04/2 Mesh-free, point-based visualization methods

		Four stages of a cutting operation in a hysteroscopy simulator. Point-based representations are better suited than traditional meshed models for handling operations that produce topological changes.

As an alternative to traditional mesh-based methods, point-based rendering and modeling have received growing attention in the computer graphics community. In recent years, we have observed the advent of high-quality rendering techniques, level-of-detail approaches for dealing with very large models, and even efficient GPU-based renderers that continue the efforts to obtain interactive rendering speeds. Point-sampled representations have also demonstrated to be an attractive approach for surface modeling and editing.

Traditional methods for soft tissue deformation have employed mesh-based representations. Operations such as cutting, tearing, or mending of tissue, however, impose efficiency problems in the presence of mesh-based representations, due to the costly maintenance of mesh connectivity information. As an alternative to mesh-based representations, Co-Me phase 2 undertakes the use of mesh-free point-based representations. It is expected that purely point-sampled approaches will have significant advantages compared to triangle or tetrahedral meshes when it comes to dynamic restructuring or topological changes.

The use of mesh-free representations will require new modeling and visualisation approaches. Most of the existing algorithms and acceleration structures for point-based rendering share various limitations making them impracticable for interactive surgical simulation. For instance, the most efficient methods include substantial preprocessing of the point cloud and store it in a hierarchical data structure, which is traversed during rendering. For deformable objects or topological changes, neighborhood relations between point samples change dynamically and demand instantaneous updates of the acceleration structures. Cutting operations very often involve sharp edges that occur along the cut boundary. Such edges have to be represented and rendered separately, since conventional point blending leads to undesired smoothing artifacts.