Cel shading, cell 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 the style of a comic book or cartoon and/or give the render a characteristic paper-like texture. There are similar techniques that can make an image look like a sketch, an oil painting or an ink painting. It appeared around the beginning of the twenty-first century. The name comes from cels (short for celluloid), clear sheets of acetate which were painted on for use in traditional 2D animation.
The cel-shading process starts with a typical 3D model. Where cel-shading differs from conventional rendering is in its non-photorealistic illumination model. Conventional smooth lighting values are calculated for each pixel and then quantized to a small number of discrete shades to create the characteristic "flat look", where the shadows and highlights appear as blocks of color rather than being smoothly mixed in a gradient.
Black "ink" outlines and contour lines can be created using a variety of methods. One popular method is to first render a black outline, slightly larger than the object itself. Back-face culling is inverted and the back-facing triangles are drawn in black. To dilate the silhouette, these back-faces may be drawn in wireframe multiple times with slight changes in translation. Alternatively, back-faces may be rendered solid-filled, with their vertices translated along their vertex normals in a vertex shader. After drawing the outline, back-face culling is set back to normal to draw the shading and optional textures of the object. Finally, the image is composited via Z-buffering, as the back-faces always lie deeper in the scene than the front-faces. The result is that the object is drawn with a black outline and interior contour lines. The term "cel-shading" is popularly used to refer to the application of this "ink" outlining process in animation and games, although originally the term referred to the flat shading technique regardless of whether the outline was applied.
The Utah teapot rendered using cel shading:
Steps 2 and 3 can be combined using multitexturing (part of texture mapping).
For a more comprehensive list, see List of cel-shaded video games.
The first documented video game to use cel shading extensively is the PlayStation title Fear Effect, released in February of 2000. Unlike later games that use the same technique, Fear Effect does not use cel shading for the entirety of its graphics; only its character models are cel shaded. The Dreamcast video game Jet Set Radio, released later the same year, used cel shading for its characters as well, and its vibrant visual style has had lasting influence on the use of cel shading in video games. Since the early 2000's many notable video games have made use of this style, such as Ōkami and The Legend of Zelda: The Wind Waker.
Cel shading, in contrast to other visual styles such as photorealism, is often used to lend a more artistic or fantastical element to a video game's environment. In developing Ōkami, director Hideki Kamiya describes his vision for the game's graphics: "I wanted to create a game with the natural beauty of the Japanese countryside... to make a world that was glistening and beautiful." Producer Atsushi Inaba recalls in a 2004 interview that Clover Studios had "abandoned the realistic style" for Ōkami as they became inspired by traditional Japanese art.
Game studios might choose a style such as cel shading in their development for reasons beyond artistic vision. Cel shaded graphics are usually simple in visual information, which can be useful in some applications. In the case of The Legend of Zelda: The Wind Waker, developer Satoru Takizawa states that using this style made it easier to "represent the mechanisms and objects for puzzles [in The Wind Waker] in a more easy-to-understand way". Takizawa also argues that photorealistic graphics, in contrast, would have "had the adverse effect of making information difficult to represent game-wise".
Main article: List of cel-shaded video games