Editing Non-photorealistic rendering

{{Short description|Style of rendering}}
[[File:Toon Shader.jpg|thumb|A normal shader (left) and an NPR shader using [[cel-shading]] (right)]]
'''Non-photorealistic rendering''' ('''NPR''') is an area of [[computer graphics]] that focuses on enabling a wide variety of expressive [[style (visual arts)|style]]s for digital art, in contrast to traditional [[computer graphics]], which focuses on [[photorealism]]. NPR is inspired by other artistic modes such as [[painting]], [[drawing]], [[technical illustration]], and [[animated cartoons]]. NPR has appeared in [[Film|movies]] and [[video game]]s in the form of [[cel shading|cel-shaded animation]] (also known as "[[cartoon|toon]]" shading) as well as in [[scientific visualization]], [[architectural illustration]] and [[experimental animation]].{{citation needed|date=September 2021}}

==History and criticism of the term==
The term ''non-photorealistic rendering'' is believed to have been coined by the [[SIGGRAPH]] 1990 papers committee, who held a session entitled "Non Photo Realistic Rendering".<ref>{{cite book |doi=10.1145/97879 |title=Proceedings of the 17th annual conference on Computer graphics and interactive techniques - SIGGRAPH '90 |year=1990 |isbn=978-0-201-50933-5 |s2cid=12199502 |last1=Baskett |first1=Forest }}{{pn|date=July 2022}}</ref><ref>{{cite thesis |last1=Schofield |first1=Simon |title=Non-photorealistic rendering: a critical examination and proposed system |date=March 1994 |url=https://eprints.mdx.ac.uk/6723/ }}</ref>

The term has received some criticism: 
* The term "[[photorealism]]" has different meanings for graphics researchers (see "[[photorealistic rendering]]") and artists.  For artists—who are the target consumers of NPR techniques—it refers to a school of painting that focuses on reproducing the effect of a [[camera lens]], with all the distortion and hyper-reflections{{Definition needed|date=May 2019}} that it creates. For graphics researchers, however, it refers to an image that is visually indistinguishable from reality.  In fact, graphics researchers lump the kinds of visual distortions that are used by photorealist painters into "non-photorealism".
* Describing something by what it is ''not'' is problematic. Equivalent (made-up) comparisons might be "non-elephant biology" or "non-geometric mathematics". NPR researchers have stated that they expect the term will disappear eventually and be replaced by the now more general term "computer graphics", with "photorealistic graphics" being the term used to describe "traditional" computer graphics.{{citation needed|date=March 2014}}
* Many techniques that are used to create 'non-photorealistic' images are not [[Rendering (computer graphics)|rendering]] techniques. They are modelling techniques, or post-processing techniques.  While the latter are coming to be known as 'image-based rendering', sketch-based modelling techniques, cannot technically be included under this heading, which is very inconvenient for conference organisers.

The first conference on non-photorealistic animation and rendering{{when|date=June 2020}} included a discussion of possible alternative names. Among those suggested were "expressive graphics", "artistic rendering", "non-realistic graphics", "art-based rendering", and "psychographics". All of these terms have been used in various research papers on the topic, but the "non-photorealistic" term seems to have nonetheless taken hold.

The first technical meeting dedicated to NPR was the [[Association for Computing Machinery|ACM]]-sponsored Symposium on Non-Photorealistic Rendering and Animation<ref>{{cite web|url=http://www.npar.org|title=npar|website=www.npar.org|access-date=20 April 2018|archive-url=https://web.archive.org/web/20020330144834/http://npar.org/ |archive-date=30 March 2002 |url-status=usurped}}</ref>(NPAR) in 2000.  NPAR is traditionally co-located with the Annecy Animated Film Festival,<ref>{{cite web|url=http://www.annecy.org|title=Site officiel du Festival et du Marché international du film d'animation d'Annecy|website=www.annecy.org|access-date=20 April 2018}}</ref> running on even numbered years. From 2007 onward, NPAR began to also run on odd-numbered years, co-located with ACM [[SIGGRAPH]].<ref>{{cite web |url=http://siggraph.org/ |title=Archived copy |website=siggraph.org |access-date=13 January 2022 |archive-url=https://web.archive.org/web/19961221040900/http://siggraph.org/ |archive-date=21 December 1996 |url-status=dead}}</ref>

