Contour lines can be an important visual cue to distinguish objects and accentuate their forms and spatial relationship. Contour lines are especially useful for cartoon animation production.

Contours can be placed at discontinuities of depth or surface orientation, between different materials, or where the color contrast is high. The contour lines are anti-aliased, and there can be several levels of contours created by reflection or seen through semitransparent materials.

The contours can be different for each material, and some materials can have no contours at all. The color and thickness of the contours can depend on geometry, position, illumination, material, frame number, and various other parameters.

The resulting image may be output as a pure contour image, a contour image composited onto the regular image (in raster form in any of the supported formats), or as a PostScript file.

It is not possible to render contours in a scene with motion blur or rasterizer (formerly called Rapid Motion), and jittering must be turned off. Contour rendering is multithreaded, but cannot make use of slave hosts. Contour rendering requires the default box 1 1 filter.

Contour shaders are called while the normal color image is created. Contours are computed using information stored by a contour store shader. The contour store shader is called once for each intersection of a ray with a material. The position of contours are determined by a contour contrast shader. It compares the two sets of information for a pair of points, and decides whether there should be a contour between the points. The color and thickness of the contours are determined by contour shaders.

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