A haptic object-oriented texture rendering system

In the questfor visual realism in computer graphics, surface textures are generated for objects based on a texture image or some procedural model. Similar approaches can be used to make objects feel more realistic with a haptic intelface. By using an object oriented approach, a sofhvare structure was created to allow the inclusion of various texture rendering algorithms for a 3 DOF haptic device. 1. Texture Rendering System The basis for haptic texture generation is found in computer graphics texture rendering. A common technique for generating complex textures, is to map a texture image onto a surface, i.e., texture mapping. Due to the large memory requirement, and aliasing problems associated with texture mapping, procedural approaches were developed. A procedural approach uses a model or algorithm for a texture, usually controlled through a few parameters. The first implementation of haptic textures was achieved by Minsky, et al. [2], which used a texture mapping procedure for a 2 DOF force reflecting joystick. Another method was presented by Siira and Pai [3], which added a Gaussian deviation to a temporally sampled surface. The other methods implemented with this system were presented in [l] to allow a greater variety of textures. These stochastic methods are variations of procedural graphics texturing methods, which take advantage of the local nature of haptics. For the system presented here, haptic texturing techniques are classified into two general categories: height map (e.g., Minsky’s technique), and normal force vector perturbation. Our system is implemented with a 3 DOF haptic interface (the PHANTOM TM from SensAble Technologies, Inc.), which interacts with a virtual or remote environment as a point process. For the vector perturbation method, the resnltant force vector of an object is the sum of three components: the constraint force (normal to the surface), friction force, and the texture force (normal and/or tangential components). For the height map method, the constraint/texture force is determined from the surface gradients defined by the height map at the location of the haptic interface point. Friction is then added separately. Each object in the virtual environment is represented with a C++ Object base class. This class is contains pointers to a ForceProjile class, for determining the constraint and friction forces, and a Texture class. The Texture base class is defined by the type of texture (e.g., height map or normal perturbation), and its specific parameters, including the sampling method used [ 11. If the texture uses a stochastic function, a Noise class is instantiated, which also provides repeatability, i.e., the texture is dependent on location on the surface, and flexibility. To produce a wider variation of texture, and to-insure the stability of the haptic interface, a Filter class implements various filtering techniques. The basic structure of these classes allows for the addition of new algorithms without the need to restructure the entire software environment.