Real-Time Rendering, Third Edition, 3rd Edition

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Chapter 11

Non-Photorealistic Rendering

“Using a term like ‘nonlinear science’ is like referring to the

bulk of zoology as ‘the study of nonelephant animals.’ ”

—Stanislaw Ulam

Photorealistic rendering attempts to make an image indistinguishable from

a photograph. Non-photorealistic rendering (NPR), also called stylistic

rendering, has a wide range of goals. One objective of some forms of NPR

is to create images similar to technical illustrations. Only those details

relevant to the goal of the particular application are the ones that should

be displayed. For example, a photograph of a shiny Ferrari engine may be

Figure 11.1. A variety of non-photorealistic rendering styles applied to a coffee grinder.

(Generated using LiveArt from Viewpoint DataLabs.)

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508 11. Non-Photorealistic Rendering

useful in selling the car to a customer, but to repair the engine, a simplified

line drawing with the relevant parts highlighted may be more meaningful

(as well as cheaper to print).

Another area of NPR is in the simulation of painterly styles and natural

media, e.g., pen and ink, charcoal, watercolor, etc. This is a huge field

that lends itself to an equally huge variety of algorithms that attempt to

capture the feel of various media. Some examples are shown in Figure 11.1.

Two different books give thorough coverage of technical and painterly NPR

algorithms [425, 1226]. Our goal here is to give a flavor of some algorithms

used for NPR in real time. This chapter opens with a detailed discussion of

ways to implement a cartoon rendering style, then discusses other themes

within the field of NPR. The chapter ends with a variety of line rendering

techniques.

11.1 Toon Shading

Just as varying the font gives a different feel to the text, different styles

of rendering have their own mood, meaning, and vocabulary. There has

been a large amount of attention given to one particular form of NPR, cel

or toon rendering. Since this style is identified with cartoons, it has strong

connotations of fantasy and (in the West, at least) childhood. At its sim-

plest, objects are drawn with solid lines separating areas of different solid

colors. One reason this style is popular is what McCloud, in his classic

book Understanding Comics [834], calls “amplification through simplifica-

tion.” By simplifying and stripping out clutter, one can amplify the effect

of information relevant to the presentation. For cartoon characters, a wider

audience will identify with those drawn in a simple style.

The toon rendering style has been used in computer graphics for well

over a decade to integrate three-dimensional models with two-dimensional

cel animation. It lends itself well to automatic generation by computer

because it is easily defined, compared to other NPR styles. Games such as

Okami and Cel Damage have used it to good effect. See Figure 11.2.

There are a number of different approaches to toon rendering. For

models with textures and no lighting, a solid-fill cartoon style can be ap-

proximated by quantizing the textures [720]. For shading, the two most

common methods are to fill the polygonal areas with solid (unlit) color or

to use a two-tone approach, representing lit and shadowed areas. Solid

shading is trivial, and the two-tone approach, sometimes called hard shad-

ing, can be performed by remapping traditional lighting equation elements

to different color palettes. This approach is related to the lighting model

work by Gooch et al. [155, 423, 424] for NPR technical illustration. Also,

silhouettes are often rendered in a black color, which amplifies the cartoon

look. Silhouette finding and rendering is dealt with in the next section.