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf Global Illumination using Photon Maps] {{Webarchive|url=https://web.archive.org/web/20080808140048/http://graphics.ucsd.edu/~henrik/papers/photon_map/global_illumination_using_photon_maps_egwr96.pdf |date=2008-08-08 }}" (PDF). ''Rendering Techniques ’96'': 21–30.</ref>. It is the basic principle of tracking photons released from a light source through a series of stages.<ref name=":19" />. The first pass includes the photons being released from a light source and bouncing off their first object; this map of where are the photons are located is then recorded.<ref name=":18" />. The photon map contains both the position and direction of each photon which either bounce or are absorbed.<ref name=":19" />. The second pass happens with [[Rendering (computer graphics)|rendering]] where the reflections are calculated for different surfaces.<ref name=":20">{{Cite web|url=https://web.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/photon_mapping/PhotonMapping.html|title=Photon Mapping - Zack Waters|website=web.cs.wpi.edu|access-date=2019-11-08}}</ref>. In this process, the photon map is decoupled from the geometry of the scene, meaning rendering can be calculated separately.<ref name=":18" />. It is a useful technique because it can simulate caustics, and pre-processing steps do not need to be repeated if the view or objects change.<ref name=":20" />.
== Polygonal shading ==
{{Main|Shading}}
Polygonal [[shading]] is part of the [[Rasterisation|rasterization]] process where [[3D computer graphics|3D]] models are drawn as [[2D computer graphics|2D]] pixel images.<ref name=":4">{{Cite web|url=https://cglearn.codelight.eu/pub/computer-graphics/shading-and-lighting|title=Computer Graphics: Shading and Lighting|website=cglearn.codelight.eu|access-date=2019-10-30}}</ref>. Shading applies a lighting model, in conjunction with the geometric attributes of the 3D model, to determine how lighting should be represented at each [[Fragment (computer graphics)|fragment]] (or pixel) of the resulting image.<ref name=":4" />. The [[Polygon mesh|polygons]] of the 3D model store the geometric values needed for the shading process.<ref name=":11">{{Cite web|url=http://math.hws.edu/graphicsbook/c4/s1.html|title=Introduction to Lighting|last=|first=|date=|website=|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. This information includes [[Vertex (geometry)|vertex]] positional values and [[Normal (geometry)|surface normals]], but can contain optional data, such as [[Texture mapping|texture]] and [[Bump mapping|bump]] maps.<ref>{{Cite web|url=https://www.khronos.org/opengl/wiki/Vertex_Specification#Primitives|title=Vertex Specification - OpenGL Wiki|website=www.khronos.org|access-date=2019-11-06}}</ref>.
[[File:Flatshading00.png|alt=|thumb|165x165px|An example of flat shading.]]
[[File:Gouraudshading01.png|alt=|thumb|165x165px|An example of Gouraud shading.]]
=== Flat shading ===
Flat shading is a simple shading model with a uniform application of lighting and color per polygon.<ref name=":12">{{Cite web|url=https://www.cs.brandeis.edu/~cs155/Lecture_16.pdf|title=Illumination Models and Shading|last=Foley|date=|website=|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. The color and normal of one vertex is used to calculate the shading of the entire polygon.<ref name=":4" />. Flat shading is inexpensive, as lighting for each polygon only needs to be calculated once per render.<ref name=":12" />.
=== Gouraud shading ===
[[Gouraud shading]] is a type of interpolated shading where the values inside of each polygon are a blend of its vertex values.<ref name=":4" />. Each vertex is given its own normal consisting of the average of the surface normals of the surrounding polygons.<ref name=":12" />. The lighting and shading at that vertex is then calculated using the average normal and the lighting model of choice.<ref name=":12" />. This process is repeated for all the vertices in the 3D model.<ref name=":7">{{Cite web|url=https://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/LightingAndShading.html|title=Intro to Computer Graphics: Lighting and Shading|website=www.cs.uic.edu|access-date=2019-11-05}}</ref>. Next, the shading of the edges between the vertices is calculated by [[Interpolation|interpolating]] between the vertex values.<ref name=":7" />. Finally, the shading inside of the polygon is calculated as an interpolation of the surrounding edge values.<ref name=":7" />. Gouraud shading generates a smooth lighting effect across the 3D model's surface.<ref name=":7" />.
