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. 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, clear sheets of acetate which were painted on for use in traditional 2D animation.

Basic process

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.
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.

Edge-detection method

In video games

Starting in the 2000s, cel shading became synonymous in interactive media with the style of the Dreamcast game Jet Set Radio, though Jet Set Radio wasn't the very first video game to use this technique. This rendering technique has been applied in numerous other games over the years, including such notable titles as Guilty Gear Xrd, Astral Chain, Killer7, Cel Damage, The House of the Dead III, No More Heroes, ', Auto Modellista, ', Viewtiful Joe, Ni No Kuni, Escape Dead Island, ', The Wolf Among Us, Ōkami, Ultimate Spider Man, Punch-Out!!, One Piece Mansion, ', Dragon Quest VIII, MadWorld, and Ollie King.

Lists of cel-shaded media

Video games

Some prominent games featuring cel shading include