Abstract: A cel image is rendered by a computer to have a "toon" look that simulates a traditional hand-drawn cartoon cel. The toon appearance of a cel image is characterized by the use of areas selectively colored with a fill, a highlight, or a shadow color. The present invention uses a material shader modified to return a value defining the color applied to each area in the scene being rendered as a function of a predetermined highlight threshold angle, which is compared to the angle of each light ray intersecting surfaces in the image. An edge detection convolution filter is applied to define color lines between the different color areas in the colored image. Contour lines defining the shape of an object and lines on its interior surfaces are determined by applying an edge detection convolution filter to surface normal data for the colored image. Additional smoothstep or gain filters may be applied to improve the contrast of the color lines and contour lines and to thicken the color lines.