Force and contact location shading thresholds for smoothly rendering polygonal models

Providing tactile feedback in concert with kinesthetic information can dramatically improve one´s ability to interact with and explore virtual environments. However, the addition of tactile feedback also enhances the saliency of the vertices and edges of polygonal objects, which are commonly used as model representations in haptic environments. When the polygonal model is an approximate representation of a smooth surface, the heightened response to discontinuities reduces the effectiveness of tactile feedback. This paper addresses this issue by developing a smoothing algorithm, which can be used to provide both tactile and force shading. To investigate the increased sensitivity and effectiveness of our algorithm we conducted four perceptual experiments. Each experiment differed by the type of haptic feedback and whether haptic shading was rendered. Participants were asked to discriminate between an ideal smooth cylinder and its polygonal approximation. The number of polygons used to render the comparison stimulus was increased until the two objects were indistinguishable. Our results are reported as the maximum recommended angular change between adjacent polygons for rending smooth objects. It was found that the addition of contact location feedback significantly increased the sensitivity of the user to the discontinuities present in polygonal models. Use of shading algorithms was able to significantly reduce the sensitivity to edges in the environment, which allows an increase in the angle between adjacent polygonal surfaces. Our results can be used as a guideline for constructing polygon models that are meant to feel smooth.