Cel-shaded animation

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Object with a basic cel-shader (also known as a toon shader) and border detection.
Object with a basic cel-shader (also known as a toon shader) and border detection.

Cel-shaded animation (also called cel-shading or toon shading) is a type of non-photorealistic rendering designed to make computer graphics appear to be hand-drawn. Cel-shading is often used to mimic the style of a comic book or cartoon. It is a somewhat recent addition to computer graphics, most commonly turning up in console video games. Though the end result of cel-shading has a very simplistic feel like that of hand-drawn animation, the process is complex. The name comes from the clear sheets of acetate, called cels, that are painted on for use in traditional 2D animation, such as Disney classics.

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The cel-shading process starts with a typical 3D model. The difference occurs when a cel-shaded object is drawn on-screen. The rendering engine only selects a few shades of each color for the object, producing a flat look. This is not the same as using only a few shades of texture for an object, as lighting and other environmental factors would come into play and ruin the effect. Therefore, cel-shading is often implemented as an additional rendering pass after all other rendering operations are completed.

In order to draw black ink lines outlining an object's contours, the backface culling is inverted to draw back-faced triangles with black-colored vertices. The vertices must be drawn multiple times with a slight change in translation to make the lines "thick". This produces a black-shaded silhouette. The back-face culling is then 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 even contours that reside inside the object's surface in screen space.

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 is drawn with a basic texture
  3. Shading

Steps 2 and 3 can be combined using multi-texturing (see texture mapping).

The Legend of Zelda: The Wind Waker is well-known for its cel-shaded style.
The Legend of Zelda: The Wind Waker is well-known for its cel-shaded style.

The first 3D video game to feature true real-time cel-shading was Jet Set Radio (2000) for the Sega Dreamcast. Another game, Fear Effect for the Sony PlayStation, was released in 2000 and was noted for its use of dramatic textures to give an anime appearance to its characters, but lacked outlines and dynamic light-sourcing. Games before Fear Effect have used textures in a similar fashion, but not as starkly apparent or stylized as the game. Wacky Races, released on Dreamcast a few months before Jet Set Radio, featured an outline effect often mistaken for cel-shading, but the game actually used traditional shading techniques. The are several other games, such as Megaman Legends, that used static cel-shaded textures (without real-time lighting) in the same vein as Fear Effect created before the previous examples, but the effect created was not as strongly noticeable as the previous examples.

In the years following Jet Set Radio, numerous other cel-shaded games were introduced during a minor fad involving cel-shaded graphics, yet only a few would fully match or surpass its mainstream appeal. The next games with cel-shading to capture the industry's attention in some form were 2002's Jet Set Radio Future and Sly Cooper and the Thievius Raccoonus. Over time, more cel-shaded titles such as Dark Chronicle, Cel Damage, Klonoa 2, the Viewtiful Joe series, and XIII were released with positive feedback, though none were considered blockbusters in terms of sales figures. Originally the only cel-shaded games to receive both positive ratings and sales after Sly Cooper and the Thievius Raccoonus were The Legend of Zelda: The Wind Waker, Sly 2: Band of Thieves, and Tales of Symphonia.

The game Ōkami uses cel-shading to give the impression of a world inside a painting.
The game Ōkami uses cel-shading to give the impression of a world inside a painting.

Originally, The House of the Dead III (HotD3) for the Microsoft Xbox was cel-shaded. Early in HotD3’s development Sega released screenshots of the then current cel-shaded graphics to the gaming community. Shortly after those initial screenshots were released, Sega announced that they were dropping the cel-shaded graphics in favor of conventional graphic techniques. There are several suspected reasons for Sega's change of heart, the most popular and most likely is that the screenshots met much negative response from gamers who disliked the cel-shaded graphical style. Many gamers claimed the cel-shading was used purely as a gimmick in an attempt to sell more games. HotD3 was a bloody, gory and very violent light gun game which featured zombies and other mutated and deformed creatures. Many felt the cel-shaded look clashed greatly with the game's themes and content.

More recently, handheld consoles, previously not realistically powerful enough to render a fully 3D world, have made use of cel-shading. Most notable is the Tony Hawk series and The Legend of Zelda: Phantom Hourglass on the Nintendo DS.

The use of cel-shading in video games has slowed somewhat since its inception, but the technique continues to be employed in the modern era. Recent examples include Dragon Quest VIII, Rogue Galaxy Killer 7,The Legend of Zelda: Phantom Hourglass , Metal Gear Acid 2, and Ōkami.

Main article: List of cel-shaded video games

Some of the more prominent games that have featured cel-shaded graphics:

In addition, many prominent movies, television programs, and commercials also use cel-shading:

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Interpolated rotoscoping can be used to create a similar effect, but the source material does not need to be computer generated. In addition, rotoscoping is applied to an existing image, whereas cel-shading is applied during the generation of the image.

Retrieved from "http://en.wikipedia.org/wiki/Cel-shaded_animation"
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