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From Wikipedia, the free encyclopedia

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, which are painted on for use in traditional 2D animation, such as Disney classics.[1]

Process

The cel-shading process starts with a typical 3D model. Where cel-shading differs from conventional rendering is in its use of non-photorealistic lighting. Conventional (smooth) lighting values are calculated for each pixel and then mapped to a small number of discrete shades to create the characteristic flat look – where the shadows and highlights appear more like blocks of color rather than mixed in a smooth way.

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. Backface 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. Alternately, 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. Popularly, this "ink" outline applied to animation and games is what’s called cel shading, while originally the term referred to the shading technique, regardless of whether outline is being applied or not.

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).

Another outlining technique is to use 2D image-processing. First, the scene is rendered (with cel-shading) to a screen-sized color texture:

Cel shading no outlines.png

Then, the scene's depth and world-space surface normal information are rendered to screen-sized textures:

Cel shading depth.png

Cel shading normals.png

A Sobel filter or similar edge-detection filter is applied to the normal/depth textures to generate an edge texture. Texels on detected edges are black, while all other texels are white:

Cel shading edge detection.png

Finally, the edge texture and the color texture are composited to produce the final rendered image:

Cel shading composite final image.png

Contrary to most image-processing techniques, the performance penalty for this method is not affected by scene complexity.

References

  1. ^ Celshader.com FAQ Retrieved August 2, 2005.

External links

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Gaming

Up to date as of February 01, 2010
(Redirected to Cel-shaded animation article)

From Wikia Gaming, your source for walkthroughs, games, guides, and more!

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.

Appearances

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

Smallwikipedialogo.png This page uses content from Wikipedia. The original article was at Cel-shaded animation. The list of authors can be seen in the page history. As with Wikia Gaming, the text of Wikipedia is available under the Creative Commons Attribution-Share Alike 3.0 (unported) license. The content might also be available under the terms of the GNU Free Documentation License.

This article uses material from the "Cel-shaded animation" article on the Gaming wiki at Wikia and is licensed under the Creative Commons Attribution-Share Alike License.

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