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. 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
graphics complex of a seashell with toon shading modeled in Mathematica 13.1
graphics complex of a seashell with toon shading modeled in Mathematica 13.1

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 shading algorithm. 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:

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.
Finally, 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.

The Sega Dreamcast title Jet Set Radio, which was revealed at the 1999 Tokyo Game Show, drew media attention for its cel-shaded style.[4][5] It used cel-shading for its characters and its vibrant visual style has had a lasting influence on the use of cel-shading in video games. Since the early 2000s, many notable video games have made use of this style, such as The Legend of Zelda: The Wind Waker (2002) and Ōkami (2006).

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 described 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."[6] 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.[6]

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 allowed to “represent the mechanisms and objects for puzzles [in The Wind Waker] in a more easy-to-understand way.”[7] Takizawa also argues that photorealistic graphics, in contrast, would have “had the adverse effect of making information difficult to represent game-wise.”[7]

Lists of cel-shaded media

Film

Television

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

See also

References
  1. "Stylized Rendering Post Processing". docs.unrealengine.com. Retrieved 2019-09-13.
  2. Hachigian, Jennifer. "Celshader.com FAQ". Archived from the original on February 23, 2022. Retrieved August 2, 2005.
  3. Luque, Raul (December 2012). The Cel Shading Technique (PDF). Retrieved December 2, 2014.
  4. "IGNDC Interviews The Creators of Jet Grind Radio". IGN. 26 May 2000. Retrieved 12 August 2021.
  5. "TGS: Sega Shows Jet Grind Radio". GameSpot. September 19, 1999. Archived from the original on July 18, 2014. Retrieved August 12, 2009.
  6. "Okami – 2004 Developer Interviews". shmuplations.com. Retrieved 2021-06-01.
  7. "Iwata Asks: The Legend of Zelda: The Wind Waker HD". iwataasks.nintendo.com. Retrieved 2021-06-01.
  8. Tadeo, Jericho (August 10, 2021). "Why Some People Are Bothered By Marvel's What If Animation Style". Screen Rant. Archived from the original on August 11, 2021. Retrieved August 29, 2021.
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