=== Phong shading ===
[[Phong shading]], similar to Gouraud shading, is another type of interpolative shading that blends between vertex values to shade polygons.<ref name=":13">{{Cite web|url=http://www.hao-li.com/cs420-fs2018/slides/Lecture05.2.pdf|title=Shading in OpenGL|last=Li|first=Hao|date=2018|website=|url-status=live|archive-url=|archive-date=}}</ref>. The key difference between the two is that Phong shading interpolates the [[vertex normal]] values over the whole polygon before it calculates its shading.<ref name=":12" />. This contrasts with Gouraud shading which interpolates the already shaded vertex values over the whole polygon.<ref name=":13" />. Once Phong shading has calculated the normal of a fragment (pixel) inside the polygon, it can then apply a lighting model, shading that fragment.<ref name=":12" />. This process is repeated until each polygon of the 3D model is shaded.<ref name=":13" />.
== Lighting effects ==
=== Caustics ===
{{Main articles|Caustic (optics)}}
[[Caustic (optics)|Caustics]] are a lighting effect of reflected and refracted light moving through a medium.<ref name=":14">{{Cite web|url=https://developer.nvidia.com/gpugems/GPUGems/gpugems_ch02.html|title=GPU Gems|website=NVIDIA Developer|language=en|access-date=2019-10-30}}</ref>. They appear as ribbons of concentrated light and are often seen when looking at bodies of water or glass.<ref name=":15">{{Cite web|url=https://www.dualheights.se/caustics/caustics-water-texturing-using-unity3d.shtml|title=Caustics water texturing using Unity 3D|website=www.dualheights.se|access-date=2019-11-06}}</ref>. Caustics can be implemented in 3D graphics by blending a caustic [[Texture mapping|texture map]] with the texture map of the affected objects.<ref name=":15" />. The caustics texture can either be a static image that is animated to mimic the effects of caustics, or a [[Real-time computing|Real-time]] calculation of caustics onto a blank image.<ref name=":15" />. The latter is more complicated and requires backwards [[Ray tracing (graphics)|ray tracing]] to simulate photons moving through the environment of the 3D render.<ref name=":14" />. In a photon mapping illumination model, [[Monte Carlo sampling|Monte Carlo]] sampling is used in conjunction with the ray tracing to compute the intensity of light caused by the caustics.<ref name=":14" />.
=== Reflection mapping ===
{{Main articles|Reflection mapping}}
Reflection mapping (also known as environment mapping) is a technique which uses 2D environment maps to create the effect of [[Reflectance|reflectivity]] without using ray tracing.<ref name=":5">{{Cite web|url=https://cglearn.codelight.eu/pub/computer-graphics/environment-mapping|title=Computer Graphics: Environment Mapping|website=cglearn.codelight.eu|access-date=2019-11-01}}</ref>. Since the appearances of reflective objects depend on the relative positions of the viewers, the objects, and the surrounding environments, graphics algorithms produce reflection vectors to determine how to color the objects based on these elements.<ref>{{Cite web|url=http://web.cse.ohio-state.edu/~wang.3602/courses/cse5542-2013-spring/17-env.pdf|title=Environment Mapping|last=Shen|first=Han-Wei|date=|website=|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. Using 2D environment maps rather than fully rendered, 3D objects to represent surroundings, reflections on objects can be determined using simple, computationally inexpensive algorithms.<ref name=":5" />.
=== Particle systems ===
{{Main articles|Particle system}}
Particle systems use collections of small [[Particle|particles]] to model chaotic, high-complexity events, such as fire, moving liquids, explosions, and moving hair.<ref name=":6">{{Cite web|url=http://web.engr.oregonstate.edu/~mjb/cs491/Handouts/particlesystems.2pp.pdf|title=Particle Systems|last=Bailey|first=Mike|date=|website=|url-status=live|archive-url=|archive-date=|access-date=}}</ref>. Particles which make up the complex animation are distributed by an emitter, which gives each particle its properties, such as speed, lifespan, and color.<ref name=":6" />. Over time, these particles may move, change color, or vary other properties, depending on the effect.<ref name=":6" />. Typically, particle systems incorporate [[randomness]], such as in the initial properties the emitter gives each particle, to make the effect realistic and non-uniform.<ref name=":6" /><ref>{{Cite web|url=https://web.cs.wpi.edu/~matt/courses/cs563/talks/psys.html|title=Particle Systems|website=web.cs.wpi.edu|access-date=2019-11-01}}</ref>.
== See also ==
| 