The Algorithms For Rendering And Editing Of Heterogeneous Translucent Materials

The rendering and editing of translucent materials takes an important role in the research of realistic graphics, especially the realtime rendering and interactive editing of heterogeneous translucent materials, which is always the frontier topic in recent years. The complexity of rendering is mainly caused by the subsurface scattering effect in the translucent materials. Subsurface scattering means that light transport takes place not only on the surface of the materials, but also interior, while heterogeneity indicates the spatial-variant distribution of different scattering properties. If the light scatters many times inside the heterogeneous materials, it needs a large computation to simulate the light behavior accurately, which is an impossible mission for today's personal computers. For the heterogeneous materials specifically, there is no accurate approximation model to describe the relationship between incoming and outgoing radiance. The inefficiency of rendering directly leads to the difficulty of editing. Besides, given the volumetric distribution of the physical scattering property, it is not intuitive to get the editing effect, which is also a challenge" for editing. Up till now few work is done on directly editing the bidirectional subsurface scattering reflectance function(BSSRDF) itself. The realtime rendering and interactive editing algorithms in this thesis are designed for various multiple-scattering-dominant heterogeneous translucent materials.In the rendering part, this thesis mainly focused on two categories of materials. One category is the heterogeneous materials with mesostructure, which has small geometric details and texture information over the surface, basically synthesized from physical scattering properties. The other category is the measured data by capturing the real-world samples, or by simulation.In the case of the first category of materials, mesostructure over the surface can cause such visual effects as occlusion, self-shadow and inter-reflection, while in the interior, it meets with the diffusion approximation. The main contribution of the first rendering algo- rithm is that it proposed a realtime approach for heterogeneous materials with mesostruc-ture. In the mean time, it also provided the optimized storage scheme for compressed BSSRDF data. The main idea of this algorithm is to model the heterogeneous translucent materials with surface mesostructure using a two-layer model, and apply precomputed radiance transfer as well as graphics hardware accleration effectively.In the case of the second category of materials, we proposed a realtime rendering algorithm for dynamic scenes. The key novelty of this algorithm is that it can provide realtime framerates and high quality visual effects, and is also flexible for several representations of BSSRDFs. Moreover, it does not need any precomputation, which means any change of underlying mesh and material is possible. Thus it can also be integrated into the editing system of heterogeneous translucent materials. The evaluation of surface radiance is a per-vertex approach. According to the observation that subsurface scattering decays locally along the distance, this algorithm takes full advantage of the parallelism of modem graphics hardware. It follows the rendering equation and evaluates the radiance of each outgoing surface point via hierarchical organization and reasonable optimization.In the editing part, this thesis originally proposed a representation(SubEdit) for editing the heterogeneous translucent materials, and developed an editing framework based on it, including the meta editing operations and their extended complex combinations. The key idea of this work is decoupling the non-local subsurface scattering effect into local scattering profiles, thus it can achieve interactivity. This representation also fits measured BSSRDF data well, and is capable to generate new materials based on simple inputs.The rendering and editing algorithms for heterogeneous translucent materials have a wide prospect for applications. The rendering framework can be used in rendering applications of realistic scenes such as games and simulations. The editing system can also be applied in modeling softwares and areas of computer-aided design, helping artists to generate high visual quality editing results.