Feature-preserving simplification and sketch-based creation of 3D models

A prototype of an innovative 3D thumbnail system for managing large 3D mesh databases is presented in this research. The goal is to provide an online 3D model exhibit page where the user can browse multiple 3D thumbnails interactively and efficiently.;An overall system framework for a large scale 3D repository is described. It includes an offline process and an online process. For the offline process, a 3D mesh is first decomposed into several significant components. For each decomposed part, its skeleton and body measurements are extracted and saved as the shape descriptor. Subsequently, its thumbnail is created according to the shape descriptor and saved as the thumbnail descriptor. In the online process, according to user's preference, the system can either render the 3D thumbnail directly with its pre-generated thumbnail descriptor or re-generate the 3D thumbnail descriptor based on a pre-generated shape descriptor without starting from the scratch. As a result, the data size of a thumbnail descriptor is much less than its original mesh and can be downloaded quickly. Rendering a simplified thumbnail demands less hardware resource, and the online thumbnail viewer can display multiple 3D thumbnails simultaneously within a few seconds.;Furthermore, we develop two feature-preserving thumbnail creation techniques. They are the surface-based and the voxel-based methods. For the surface-based technique, a 3D polygonal mesh is decomposed by a visual salience-guided mesh decomposition approach that identifies and preserves significant components. For each decomposed part, its skeleton and body measurements are extracted after the PCA transformation. Then, a coarse-to-fine primitive approximation algorithm is used to create the 3D thumbnail. Moreover, a customized deformable primitive, called the d-cylinder, is designed for approximating the shape better and fining the appearance of the resultant thumbnail. We generate the 3D thumbnail with a different number of d-cylinders so that the thumbnail can represent a simplified mesh in different level of details successfully. The processing time of each process and the file size of the 3D thumbnail descriptor are given to show the efficiency of the surface-based approach.;In the voxel-based approach, a polygonal model is first rasterized into a volumetric model and a coarse skeleton is extracted with a thinning operation. The skeleton derived from the thinning process is further refined to meet the required accuracy. Subsequently, the skeleton is classified to significant groups, and the volumetric model is decomposed into significant parts accordingly. As compared with the surface-based approach, the voxel-based approach can preserve more features of the model and decompose the model more precisely. Thus, the significant components of the original model can be preserved better in the 3D thumbnails while the model is extremely simplified. A thorough performance comparison between the surface-based and the voxel-based techniques is conducted.;Finally, the framework is extended to sketch-based 3D modeling. We present a sketch-based 3D system that provides a set of tools for users to create simple 3D models from 2D sketches easily. A user-drawn 2D sketch is refined and then approximated by 3D fitting primitives. There are five new customized elements developed for approximation: the open-tube, the closed-tube, the ellipsoid, the prism, and the complex-prism. When a 3D model is composed of multiple parts, they can be grouped hierarchically using editing tools. Furthermore, the system embeds a 3D character with a hierarchical skeleton for the purpose of animation.