Research On Illumination Technologies In 3D Virtual Exhibition

With the development of virtual reality and internet technologies,3D virtual exhibition applications have become increasingly widespread. Illumination technologies are one of the key technologies in virtual exhibition system. The realistic lighting effects created by them are important sources of users'immersive experience. For different application purposes, exhibition system sets different illumination environments to create corresponding lighting effects. Affected by many factors in exhibition, such as model composition and light composition, traditional illumination technologies are not fully applicable in these environments. In this dissertation, three representative illumination environment settings are discussed. They are illumination by simple point lights, illumination by many area lights and interactive global illumination by an amount of area lights respectively. The drawbacks of current technologies in dealing with these environments are analyzed and proper solutions are proposed.Point light illumination is carried out by graphics hardware rendering pipeline. The efficiency is limited by the rendering speed of scene models, so model simplification technologies are usually introduced to accelerate this progress. To overcome the drawback that traditional simplification algorithms can't preserve model's discontinuous appearance attributes, the cost function is modified to delay the generation of appearance distortion and preserve model's discontinuous appearance attributes. A corresponding multiresolution representation structure is also given to store simplification information. By using this structure, exhibition system can rebuild simplified model at any resolution in real time. Shadow is an important feature of point light illumination, and shadow map technologies are important means of shadow generation. To reduce the rebuilding overhead of shadow map caused by avatar movement, model separation strategy is adopted to rebuild shadow map by stages and to guarantee shadow map construction efficiency. The experiment results prove that our solution combined of model simplification and shadow map technologies in this dissertation can generate lighting images with shadow effects in real time.For many area lights illumination, the drawback that traditional methods will introduce sampling errors is analyzed theoretically and a direct clustering method on area lights is proposed to avoid error generation and improve illumination efficiency. An area light tree is pre-computed to organize all lights in the scene. For every pixel point, a proper area light cluster partition is located by traversing this tree. The sum of estimated intensity of each light cluster is used as pixel point's direct illumination intensity. The experiment results show that our method is more efficient and can generate more fidelity results than traditional light sampling based methods. The implementation of traditional interactive global illumination algorithms is influenced by various factors. When the scene composition is complex, e.g. a certain number of area lights, the avatar in walking-through status and exhibiting models with considerable complexity, these algorithms cannot achieve desired rendering speed. To support interactive global illumination computation in such complex environment, we propose to improve existing illumination technologies in both software and hardware levels. In software level, a model separation strategy is used to reduce the influence on lighting data by avatar's movement. In hardware level, a highly-powered GPU server is chosen as the concrete illumination computing platform. The data transfer progress among different platforms and the concrete illumination computing progress are improved to eliminate data transfer delay, increase computing parallelism and guarantee illumination efficiency. The experiment results prove that our interactive global illumination system has nice flexibility on such complex environment and it can create realistic global lighting effects in nearly real time.As these three illumination environment settings are very common in the field of virtual reality, our illumination technologies are also applicable to other virtual reality applications. Thus they have good practical values and reference values.