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Recent advances in real-time rendering have allowed the GPU implementation of traditionally CPU-restricted algorithms, often with performance increases of an order of magnitude or greater. Such gains are achieved by leveraging the large-scale parallelism of the GPU towards applications that are well-suited for these streaming architectures. By contrast, mesh simplification has traditionally been viewed as a non-interactive process not readily amenable to GPU acceleration. We demonstrate how it becomes practical for real-time use through our method, and that the use of the GPU even for offline simplification leads to significant increases in performance. Our approach for mesh decimation adopts a vertexclustering method to the GPU by taking advantage of a new addition to the rendering pipeline -the geometry shader stage. We present a novel general-purpose data structure designed for streaming architectures called the probabilistic octree, which allows for much of the flexibility of offline implementations, including sparse encoding and variable level-of-detail. We demonstrate successful use of this data structure in our GPU implementation of mesh simplification. We can generate adaptive levels of detail by applying non-linear warping functions to the cluster map in order to improve resulting simplification quality. Our GPU-accelerated approach enables simultaneous construction of multiple levels of detail and outof-core simplification of extremely large polygonal meshes.

In this chapter we will cover approaches for creating visually complex, rich interactive environments as a case study of developing the world of ATI "ToyShop" demo. We will discuss the constraints for developing large immersive worlds in real-time, and go over the considerations for developing lighting environments for such scene rendering. Rainspecific effects in city environments will be presented. We will overview the lightning system used to create illumination from the lightning flashes, the high dynamic range rendering techniques used, various approaches for rendering rain effects and dynamic water simulation on the GPU. Methods for rendering reflections in real-time will be illustrated. Additionally, a number of specific material shaders for enhancing the feel of the rainy urban environment will be examined.

Level-of-detail (LOD) rendering is a key optimization used by modern video game engines to achieve high-quality rendering with fast performance. These LOD systems require simplified shaders, but generating simplified shaders remains largely a manual optimization task for game developers. Prior efforts to automate this process have taken hours to generate simplified shader candidates, making them impractical for use in modern shader authoring workflows for complex scenes. We present an end-to-end system for automatically generating a LOD policy for an input shader. The system operates on shaders used in both forward and deferred rendering pipelines, requires no additional semantic information beyond input shader source code, and in only seconds to minutes generates LOD policies (consisting of simplified shader, the desired LOD distance set, and transition generation) with performance and quality characteristics comparable to custom hand-authored solutions. Our design contributes new shader simplification transforms such as approximate common subexpression elimination and movement of GPU logic to parameter bind-time processing on the CPU, and it uses a greedy search algorithm that employs extensive caching and upfront collection of input shader statistics to rapidly identify simplified shaders with desirable performance-quality trade-offs.

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