==3D==
[[File:Planar core assembly exploded.png|thumb|An example of NPR used for [[technical illustration]]s]]
Three-dimensional NPR is the style that is most commonly seen in video games and movies.  The output from this technique is almost always a [[polygonal modeling|3D model]] that has been modified from the original input model to portray a new artistic style.  In many cases, the [[solid geometry|geometry]] of the model is identical to the original geometry, and only the material applied to the surface is modified. With increased availability of programmable [[GPU]]'s, [[shaders]] have allowed NPR effects to be applied to the [[rasterised]] image that is to be displayed to the screen.<ref>{{cite book |last1=Drew |first1=Card |chapter=Non-Photorealistic Rendering with Pixel and Vertex Shaders |date=2002 |pages=319–333 |citeseerx=10.1.1.360.918 |editor1-first=Wolfgang F. |editor1-last=Engel |title=Direct3D Shaderx: Vertex & Pixel Shader Tips and Techniques |series=87Wordware Game Developer's Library |publisher=Wordware Publishing |isbn=978-1-55622-041-8 }}</ref> The majority of NPR techniques applied to 3D geometry are intended to make the scene appear two-dimensional.

NPR techniques for 3D images include [[cel shading]] and [[Gooch shading]]. 

Many methods can be used to draw stylized outlines and strokes from 3D models, including [[occluding contours]] and [[Suggestive contours]].<ref>{{cite journal |last1=Bénard |first1=Pierre |last2=Hertzmann |first2=Aaron |title=Line Drawings from 3D Models: A Tutorial |journal=Foundations and Trends in Computer Graphics and Vision |date=2019 |volume=11 |issue=1–2 |pages=1–159 |doi=10.1561/0600000075 |arxiv=1810.01175 |s2cid=52912187 }}</ref>

For enhanced legibility, the most useful [[technical illustration]]s for [[technical communication]] are not necessarily photorealistic.  Non-photorealistic renderings, such as [[exploded view]] diagrams, greatly assist in showing placement of parts in a complex system.

Cartoon rendering, also called [[cel shading]] or toon shading, is a non-photorealistic rendering technique used to give 3D computer graphics a flat, cartoon-like appearance. Its defining feature is the use of distinct shading colors rather than smooth gradients, producing a look reminiscent of comic books or animated films. This technique is often used to blend 3D objects and environments with  2D hand-animated elements while maintaining a consistent look. [[Treasure Planet]] movie by Disney is an example of blending these techniques.<ref>{{Cite web |last=Hall |first=Cameron Roy |date=2023-01-21 |title=Treasure Planet's Animation Was Even More 3D Than It Seems |url=https://www.slashfilm.com/1168766/treasure-planets-animation-was-even-more-3d-than-it-seems/ |access-date=2024-10-27 |website=SlashFilm |language=en-US}}</ref> 

==2D==
[[File:Cradle Mountain Behind Dove Lake painted.jpg|thumb|300px|right|A non-photorealistic rendering of an existing 2D (photographic) image]]
[[File:Cradle Mountain Behind Dove Lake.jpg|thumb|300px|Original here]]
The input to a two dimensional NPR system is typically an image or video. The output is a typically an artistic rendering of that input imagery (for example in a watercolor, painterly or sketched style) although some 2D NPR serves non-artistic purposes e.g. data visualization.

The artistic rendering of images and video (often referred to as ''image stylization''<ref>{{cite book |doi=10.1007/978-1-4471-4519-6 |title=Image and Video-Based Artistic Stylisation |series=Computational Imaging and Vision |year=2013 |volume=42 |isbn=978-1-4471-4518-9 |s2cid=40656135 |editor1-first=Paul |editor1-last=Rosin |editor2-first=John |editor2-last=Collomosse }}{{pn|date=July 2022}}</ref>) traditionally focused upon heuristic algorithms that seek to simulate the placement of brush strokes on a digital canvas.<ref>{{cite journal |last1=Kyprianidis |first1=Jan Eric |last2=Collomosse |first2=John |last3=Wang |first3=Tinghuai |last4=Isenberg |first4=Tobias |title=State of the 'Art': A Taxonomy of Artistic Stylization Techniques for Images and Video |journal=IEEE Transactions on Visualization and Computer Graphics |date=May 2013 |volume=19 |issue=5 |pages=866–885 |doi=10.1109/TVCG.2012.160 |pmid=22802120 |s2cid=2656810 |url=https://hal.inria.fr/hal-00781502/file/Kyprianidis_2013_SAT.pdf }}</ref>

Arguably, the earliest example of 2D NPR is [[Paul Haeberli]]'s '[[paint by number|Paint by Numbers]]' at [[SIGGRAPH]] 1990. This (and similar interactive techniques) provide the user with a canvas that they can "paint" on using the cursor — as the user paints, a stylized version of the image is revealed on the canvas. This is especially useful for people who want to simulate different sizes of brush strokes according to different areas of the image.

