Real-Time Smooth Shadow Rending Of Mesh Surfaces

Shadow can strengthen the realism in three-dimensional virtual scene drawing. Therefore, shadow drawing has always been a focus area in computer graphics. Polygon mesh is one of most important forms to approximate the smooth surface of three-dimensional objects and has been widely used in computer games and animations. Given that mesh model is the approximation of smooth curved surface, so drawing shadow directly would result in some bad effects:shadow boundary is not smooth; shadow boundary aliased, to which human eyes are very sensitive.Thus, the main issue of this thesis is how to draw mesh model to obtain smooth shadow boundary. In addition, for the pursuit of real-time rendering and rendering shadow efficiently, how to implement shadow generating algorithms on GPU is another important issue in this paper. Because human eyes are not sensitive to the soft shadow boundary, therefore in our thesis, we just discuss the generation of hard shadow.Now, there are mainly two ways to create dynamic shadow effects. The first way is the Shadow Volume Algorithm [2], which was originally proposed by Crow in 1997. It is one of the most commonly used shadow drawing algorithms in current computer animations and games. The second way is Shadow Mapping Algorithm [13], which is originally proposed by Williams in 1978. It is a method based on image space, requiring rendering each shadow projection light two times to identify whether the pixel is in the shadow. This kind of method is to calculate shadow boundary in image space; as a result, it contributes to the simplicity of computing, however, the accuracy is limited by cache resolution, so it will lead to a lower accuracy than that of shadow volume algorithm in real-time rendering applications. Most current generation technologies are based on the two algorithms, whose purpose is to eliminate aliasing phenomenon in shadows rendering. Technologies based on shadow volume algorithm are very accurate, generating precise shadows, but the cost of computing resources is higher. Technologies based on Shadow Mapping Algorithm solves Aliasing phenomenon by modifying shadow mapping structure. The essential thought made no change.Because our algorithm uses coarse mesh models as input, to ensure the reality effect of scene, one of processes is to smooth the input models. To reduce space occupancy, how to use mesh subdivision technology to calculate smooth shadow is another problem to solve. Moreover, in order to guarantee the real-time rendering and improve the efficiency, how to transplant smooth shadow generation algorithm on GPU is another key problem. The main contributions of our thesis can be covered as follows:(1) The silhouette smoothing process of literature is integrated into GPU graphics pipeline, after that, the geometry shader in GPU structure can be adapted to real-time render smooth contour with greatly improved rendering efficiency.(2) We propose a real-time smoothing shadow algorithm, and realize the shadow-smoothing effect of coarse mesh models based on the idea of the shadow volume algorithm.(3) GPU based shadow generation algorithm is utilized to improve the rendering efficiency. Also, the algorithm takes advantage of GPU geometry shader to effectively avoid the low rendering efficiency issue incurred by massive data exchange between CPU and GPU.Our algorithm has advantages in space occupancy and operation efficiency, but there are still some shortcomings. First of all, when point light is close to objects, and far away from projection planes. Shadows would not smooth. According to this problem, multi-resolution mesh subdivision algorithm could be used to realize the smooth shadow. Secondly, when light projects to rough surface, Shadow aliasing phenomenon also appears. To treat this problem we can use local subdivision mesh algorithm to smooth shadow effect. Finally, when there are more than one light sources, triangles generated by our algorithm will rise dramatically and occupancy huge space. In the future work, this work is one of the main problems to solve.