Real-time Rendering of Realistic Lighting Effects for Animated 3D Models

為動態三維模型提供逼真的實時光照渲染效果

Student thesis: Doctoral Thesis

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Award date14 Sep 2022

Abstract

Rendering realistic lighting effects in real-time is an important concern in computer graphics. Precomputed radiance transfer (PRT) algorithms can render high quality results in real-time. However, due to the precomputation process, most of the PRT algorithms are limited to rigid 3D models. On the other hand, ray tracing algorithms are flexible to handle animated 3D models, and the recent advances in graphics hardware, i.e. RTX, offer us the possibility to utilize ray tracing algorithms to render visually appealing results in real-time. Visually appealing though, the rendering results can still deviate from the actual one considerably. In this thesis, we present several rendering algorithms to real-time render realistic lighting effects for animated 3D models.

First, we present three approaches to solve the problems of combining dual paraboloid mapping and mipmapping. Dual paraboloid mapping is an approach to environment mapping. Its major advantage is its fast map generation speed. Mipmapping is the primary filtering tool for graphics applications when filtering is needed. However, directly applying mipmapping to dual paraboloid mapping would give us three problems. They are the discontinuity across the dual paraboloid map boundary, the non-uniform sampling problem, and the depth testing issue. These problems significantly influence the visual quality of dual paraboloid mapping and mipmapping.

The three proposed approaches solve the three problems individually based on some closed form equations derived via theoretical analysis. With these equations, our approaches handle these problems just by using dual paraboloid mapping and mipmapping differently instead of fundamentally altering their data structure. Consequently, our approaches improve the visual quality of dual paraboloid map mipmap, while preserving its fast map generation speed. The efficiency of the proposed approaches is demonstrated by using a glossy reflection application and an omnidirectional soft shadow generation application. The rendering results show that our approaches can render realistic glossy reflections and realistic soft shadows in real-time.

For direct illumination rendering applications, capturing all frequency lighting effects is essential to provide a realistic visual experience. Vectorized visibility is a powerful visibility representation algorithm that can render all frequency lighting effects in real-time. The existing rendering process of vectorized visibility requires the visibility functions to be synthesized three times, followed with the radiance evaluation, for each triangle of the 3D model. This process requires a high computational cost for the radiance evaluation. We then introduce a novel algorithm to linearly interpolate the vectorized visibility, and therefore, reduce the computation bottleneck, i.e. the radiance evaluation, to just once.

To facilitate the interpolation, we use spherical Voronoi diagram as a tool to generate the preliminary correspondence among the three sampled visibility functions for each triangle. Additional treatments are also implemented to ensure the interpolated visibility functions have smooth transitions across the 3D model. The rendering results show that our algorithm can achieve twice as fast rendering speed as the previous method, while providing a bit more accurate all frequency direct illumination results.

Although vectorized visibility together with the above algorithm provides an outstanding visual quality in real-time, as a PRT algorithm, it is limited to rigid 3D models. Lastly, we introduce a front back edge oriented line sample bounding volume hierarchy (BVH) traversal algorithm to dynamically generate vectorized visibility. This algorithm utilizes the properties of front back edges and the line sample BVH traversal to maximize the efficiency of the vectorized visibility generation. We also propose a novel data structure, namely temporal visibility, which allows us to share vectorized visibility across time and further increases the generation efficiency. On the other hand, utilizing a straightforward RTX ray tracing implementation, we further extend vectorized visibility to support global illumination rendering.

With our algorithm, we extend vectorized visibility to support animated 3D models while maintaining the outstanding visual quality. For the direct illumination rendering, with a similar processing time, our algorithm provides a visual quality improvement around 10 dB in terms of PSNR w.r.t. the state-of-the-art ray tracing algorithms. For the global illumination rendering, our algorithm just uses a fractional computational cost w.r.t. the direct illumination only rendering and obtains the close-to-reference view dependent high frequency shadows in the inter-reflections.