Subsequently, basic image processing operations using gradient operators<ref>{{cite book |doi=10.1145/258734.258893 |chapter=Processing images and video for an impressionist effect |title=Proceedings of the 24th annual conference on Computer graphics and interactive techniques - SIGGRAPH '97 |year=1997 |last1=Litwinowicz |first1=Peter |pages=407–414 |isbn=978-0-89791-896-1 |s2cid=13139308 }}</ref> or statistical moments<ref>{{cite book |doi=10.1145/340916.340923 |chapter=An algorithm for automatic painterly rendering based on local source image approximation |title=Proceedings of the first international symposium on Non-photorealistic animation and rendering - NPAR '00 |year=2000 |last1=Shiraishi |first1=Michio |last2=Yamaguchi |first2=Yasushi |pages=53–58 |isbn=978-1-58113-277-9 |s2cid=16915734 }}</ref> were used to automate this process and minimize user interaction in the late nineties (although artistic control remains with the user via setting parameters of the algorithms). This automation enabled practical application of 2D NPR to video, for the first time in the living paintings of the movie ''[[What Dreams May Come (film)|What Dreams May Come]]'' (1998).

More sophisticated image abstractions techniques were developed in the early 2000s harnessing computer vision operators e.g. image salience,<ref>{{cite book |doi=10.1109/EGUK.2002.1011281 |citeseerx=10.1.1.7.5383 |chapter=Painterly rendering using image salience |title=Proceedings 20th Eurographics UK Conference |year=2002 |last1=Collomosse |first1=J.P. |last2=Hall |first2=P.M. |pages=122–128 |isbn=978-0-7695-1518-2 |s2cid=9610948 }}</ref> or segmentation<ref>{{cite book |doi=10.1145/508530.508545 |citeseerx=10.1.1.10.1761 |chapter=Artistic Vision |title=Proceedings of the second international symposium on Non-photorealistic animation and rendering - NPAR '02 |year=2002 |last1=Gooch |first1=Bruce |last2=Coombe |first2=Greg |last3=Shirley |first3=Peter |page=83 |isbn=978-1-58113-494-0 |s2cid=1146198 }}</ref> operators to drive stroke placement. Around this time, machine learning began to influence image stylization algorithms notably [[image analogy]]<ref>{{cite book |doi=10.1145/383259.383295 |citeseerx=10.1.1.119.5127 |chapter=Image analogies |title=Proceedings of the 28th annual conference on Computer graphics and interactive techniques - SIGGRAPH '01 |year=2001 |last1=Hertzmann |first1=Aaron |last2=Jacobs |first2=Charles E. |last3=Oliver |first3=Nuria |last4=Curless |first4=Brian |last5=Salesin |first5=David H. |pages=327–340 |isbn=978-1-58113-374-5 |s2cid=2201072 }}</ref> that could learn to mimic the style of an existing artwork.

The advent of [[deep learning]] has re-kindled activity in image stylization, notably with [[neural style transfer]] (NST) algorithms that can mimic a wide gamut of artistic styles from single visual examples. These algorithms underpin mobile apps capable of the same e.g. [[Prisma (app)|Prisma]]

In addition to the above stylization methods, a related class of techniques in 2D NPR address the simulation of artistic media. These methods include simulating the [[diffusion]] of ink through different kinds of [[paper]], and also of [[pigment]]s through water for simulation of [[watercolor]].

