GPU-based Appearance Preserving Trimmed NURBS Rendering

Trimmed NURBS are the standard surface representation used in CAD/CAM systems and accurate visualization of trimmed NURBS models at interactive frame rates is of great interest for industry. To support modification and/or animation of such surfaces, a GPU-based trimming and tessellation algorithm has been developed recently. First, the NURBS is approximated with a bi-cubic hierarchy of Bézier patches on the CPU and then these are tessellated on the GPU. Since this approach only took the geometric error of an approximation into account, the various illumination artifacts introduced by the chosen bi-cubic approximation and the subsequent tessellation were neglected. Although this problem could be solved partially by calculating exact per-pixel normals on the GPU, the shading error introduced due to the bi-cubic approximation would remain. Furthermore, the long fragment shader required for per-pixel normals would lead to unacceptably low performance. In this paper we present a novel bi-cubic approximation algorithm that takes the normal approximation error into account. In addition, we also define a new error measure to calculate the required grid resolution for the bi-linear approximation. In combination, this allows GPU-based NURBS tessellation with guaranteed visual fidelity. Our new method is also capable of high quality visualization of further attributes like curvature, temperature, etc. on surfaces with little or no modification.