Tan-Chi Ho

User-assisted mesh simplification

During the last decade, many simplification methods have been proposed to generate multi-resolution meshes for real-time applications. Practitioners have found that these methods alone usually fail to produce satisfactory result when models of very low polygon count are desired. This is due to the fact that the existing methods take no semantic or functional metric into account, and moreover, each error metric has its own strength and weakness. In this paper, we propose a user-assisted mesh simplification framework that allows users to improve the quality of simplified meshes derived by any error metric. The framework consists of two stages. The first stage employs a weighting scheme that allows users to refine a unsatisfactory region to achieve a user-specified resolution. The second stage is a local refinement scheme aiming to provide a user-guided fine-tune to recover local sharp features. The proposed weighting scheme differs from the previous approaches in that the weights are used to directly reorder the edge collapsing sequence rather than weighting the collapsing cost. Such a direct reordering mechanism ensures a predictable increase of resolution in the selected region, and is both error-metric and resolution independent.

Fast rendering of dynamic clouds

We propose an efficient framework for dynamic cloud rendering. Based on the proposed simplified lighting model, the shadow relation table and metaball lighting texture database are constructed in the preprocessing, and features such as self-shadowing and light scattering can be computed quickly using table lookup at the run time. Clouds are rendered using textured billboards and alpha blending, which is further speeded up by incorporating the octree hierarchy and the hierarchical texture caching.

Interactive lighting design with hierarchical light representation

Lighting design plays a crucial role in indoor lighting design, computer cinematograph and many other applications. Computer‐assisted lighting design aims to find a lighting configuration that best approximates the illumination effect specified by designers. In this paper, we present an automatic approach for lighting design, in which discrete and continuous optimization of the lighting configuration, including the number, intensity, and position of lights, are achieved. Our lighting design algorithm consists of two major steps. The first step estimates an initial lighting configuration by light sampling and clustering. The initial light clusters are then recursively merged to form a light hierarchy. The second step optimizes the lighting configuration by alternatively selecting a light cut on the light hierarchy to determine the number of representative lights and optimizing the lighting parameters using the simplex method. To speed up the optimization computation, only illumination at scene vertices that are important to rendering are sampled and taken into account in the optimization. Using the proposed approach, we develop a lighting design system that can compute appropriate lighting configurations to generate the illumination effects iteratively painted and modified by a designer interactively.

A Skeleton-Based Approach for Consistent Segmentation Transfer

We present a skeleton-based approach for mapping consistently a segmentation of a source mesh to a target mesh. The source and target meshes are semantically similar but their geometries may be different. We assume that the segmentation of the source mesh and the skeleton correspondence between the meshes are provided. Our method first establishes the skeleton-boundary correspondence and then performs the boundary transfer procedure. The skeleton-boundary correspondence is useful for computing approximately the target boundaries. The boundary transfer procedure aims to produce the segment boundaries of the target mesh such that their geometric details are consistent to the ones of the corresponding source segment boundaries. We have applied our method for various models and compared with the state-of-the-art techniques. Our method computes segment boundaries of the target mesh which are similar to the ones of the source mesh.

A skeleton-based approach for shape correspondence

We present a novel skeleton-based approach for performing shape correspondence by exploiting the mesh geometric properties associated with the skeletons of the objects. The skeletal nodes which correspond to the similar parts of the two objects are matched. We compute a set of potential shape correspondences and then evaluate the best one. Our method may merge adjacent skeletal nodes and adjacent skeletal branches so that it can compute not only an 1-1 correspondence but also a many-many correspondence. We apply our method to various models with similar topological structures and compare with the latest techniques. Experimental results show that our method can compute acceptable shape correspondence.

Fire synthesis using basis fires and design

In this paper, we present an approach to use basis fires to design and synthesize fires in desired shapes and motions. In the preprocessing stage, each basis fire is simulated based on a physics-based fire simulator with a specific simulation configuration. The pathlines and temperatures of the basis fires are stored in a database. In the fire design stage, a user inputs a sequence of curves which represent the desired shapes of fires. In the fire synthesis stage, we use a least-square fitting method to fit the curves using the pathlines and temperatures of the basis fires. In the fire animation stage, we adopt a modified motion graph for animating the synthesized fires. We have applied our approach to synthesize fires with different shapes and motions. Experimental results show that our approach is easy to use and it can produce fires with desired shapes in an intuitive manner. The synthesized fire can be animated at real-time rates.

Aggressive region-based visibility computation using importance sampling

We present an aggressive region-based visibility sampling algorithm for general 3D scenes. The visibility portal in the scene is a cue for guiding the visibility sampling. Our algorithm extends the image space sampling algorithm by measuring the size and orientation of portals in the scene, and results in a predictable sampling mechanism of visible set in the image space. An importance visibility sampling scheme in the image space is proposed based on the visibility portals, and used to guide the sampling process. Each newly added visibility sample is placed at the position potentially visible for most missing polygons. Experiments show that our sampling approach can effectively improve the performance of visibility sampling in both the convergence rate and the visual quality compared to the previous approaches.