Research On Real-time Rendering Algorithms Of Shading And Shadows In Virtual Environment

Different real-time rendering algorithms of shading and shadows are researched in this dissertation. We concentrate on some key technologies, such as anti-aliasing algorithms of shadows based on image, rendering algorithms of shadows under omni directional light source, real-time rendering algorithms of soft shadows based on image, screen space ambient occlusion algorithms, shading algorithms in digital relief shading. The followings are what we have done in this dissertation:Firstly, we analyzed the characters of standard shadow mapping algorithm and talked out the shortcoming of it. Through analyzing aliasing problem of shadow mapping algorithm, it could be found that under-sampling is the main reason of aliasing. So the essential way to improve aliasing of shadows is enhancing the sampling of shadow map. This dissertation summarizes the characters and conditions of algorithm based on partition and warping. A new algorithm was brought forward about partition way in PSSM algorithm. Neither PSSM algorithm nor Cascaded Shadow Maps algorithm has clear partition criterions. We have brought forward furthest criterion of every part of view frustum in eye space according to measure method of shadow aliasing and get ration relationship between split project of view frustum and resolution. This dissertation provides a kind of view split algorithm which could operate easily.Secondly, according to the characters of virtual terrain environment, a new shadow algorithm of viewpoint adapting was brought forward by this dissertation. This algorithm can work out least area which light source view frustum should surround according to the relationship of view frustum in eye space and terrain. By using this algorithm, the shadow map could be utilized greatly. We take an experiment of this algorithm by using global virtual terrain environment. The outcomes of the experiment show that this algorithm can improve the aliasing of shadows. We also disscuss the characters of the rendering algorithms of shadows under omni-directional light source by using Blinn-Newell environment map, spherical environment map, cube environment map and paraboloid environment map. It is could be found that paraboloid environment map is most adapt to shadow rendering under omni-directional light source.Thirdly, evaluating criterion is brought forward according to theory analysis and photos of shadows. We talk out the algorithm which rendering soft shadow using image filtering. The factors of impacting effect and efficiency of soft shadow rendering are showed. VSM, CSM, ESM are compared each other and the characters of these algorithms are summarized. The main factors that affect efficiency of PCSS algorithm is gotten through experiments. We can conclude that the same region of depth map is sampled second time during computing the position of occluder and occlusion testing. So re-sampling is occurred in depth map. Thus we bring forward a soft shadow rendering algorithm which have unfixed penumbras. This algorithm combines the two same sampling to one sampling and predigests the course of computing occluder's position. This algorithm either gets same effect as PCSS or has higher efficiency. And when the shadows in scene are excessive, the advantages are more outstanding. The experiments show that this algorithm is more efficient than PCSS about 10 percent.Fourthly, this dissertation talks out the development and character of environment occlusion algorithms, especially summarizes the optimized algorithm brought forward recent years. We talk out the principle and characters of two ambient occlusion algorithms. We believe that HSAO algorithm has better expansibility through comparing. Different kinds of factors are analyzed by experimenting, such as sampling region of depth map, number of sampling directions, number of sampling points in radial direction, resolution of screen, orientation of viewpoint. The law that these factors affecting the HSAO is brought by this dissertation. The application of HSAO in Virtual Environment and method of acceleration by using GPU is talked out. The optimizing method of HSAO is discussed in this dissertation.Lastly, basic laws of traditional relief shading are summarized by studying the ideas of Imhof and Shizuhui. Some shading model is talked out which is used commonly in relief shading. Diffuse model and aspect based model is compared by experimenting and theory analyzing. The pros and cons of the two algorithms are analyzed. Diffuse model could render the character of plain terrain greatly and aspect based model is better in representing the rolling terrain. We bring forward four ideas about the two shading models are introduced. A new light model oriented slope, aspect and elevation. It make an experiment on this new light model using a pyramids model which slope increased with elevation and two DEM data which are differently in terrain character each other. It is concluded that this new light model could overcome effectively the drawbacks of Lambert model and aspect-based shading.