==Artistic rendering{{anchor|Artistic}}==
{{distinguish|Artist's rendition}}
'''Artistic rendering''' is the application of [[style (visual arts)|visual art style]]s to rendering. For [[photorealistic]] rendering styles, the emphasis is on accurate reproduction of light-and-shadow and the surface properties of the depicted objects, [[composition (visual arts)|composition]], or other more generic qualities. When the emphasis is on unique interpretive rendering styles, visual information is interpreted by the artist and displayed accordingly using the chosen [[art medium]] and level of [[abstraction]] in [[abstract art]].  In [[computer graphics]], interpretive rendering  styles are known as non-photorealistic rendering styles, but may be used to simplify [[technical illustration]]s. Rendering styles that combine photorealism with non-photorealism are known as [[Hyperrealism (visual arts)|hyperrealistic]] rendering styles.

==Notable films and games==
This section lists some seminal uses of NPR techniques in films, games and software. See [[cel-shaded animation]] for a list of uses of toon-shading in games and movies.

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  Please include a note explaining why new entries are notable usages of NPR, thanks.
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{| class="wikitable"
! colspan="3" | Short films
|-
| ''[[Technological Threat]]'' || 1988 || Early use of toon shading together with [[Tex Avery]]-style cartoon characters
|-
| ''Gas Planet'' || 1992 || Pencil-sketching 3D rendering by Eric Darnell
|-
| ''Fishing'' || 2000 || Watercolor-style 3D rendering David Gainey
|-
| ''RoadHead''<br />''Snack and Drink'' || 1998<br />1999 || Short films created with [[Rotoshop]] by [[Bob Sabiston]]
|-
| ''[[Ryan (film)|Ryan]]'' || 2004 || Nonlinear projection and other distortions of 3D geometry
|-
| ''[[The Girl Who Cried Flowers]]'' || 2008 || Watercolor-style rendering by Auryn
|-
! colspan="3" | Feature films
|-
| ''[[What Dreams May Come (film)|What Dreams May Come]]'' || 1998 || Painterly rendering in the "painted world" sequence
|-
| ''[[Tarzan (1999 film)|Tarzan]]'' || 1999 || First use of Disney's "Deep Canvas" system
|-
| ''[[Waking Life]]'' || 2001 || First use of rotoshop in a feature film
|-
| ''[[A Scanner Darkly (film)|A Scanner Darkly]]'' || 2006 || "a 15-month animation process"
|-
|[[Spider-Man: Into the Spider-Verse]]
|2018
|Creators wanted to create the feeling that the viewer  "walked inside a comic book"<ref>{{Cite web |last=Weintraub |first=Steven |date=2017-12-11 |title=Here's How Peter Parker Factors into Spider-Man: Into the Spider-Verse |url=https://collider.com/peter-parker-spider-man-into-the-spider-verse-details/ |access-date=2024-10-28 |website=Collider |language=en}}</ref>
|-
|''[[Spider-Man: Across the Spider-Verse]]''
|2023
|Incorporated NPR techniques to develop several different animation styles, including impressionistic watercolor style.<ref>{{Cite web |last=Croll |first=Ben |date=2022-06-13 |title=‘Spider-Verse’: Enigmatic New Villain The Spot Unveiled With First Look Image at Annecy |url=https://variety.com/2022/film/news/spider-verse-the-spot-sony-annecy-1235292383/ |access-date=2024-10-28 |website=Variety |language=en-US}}</ref> 
|-
! colspan="3" | Video games
|-
| ''[[Jet Set Radio]]'' || 2000 || Early use of toon-shading in video games
|-
| ''[[The Legend of Zelda: The Wind Waker]]'' || 2002 || One of the most well-known cel-shaded games
|-
| ''[[Valkyria Chronicles]]'' || 2008 || Uses a number of NPR techniques in the game, including a sketch-like shading method
|-
| ''[[XIII (video game)|XIII]]'' || 2003 || A game made as "comic"-like as possible
|-
| ''[[Ōkami]]'' || 2006 || A game whose visuals emulate the style of [[Ink wash painting|sumi-e]] (Japanese ink wash painting)
|-
| ''[[Guilty Gear Xrd]]''
|2014
|Fighting game using cel-shaded 3D characters with limited animation to imitate the look of 2D sprites
|-
| ''[[Return of the Obra Dinn]]'' || 2018 || A 3D game rendered in a unique monochrome, pointillist style
|-
| ''[[Manifold Garden]]'' || 2019 || A 3D puzzle game using [[impossible geometry]], notable for its novel edge-shading techniques.<ref>{{cite book |doi=10.1145/3388767.3407385 |chapter=That's a wrap: Manifold Garden rendering retrospective |title=Special Interest Group on Computer Graphics and Interactive Techniques Conference Talks |year=2020 |last1=Brussee |first1=Arthur |last2=Saraev |first2=Andrew |last3=Chyr |first3=William |pages=1–2 |isbn=978-1-4503-7971-7 |s2cid=221178781 }}</ref>
|-
! colspan="3" |Software for non-photorealistic rendering
|-
|''[[SketchUp]]''
|2000
|Sketch-like modelling ''software'' with toon rendering
|-
|''[[E-on Vue|Vue Xstream]]''
|2015
|3D environment creation ''software'' featuring an NPR renderer with various traditional art style emulating presets
|}

