A Versatile Interactive 3D Brush Model William V. Baxter and Ming C. Lin University of North Carolina at Chapel Hill Department of Computer Science Chapel Hill, NC, USA {baxter,lin}@cs.unc.edu http://gamma.cs.unc.edu/BRUSH Abstract We present a flexible modeling approach capable of re- alistically simulating many varieties of brushes commonly used in painting. Our geometric model of brush heads is a combination of subdivision surfaces and hundreds of indi- vidual bristles represented by thin polygonal strips. We ex- ploit bristle-to-bristle coherence, simulating only a fraction of the bristles and using interpolation for the remainder. Our dynamic model incorporates realistic physically-based deformation, including anisotropic friction, brush plasticity, and tip spreading. We use an energy minimization frame- work with a novel geometric representation of the brush head to generate a wider variety of brushes. Finally, we have developed an improved haptic model that provides re- alistic force feedback, directly related to the results of the brush dynamic simulation. Using this model, we are able to simulate a wide range of brush styles and create an excellent variety of strokes such as the crisp, curvy strokes of Western decorative painting, or rough scratchy strokes like certain Oriental calligraphy. We have also developed an exporter for a popular free 3D mod- eling package that makes it easier for non-programmers to create any desired style of brush, real or fanciful. 1. Introduction “There is no item of greater importance to the successful execution of a painting than a sufficient quantity of the very- highest-grade brushes that it is possible to find.” [7]. So be- gins Mayer’s discussion of brushes in The Artist’s Hand- book. Having the proper brushes is critical to good paint- ing. A good set of brushes can enable a competent artist to quickly create virtually any effect he or she can imag- ine, from the intricate detail of cresting waves, leafy trees and delicate flower petals, to wispy billowing clouds, and the subtly blended shifting hues in a sunset. Digital artists can benefit greatly from having this expressive power avail- able in digital painting programs. Figure 1. A painting created using our new brush models. Notice the subtle shading and shape of the petals which were each created with a single, complex stroke. Though there are many types of brushes commonly used in painting (see Figure 2), they share certain properties that make them effective tools for applying paint to a sur- face in accordance with an artist’s intentions. Artistic ref- erences such as [7] describe desirable attributes of high- quality brushes with terms such as “elasticity”, “durabil- ity” and “ability to maintain a point”. For a virtual brush, however, the greatest challenge is capturing the most ba- sic physical attributes such as stability, passivity, interactiv- ity, and proper reaction to friction, which can all be taken