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 has focused on photorealism, NPR is inspired by artistic styles such as painting, drawing, technical illustration, and animated cartoons. NPR has appeared in movies and video games in the form of "toon shaders," as well as in architectural illustration and experimental animation. An example of a modern use of this method is that of Cel-shaded animation.

Terminology

The term "non-photorealistic rendering" was probably coined by David Salesin and George Winkenbach in a 1994 paper. It is widely disliked by researchers in the area, because:

• The term "photorealism" means something different to graphics researchers than it does to artists, who are the target consumers of NPR techniques. For artists, 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 involves. For graphics researchers, 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 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 more general term "computer graphics", with "photorealistic graphics" being 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 term NPR seems to have none-the-less taken hold.

Three-Dimensional NPR Techniques

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. The majority of NPR techniques applied to 3D geometry are intended to make the scene appear two-dimensional.

Two-Dimensional NPR Techniques

The input to a two-dimensional NPR system is most commonly an image; however, there are systems that take 3D geometry information as input and produce a 2D image or video as output. Again, many of the systems are intended to mimic a desired artistic style, such as watercolor, impressionism, or pen and ink drawing.

NPR for Enhancing Legibility

The most useful illustrations in technical illustrations are not necessarily photorealistic. Non-photorealistic renderings, such as exploded view diagrams, greatly assist in showing placement of parts in a complex system.

Interactive Techniques

Users who are interested in having much more control in the NPR process may be more interested in interactive techniques. Many of these NPR systems 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.

Simulating the Artistic Media

In contrast to the methods mentioned previously, another technique in NPR is simulating the painter's medium. Methods include simulating the diffusion of ink through different kinds of paper, and also of pigments through water for simulation of watercolor.

The Artistic Process

The process of creating images using NPR is quite different from the normal artistic process, despite producing arguably similar results. Before creating a piece of art from a scene, the artist is likely to take into account:

• a philosophical representation of the object: what sort of ideas they would like their art to convey, and what techniques to use to achieve their purpose
• a physical representation of the object: its geometry, color, and relationship with surrounding objects

A computer is incapable of generating a philosophical representation of the object, so the user of the system must instead specify what sort of artistic technique they would like to use. A physical representation of the image is difficult to produce, but can be done automatically by a computer. NPR systems employ many techniques within the realm of image processing, especially edge detection and image segmentation, in an attempt to produce meaningful physical data during image preprocessing.

References

Some key papers in the development of NPR are:

• "Paint by Numbers: Abstract Image Representations", by Paul Haeberli, SIGGRAPH 90
• "Comprehensible rendering of 3-D shapes", by Saito and Takahashi, SIGGRAPH 90
• "Computer-Generated Pen-and-Ink Illustration", by Winkenbach and Salesin, SIGGRAPH 94
• "Interactive Pen-and-Ink Illustration", by Salisbury, Anderson, Barzel, Salesin, SIGGRAPH 94
• "Painterly Rendering for Animation", by Barb Meier, SIGGRAPH 96

External links

• www.cybercoalition.com More examples of NPR and cel shading
• Paul Haeberli's Impressionist applet
• Stylized Depiction in Computer Graphics: An annotated survey of online NPR resources
• NPAR conference

Computer graphics

Non-photorealistic Rendering | Nem fotorealisztikus képszintézis | 非真实感绘制