The morphological development of a wood burl shader

In the field of computer graphics, shaders provide an interface between lights and surfaces, giving the appearance of metal, plastic, wood, etc. As the field progresses, more and more shaders are required to simulate a wider and wider variety of materials. We present a new shader for the simulation of wood burl, a complex material used in furniture, art, car interiors, and a host of other luxury items. This shader was developed through a morphological approach - a study of the original material, its structure, and growth. Consequently, research began with a thorough look at wood burl, polished and unpolished, in an assortment of different species. We discovered the appearance can be broken into three sub-appearances - knots, curl, and a subtle undergrain. These three sub-appearances interact to create the characteristic swirls and whorls of burl. For the subtle undergrain, we used a common oak shader, added noise, and faded it into the background. We then developed a system of randomly placing points through the material to act as knots. Since the knots grow and distort the surrounding grain, we used distance-scaled forces to push the surface coordinates around and between all the knots. When the oak shader is applied, it appears to swirl and curl around the knots, much like a stream between rocks. This created the first level of curl or swirly grained wood, but one level alone appeared flat. To solve this, we procedurally blended levels of curl to give a look of increased depth. Finally, we added reflection, gloss, and other surface properties to give a look of warmth and polish. All of these properties are controlled by a set of parameters in the shader's interface. By adjusting these parameters, the user can emulate a variety of different burl types.