==References==
{{Reflist}}

==Further reading==
Some key papers in the development of NPR are:
* {{cite journal |doi=10.1145/97880.97902 |citeseerx=10.1.1.94.8782 |title=Paint by numbers: Abstract image representations |year=1990 |last1=Haeberli |first1=Paul |journal=ACM SIGGRAPH Computer Graphics |volume=24 |issue=4 |pages=207–214 }}
* {{cite journal |doi=10.1145/97880.97901 |citeseerx=10.1.1.83.4139 |title=Comprehensible rendering of 3-D shapes |year=1990 |last1=Saito |first1=Takafumi |last2=Takahashi |first2=Tokiichiro |journal=ACM SIGGRAPH Computer Graphics |volume=24 |issue=4 |pages=197–206 }}
* {{cite thesis |last1=Cockshott |first1=Malcolm Tunde |title=Wet and Sticky: A Novel Model for Computer-Based Painting |date=1991 |id={{ProQuest|2124902248}} |url=https://theses.gla.ac.uk/78293/ }}
* {{cite book |doi=10.1145/192161.192184 |chapter=Computer-generated pen-and-ink illustration |title=Proceedings of the 21st annual conference on Computer graphics and interactive techniques - SIGGRAPH '94 |year=1994 |last1=Winkenbach |first1=Georges |last2=Salesin |first2=David H. |pages=91–100 |isbn=978-0-89791-667-7 |s2cid=3234749 }}
* {{cite book |doi=10.1145/192161.192185 |chapter=Interactive pen-and-ink illustration |title=Proceedings of the 21st annual conference on Computer graphics and interactive techniques - SIGGRAPH '94 |year=1994 |last1=Salisbury |first1=Michael P. |last2=Anderson |first2=Sean E. |last3=Barzel |first3=Ronen |last4=Salesin |first4=David H. |pages=101–108 |isbn=978-0-89791-667-7 |s2cid=1098501 }}
* {{cite book |doi=10.1145/237170.237288  |citeseerx=10.1.1.126.7091 |chapter=Painterly rendering for animation |title=Proceedings of the 23rd annual conference on Computer graphics and interactive techniques - SIGGRAPH '96 |year=1996 |last1=Meier |first1=Barbara J. |pages=477–484 |isbn=978-0-89791-746-9 |s2cid=1199189 }}
* {{cite book |doi=10.1145/280814.280950  |citeseerx=10.1.1.435.1775 |chapter=A non-photorealistic lighting model for automatic technical illustration |title=Proceedings of the 25th annual conference on Computer graphics and interactive techniques - SIGGRAPH '98 |year=1998 |last1=Gooch |first1=Amy |last2=Gooch |first2=Bruce |last3=Shirley |first3=Peter |last4=Cohen |first4=Elaine |pages=447–452 |isbn=978-0-89791-999-9 |s2cid=1411270 }}

==External links==
*[https://web.archive.org/web/20070512002403/http://java.cms.livjm.ac.uk/homepage/staff/cmsdengl/ws/ Tunde Cockshott's Wet and Sticky revisited]
*[http://www.red3d.com/cwr/npr/ Stylized Depiction in Computer Graphics: An annotated survey of online NPR resources]
*[http://www.npar.org/ NPAR conference]
*[http://blendernpr.org Blender NPR: Dedicated to Stylize and Non-Photorealistic Rendering]
*[http://www.cs.cf.ac.uk/npr/ Online image based NPR system]

{{Animation}}

{{DEFAULTSORT:Non-Photorealistic Rendering}}
[[Category:3D rendering]]
[[Category:Computer graphics]]