Contour lines for volumetric objects

Systems and methods automatically generate contours on an illustrated object for performing an animation. Contour lines are generated on the surface of the object according to criteria related to the shape of the surface of the object. Points of the contour lines that are occluded from a virtual camera are identified. The occluded points are removed to generate visible lines. The visible lines are extruded to define a three-dimensional volume defining contours of the object. The object itself, along with the three-dimensional volume, are illuminated and rendered. The parameters defining the opacity and color of the contour may differ from corresponding parameters of the rest of the object, so that the contours stand out and define portions of the object. The contours are useful in contexts such as defining areas of an object that is fuzzy or cloudy in appearance, as well as creating certain artistic effects.