Non-photorealistic rendering (NPR) is an area of
computer graphics
Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
that focuses on enabling a wide variety of expressive
style
Style is a manner of doing or presenting things and may refer to:
* Architectural style, the features that make a building or structure historically identifiable
* Design, the process of creating something
* Fashion, a prevailing mode of clothing ...
s for digital art, in contrast to traditional
computer graphics
Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
, which focuses on
photorealism. NPR is inspired by other artistic modes such as
painting
Painting is the practice of applying paint, pigment, color or other medium to a solid surface (called the "matrix" or "support"). The medium is commonly applied to the base with a brush, but other implements, such as knives, sponges, and ai ...
,
drawing,
technical illustration
Technical Illustration is illustration meant to visually communicate information of a technical nature. Technical illustrations can be components of technical drawings or diagrams. Technical illustrations in general aim "to generate expressive ...
, and
animated cartoons. NPR has appeared in
movies
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
and
video game
Video games, also known as computer games, are electronic games that involves interaction with a user interface or input device such as a joystick, controller, keyboard, or motion sensing device to generate visual feedback. This fee ...
s in the form of
cel-shaded animation
Cel shading or toon shading is a type of non-photorealistic rendering designed to make 3-D computer graphics appear to be flat by using less shading color instead of a shade gradient or tints and shades. A cel shader is often used to mimic ...
(also known as "
toon" shading) as well as in
scientific visualization
Scientific visualization ( also spelled scientific visualisation) is an interdisciplinary branch of science concerned with the visualization of scientific phenomena. Michael Friendly (2008)"Milestones in the history of thematic cartography, s ...
,
architectural illustration
Architectural rendering, architectural illustration, or architectural visualization is the art of creating three-dimensional images or animations showing the attributes of a proposed architectural design.
Computer generated renderings
Images tha ...
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".
The term has received some criticism:
* The term "
photorealism" has different meanings for graphics researchers (see "
photorealistic rendering
__NOTOC__
Within the field of computer graphics, unbiased rendering refers to any rendering technique that does not introduce systematic error, or bias, into the radiance approximation. The term refers to statistical bias, not the broader mean ...
") 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, running on even numbered years. From 2007 onward, NPAR began to also run on odd-numbered years, co-located with ACM
SIGGRAPH.
3D
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
Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is ...
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
A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, mobi ...
's,
shaders
In computer graphics, a shader is a computer program that calculates the appropriate levels of light, darkness, and color during the rendering of a 3D scene - a process known as ''shading''. Shaders have evolved to perform a variety of speci ...
have allowed NPR effects to be applied to the
rasterised
In computer graphics, rasterisation (British English) or rasterization (American English) is the task of taking an image described in a vector graphics format (shapes) and converting it into a raster image (a series of pixels, dots or lines, whi ...
image that is to be displayed to the screen. 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
Gooch shading is a non-photorealistic rendering technique for shading objects. It is also known as "cool to warm" shading, and is widely used in technical illustration. History
Gooch shading was developed by Amy Gooch et al. at the University of ...
.
Many methods can be used to draw stylized outlines and strokes from 3D models, including
occluding contours and
Suggestive contours.
For enhanced legibility, the most useful
technical illustration
Technical Illustration is illustration meant to visually communicate information of a technical nature. Technical illustrations can be components of technical drawings or diagrams. Technical illustrations in general aim "to generate expressive ...
s for
technical communication
Technical communication is used to convey scientific, engineering, or other technical information. Individuals in a variety of contexts and with varied professional credentials engage in technical communication. Some individuals are designated as ...
are not necessarily photorealistic. Non-photorealistic renderings, such as
exploded view
An exploded-view drawing is a diagram, picture, schematic or technical drawing of an object, that shows the relationship or order of assembly of various parts.
It shows the components of an object slightly separated by distance, or suspended in ...
diagrams, greatly assist in showing placement of parts in a complex system.
2D
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'') traditionally focused upon heuristic algorithms that seek to simulate the placement of brush strokes on a digital canvas.
Arguably, the earliest example of 2D NPR is
Paul Haeberli
Paul E. Haeberli is an American computer graphics programmer and researcher.
Biography
Paul Haeberli studied for a Bachelor of Science degree in electrical engineering at the University of Wisconsin–Madison, United States.
Haeberli was recruite ...
'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 or statistical moments 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, or segmentation operators to drive stroke placement. Around this time, machine learning began to influence image stylization algorithms notably
image analogy 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
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
of ink through different kinds of
paper
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distrib ...
, and also of
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s through water for simulation of
watercolor
Watercolor (American English) or watercolour (British English; see spelling differences), also ''aquarelle'' (; from Italian diminutive of Latin ''aqua'' "water"), is a painting method”Watercolor may be as old as art itself, going back to ...
.
Artistic rendering
Artistic rendering is the application of
visual art styles to rendering. For
photorealistic
Photorealism is a genre of art that encompasses painting, drawing and other graphic media, in which an artist studies a photograph and then attempts to reproduce the image as realistically as possible in another medium. Although the term can be ...
rendering styles, the emphasis is on accurate reproduction of light-and-shadow and the surface properties of the depicted objects,
composition
Composition or Compositions may refer to:
Arts and literature
*Composition (dance), practice and teaching of choreography
*Composition (language), in literature and rhetoric, producing a work in spoken tradition and written discourse, to include v ...
, 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
Art is a diverse range of human activity, and resulting product, that involves creative or imaginative talent expressive of technical proficiency, beauty, emotional power, or conceptual ideas.
There is no generally agreed definition of wha ...
and level of
abstraction in
abstract art. In
computer graphics
Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
, interpretive rendering styles are known as non-photorealistic rendering styles, but may be used to simplify
technical illustration
Technical Illustration is illustration meant to visually communicate information of a technical nature. Technical illustrations can be components of technical drawings or diagrams. Technical illustrations in general aim "to generate expressive ...
s. 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
Cel shading or toon shading is a type of non-photorealistic rendering designed to make 3-D computer graphics appear to be flat by using less shading color instead of a shade gradient or tints and shades. A cel shader is often used to mimic ...
for a list of uses of toon-shading in games and movies.
References
Further reading
Some key papers in the development of NPR are:
*
*
*
*
*
*
*
External links
Tunde Cockshott's Wet and Sticky revisitedStylized Depiction in Computer Graphics: An annotated survey of online NPR resourcesNPAR conferenceBlender NPR: Dedicated to Stylize and Non-Photorealistic RenderingOnline image based NPR system
{{DEFAULTSORT:Non-Photorealistic Rendering
3D rendering
Computer graphics