Cel shading

An example of cel-shading from The Legend of Zelda: The Wind Waker

Cel-shaded animation or cel-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 relatively 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.

Process

The cel-shading process starts with 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 back-face culling is inverted to draw back-faced triangles with black-coloured 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.

History

The first video game to feature cel-shading was Jet Grind Radio for the Sega Dreamcast. Its release in the year 2000 was marked with high praise and average sales. As the industry evolved, many video game developers took notice of the game's stylish touches and began to implement cel-shading in their own projects. Cel Damage, an Xbox launch title, was one such attempt, though the game received mediocre reviews and sales. Auto Modellista and Robotech: Battlecry were two other cel-shaded ventures which met similar fates. Jet Set Radio Future, the sequel to Jet Grind Radio, was critically raved but had poor sales.

The next game with cel-shading to capture the market was Sly Cooper and the Thievius Raccoonus for the Sony Playstation 2. Over time, more cel-shaded titles such as Dark Cloud 2, Magic Pengel, and the Viewtiful Joe series were released to positive reviews, though none were considered to be blockbusters in terms of sales figures. The only cel-shaded games to receive both good ratings and sales after Sly Cooper and the Thievius Raccoonus were The Legend of Zelda: The Wind Waker and Sly 2: Band of Thieves.

Though the use of cel-shading has ebbed somewhat since its inception, the technique continues to be employed in games during the modern era. Recent examples include Dragon Quest VIII, Killer 7, and Metal Gear Acid 2.

Appearances

Some of the prominent games and films that have featured cel-shading graphics include: