Computer graphics lighting: Difference between revisions

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{{short description|Simulation of light in computer graphics}}

'''Computer graphics lighting''' isencompasses the collectionrange of techniques used to simulate light inwithin [[computer graphics]] scenes. WhileThese lightingmethods techniques offer flexibilityvary in the[[computational level of detail and functionality availablecomplexity]], theyoffering alsoartists operateflexibility atin differentboth levelsvisual of computational demanddetail and [[Computational complexity|complexity]]performance. Graphics artistsprofessionals can chooseselect from a varietywide array of [[light sourcessource]]s, models[[lighting model]]s, [[shading]] techniques, and effects to suitmeet the needsspecific requirements of each applicationproject.

== Light sources ==

Light sources allow for different ways to introduce light into graphics scenes.<ref>{{Cite news|url=https://garagefarm.net/blog/light-the-art-of-exposure|title=Light: The art of exposure|date=2020-11-11|work=GarageFarm|access-date=2020-11-11|language=en-US}}</ref><ref name=":727">{{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=":7372">{{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 over the scale of the scene, 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 on Earth.<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">{{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> The light becomes more intense as the viewer gets 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" />

=== Area ===

Area lights are 3D objects which emit light. Whereas point lights and spot lights sources are considered infinitesimally small points, area lights are treated as physical shapes.<ref>{{cite conference |last1=Lagarde |first1=Sebastien |author-link1= |last2=de Rousiers |first2=Charles |author-link2= |date=Summer 2014 |title=Moving Frostbite to Physically Based Rendering 3.0 |url=https://www.ea.com/frostbite/news/moving-frostbite-to-pb |conference=SIGGRAPH |___location= |publisher= |pages= |id= |book-title=}}</ref> Area light produce softer shadows and more realistic lighting than point lights and spot lights.<ref>{{Cite book |last1=Pharr |first1=Matt |title=Physically Based Rendering: From Theory to Implementation |last2=Humphreys |first2=Greg |last3=Wenzel |first3=Jakob |publisher=Morgan Kaufmann |year=2016 |isbn=978-0128006450 |edition=3rd |language=English}}</ref>

=== 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" />

=== Lightwarp ===

A lightwarp is a technique of which an object in the geometrical world [[refracts]] light based on the [[Unit vector|direction]] and [[Intensity (physics)|intensity]] of the light. The light is then [[Wave function|warped]] using an ambient diffuse term with a range of the [[color spectrum]]. The light then may be [[reflectively]] scattered to produce a higher [[depth of field]], and [[refracted]]. The technique is used to produce a [[Style_(visual_arts)#Stylization|unique rendering style]] and can be used to limit [[overexposure]] of objects. Games such as [[Team Fortress 2]] use the rendering technique to create a [[cartoon]] [[cel shaded]] stylized look.<ref>{{Cite book|chapter-url=https://hal.inria.fr/inria-00449828|chapter=Radiance Scaling for Versatile Surface Enhancement|first1=Romain|last1=Vergne|first2=Romain|last2=Pacanowski|first3=Pascal|last3=Barla|first4=Xavier|last4=Granier|first5=Christophe|last5=Schlick|title=Proceedings of the 2010 ACM SIGGRAPH symposium on Interactive 3D Graphics and Games |date=February 19, 2010|pages=143–150 |publisher=ACM|via=hal.inria.fr|doi=10.1145/1730804.1730827|isbn=9781605589398 |s2cid=18291692 }}</ref>

===HDRI===

HDRI stands for High dynamic range image and is a 360° image that is wrapped around a [[3D modeling|3D model]] as an outdoor setting and uses the sun typically as a light source in the sky. The [[Texture mapping|textures]] from the model can reflect the direct and [[Shading#Ambient lighting|ambient light]] and colors from the HDRI.<ref>{{cite web | url=https://visao.ca/what-is-hdri/#:~:text=High%20dynamic%20range%20images%20are,look%20cartoonish%20and%20less%20professional. | title=What are HDRI images? | date=13 January 2021 }}</ref>

== Lighting interactions ==

In computer graphics, the overall effect of a light usuallysource consistson an object is determined by the combination of multiplethe object's interactions with it usually described by at least three main 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=":838">{{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|author-link=Nancy Pollard|date=Spring 2004}}</ref> After light strikes an object, it is [[Reflection (computer graphics)|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">{{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>

=== Ambient ===

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==== Phong illumination model ====

{{Main|Phong reflection model}}

One of the most common shadingreflection 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" />

==== Blinn-Phong illumination model ====

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{{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|bibcode=1989Sci...244..166G |s2cid=46575183 }}</ref>

==== Radiosity ====

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==== Photon mapping ====

{{Main|Photon mapping}}

[[Photon]] mapping was created as a two-pass global illumination algorithm that is more efficient than raytracingray tracing.<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 Computer Graphics, Section 4.1 -- Introduction to Lighting|website=math.hws.edu}}</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.]]

[[File:Phongshading00.png|alt=|thumb|165x165px|An example of Phong shading.]]

=== Flat shading ===

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=== 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 ===

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== Lighting effects ==

[[File:Miroir-cercle.jpg|thumb|A reflective material demonstrating caustics.]]

=== Caustics ===

{{Main articles|Caustic (optics)}}

[[Caustic (optics)|Caustics]] are a lightingan effect of light reflected and refracted lightin movinga medium with curved interfaces or reflected throughoff a mediumcurved surface.<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 ===

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== See also ==

*[[Per-pixel lighting]]

*[[Computer graphics]]

==References==

{{reflist}}

{{Computer graphics}}

{{DEFAULTSORT:Computer Graphics Lighting}}

[[Category:3D rendering]]

[[Category:Lighting| ]]

[[Category:Shading| ]]