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 styles 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 movies and video games in the form of cel-shaded animation (also known as "toon" shading) as well as in scientific visualization, architectural illustration and experimental animation.

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".[1][2]

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 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.
  • Many techniques that are used to create 'non-photorealistic' images are not 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 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 ACM-sponsored Symposium on Non-Photorealistic Rendering and Animation (NPAR) in 2000. NPAR is traditionally co-located with the Annecy Animated Film Festival,[3] running on even numbered years. From 2007 onward, NPAR began to also run on odd-numbered years, co-located with ACM SIGGRAPH.[4]

3D

An example of NPR used for technical illustrations

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 3D model that has been modified from the original input model to portray a new artistic style. In many cases, the 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.[5] 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.[6]

For enhanced legibility, the most useful technical illustrations 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.

2D

A non-photorealistic rendering of an existing 2D (photographic) image
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[7]) traditionally focused upon heuristic algorithms that seek to simulate the placement of brush strokes on a digital canvas.[8]

Arguably, the earliest example of 2D NPR is Paul Haeberli's '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[9] or statistical moments[10] 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 (1998).

More sophisticated image abstractions techniques were developed in the early 2000s harnessing computer vision operators e.g. image salience,[11] or segmentation[12] operators to drive stroke placement. Around this time, machine learning began to influence image stylization algorithms notably image analogy[13] 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

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 pigments through water for simulation of watercolor.

Artistic rendering

Artistic rendering is the application of visual art styles 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, 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 illustrations. Rendering styles that combine photorealism with non-photorealism are known as 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.

Short films
Technological Threat1988Early use of toon shading together with Tex Avery-style cartoon characters
Gas Planet1992Pencil-sketching 3D rendering by Eric Darnell
Fishing2000Watercolor-style 3D rendering David Gainey
RoadHead
Snack and Drink		1998
1999		Short films created with Rotoshop by Bob Sabiston
Ryan2004Nonlinear projection and other distortions of 3D geometry
The Girl Who Cried Flowers2008Watercolor-style rendering by Auryn
Feature films
What Dreams May Come1998Painterly rendering in the "painted world" sequence
Tarzan1999First use of Disney's "Deep Canvas" system
Waking Life2001First use of rotoshop in a feature film
A Scanner Darkly2006"a 15-month animation process"
Video games and other software
Jet Set Radio2000Early use of toon-shading in video games
SketchUp2000Sketch-like modelling software with toon rendering
The Legend of Zelda: The Wind Waker2002One of the most well-known cel-shaded games
Valkyria Chronicles2008Uses a number of NPR techniques in the game, including a sketch-like shading method
XIII2003A game made as "comic"-like as possible
Ōkami2006A game whose visuals emulate the style of 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
Vue Xstream20153D environment creation software featuring an NPR renderer with various traditional art style emulating presets
Return of the Obra Dinn2018A 3D game rendered in a unique monochrome, pointillist style
Manifold Garden2019A 3D puzzle game using impossible geometry, notable for its novel edge-shading techniques.[14]

References

  1. ↑ Baskett, Forest (1990). Proceedings of the 17th annual conference on Computer graphics and interactive techniques - SIGGRAPH '90. doi:10.1145/97879. ISBN 978-0-201-50933-5. S2CID 12199502.
  2. ↑ Schofield, Simon (March 1994). Non-photorealistic rendering: a critical examination and proposed system (Thesis).
  3. ↑ "Site officiel du Festival et du Marché international du film d'animation d'Annecy". www.annecy.org. Retrieved 20 April 2018.
  4. ↑ "Archived copy". siggraph.org. Archived from the original on 21 December 1996. Retrieved 13 January 2022.{{cite web}}: CS1 maint: archived copy as title (link)
  5. ↑ Drew, Card (2002). "Non-Photorealistic Rendering with Pixel and Vertex Shaders". In Engel, Wolfgang F. (ed.). Direct3D Shaderx: Vertex & Pixel Shader Tips and Techniques. 87Wordware Game Developer's Library. Wordware Publishing. pp. 319–333. CiteSeerX 10.1.1.360.918. ISBN 978-1-55622-041-8.
  6. ↑ Bénard, Pierre; Hertzmann, Aaron (2019). "Line Drawings from 3D Models: A Tutorial". Foundations and Trends in Computer Graphics and Vision. 11 (1–2): 1–159. arXiv:1810.01175. doi:10.1561/0600000075. S2CID 52912187.
  7. ↑ Rosin, Paul; Collomosse, John, eds. (2013). Image and Video-Based Artistic Stylisation. Computational Imaging and Vision. Vol. 42. doi:10.1007/978-1-4471-4519-6. ISBN 978-1-4471-4518-9. S2CID 40656135.
  8. ↑ Kyprianidis, Jan Eric; Collomosse, John; Wang, Tinghuai; Isenberg, Tobias (May 2013). "State of the 'Art': A Taxonomy of Artistic Stylization Techniques for Images and Video" (PDF). IEEE Transactions on Visualization and Computer Graphics. 19 (5): 866–885. doi:10.1109/TVCG.2012.160. PMID 22802120. S2CID 2656810.
  9. ↑ Litwinowicz, Peter (1997). "Processing images and video for an impressionist effect". Proceedings of the 24th annual conference on Computer graphics and interactive techniques - SIGGRAPH '97. pp. 407–414. doi:10.1145/258734.258893. ISBN 978-0-89791-896-1. S2CID 13139308.
  10. ↑ Shiraishi, Michio; Yamaguchi, Yasushi (2000). "An algorithm for automatic painterly rendering based on local source image approximation". Proceedings of the first international symposium on Non-photorealistic animation and rendering - NPAR '00. pp. 53–58. doi:10.1145/340916.340923. ISBN 978-1-58113-277-9. S2CID 16915734.
  11. ↑ Collomosse, J.P.; Hall, P.M. (2002). "Painterly rendering using image salience". Proceedings 20th Eurographics UK Conference. pp. 122–128. CiteSeerX 10.1.1.7.5383. doi:10.1109/EGUK.2002.1011281. ISBN 978-0-7695-1518-2. S2CID 9610948.
  12. ↑ Gooch, Bruce; Coombe, Greg; Shirley, Peter (2002). "Artistic Vision". Proceedings of the second international symposium on Non-photorealistic animation and rendering - NPAR '02. p. 83. CiteSeerX 10.1.1.10.1761. doi:10.1145/508530.508545. ISBN 978-1-58113-494-0. S2CID 1146198.
  13. ↑ Hertzmann, Aaron; Jacobs, Charles E.; Oliver, Nuria; Curless, Brian; Salesin, David H. (2001). "Image analogies". Proceedings of the 28th annual conference on Computer graphics and interactive techniques - SIGGRAPH '01. pp. 327–340. CiteSeerX 10.1.1.119.5127. doi:10.1145/383259.383295. ISBN 978-1-58113-374-5. S2CID 2201072.
  14. ↑ Brussee, Arthur; Saraev, Andrew; Chyr, William (2020). "That's a wrap: Manifold Garden rendering retrospective". Special Interest Group on Computer Graphics and Interactive Techniques Conference Talks. pp. 1–2. doi:10.1145/3388767.3407385. ISBN 978-1-4503-7971-7. S2CID 221178781.

Further reading

Some key papers in the development of NPR are:

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