Cel shading

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 the style of a comic book or cartoon and/or give the render a characteristic paper-like texture.[1] 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.[2]

A representation of a space suit from The Adventures of Tintin comic Explorers on the Moon with a basic cel shader (also known as a toon shader) and border detection

Basic process
Cel-shaded rendering of two isosurfaces of the probability density of a particle in a box

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.

Outlines

Wireframe method

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.[3]

The Utah teapot rendered using cel shading:

  1. The back faces are drawn with thick lines
  2. The object faces are drawn using a single color
  3. Shading is applied

Steps 2 and 3 can be combined using multitexturing (part of texture mapping).

Edge-detection method
2D image processing outlining technique
First, the scene is rendered with cel shading to a screen-sized color texture.
Then depth-information of the scene is rendered to a screen-sized texture.
World-space surface normals are rendered as a screen-sized texture.
And then...
A Sobel filter or similar edge-detection filter is applied to the normal and depth textures to generate an edge texture. Texels on detected edges are black, while all other texels are white.
Finally, the edge texture and the color texture are composited to produce the final rendered image.

In video games
For a more comprehensive list, see List of cel-shaded video games.

Starting in the 2000s, cel shading became synonymous in interactive media with the style of the Dreamcast game Jet Set Radio, but it has been applied in numerous other games over the years, including more recent titles such as Cel Damage, The House of the Dead III, No More Heroes and Grand Theft Auto: Chinatown Wars. Other notable examples include The Legend of Zelda: The Wind Waker, Viewtiful Joe, Ni No Kuni, Escape Dead Island, The Legend of Zelda: Breath of the Wild, The Wolf Among Us, Ōkami, Ultimate Spider Man, and Punch-Out!!.

Lists of cel-shaded media

Video games
Main article: List of cel-shaded video games

Some prominent games featuring cel shading include

Commercials

Television

Film
  • Appleseed, a 2004 Japanese animated film.
  • Mutafukaz: Operation Blackhead, a 2002 crime short film that became the basis for the comic book for the same name produced by Run.

See also

References
  1. "Stylized Rendering Post Processing". docs.unrealengine.com. Retrieved 2019-09-13.
  2. Hachigian, Jennifer. "Celshader.com FAQ". Retrieved August 2, 2005.
  3. Luque, Raul (December 2012). The Cel Shading Technique (PDF). Retrieved December 2, 2014.

(Wayback Machine copy)

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