基于光源空间透视的平行分割阴影图算法

Parallel Split Shadow Map Algorithm Based on Light Space Perspective

随着地理信息系统应用的深入,在真三维空间中处理问题已成为一种趋势,而实时阴影对提高三维场景的真实感和沉浸感非常重要。目前主流的平行分割阴影图算法对大尺度场景兼容性好,但在细节层次的渲染方面质量不高,而光源空间透视阴影图算法在细节层次渲染方面精度很高,但是对大尺度场景难以覆盖。针对该问题,提出了一种基于光源空间透视的平行分割阴影图算法,也称为光源透视平行分割阴影图算法。该方法将光源空间透视阴影图算法中透视锥体的透视变换矩阵应用于平行分割阴影图算法对各个子视锥体的渲染阶段并生成产生形变的阴影深度图,通过该方法减小其阴影走样误差。并且在计算透视变换矩阵的过程中,提出了一种确定阴影图形变程度的算法,使该算法更好地适应于不同大小的场景。实验结果表明,该算法通过提高视锥体近平面附近阴影采样率有效地解决了在平行分割阴影图算法中存在的阴影透视走样的问题。在对大规模场景的渲染方面,该算法在细节层次的表现要明显优于平行分割阴影图算法,满足了大尺度场景和精细建筑模型的阴影渲染需求。

With the deeper application of geographic information system,to solve problem in real 3Dspace has come to be a trend.It is very important for real-time shadow rendering to improve the sense of reality and immersive of virtual 3Dscene.For now,Parallel Split Shadow Maps（PSSM）algorithm is a powerful method which is widely used for large-scale virtual environments rendering.But the detail of the shadow is kind of rough,compared with other shadow map algorithm with perspective projection parameterization method.On the other side,the Light Space Perspective Shadow Maps（LisPSM）is good at rendering the detail of shadow.But it cannot cover the large-scale scene,due to the limitation of hardware.So to improve the shadow performance of large-scale virtual scene,the advantages of both methods are combined,and Parallel Split Shadow Maps algorithm on Light Space Perspective is proposed,which is also called Light Perspective Parallel Split Shadow Maps（LPPSSM）.The realization of this algorithm is to use LisPSM method to render every child view frustum,after the split of parent frustum which is done by PSSM method.With the utility of perspective transformation,the shadow map aliasing error is reduced greatly and the shadow map perspective aliasing error is removed.During the process of perspective transformation matrix′s calculation,another algorithm is proposed to help calculate a parameter,which is used to determine the strength the shadow map wraps.And this can help to make this method to suit different size of scenes,and combining LisPSM algorithm and PSSM algorithm in a much better way.By making a comparison between LisPSM algorithm,PSSM algorithm and LPPSSM algorithm,the shadow map shape of LPPSSM algorithm is similar to LisPSM algorithm,which is much different from PSSM method.The shape of former one is like a trapezoid,while the other one is like a rectangle.And the shadow quality is different too.From the result of the test,it can be seen clearly that the shadow quality of LPPSSM algorithm is much higher than the other ones.By combining the advantages of LisPSM algorithm and PSSM algorithm,the LPPSSM algorithm is proposed.This algorithm can help to solve the problem of low shadow quality when using PSSM algorithm to render a large scale 3Dscene.And the algorithm proposed to calculate the parameter of shadow wrap can make the LisPSM algorithm match to PSSM algorithm much better.The result of test shows that LPPSSM algorithm solved the shadow map perspective aliasing problem,improved the shadow performance,and satisfied the need of rendering large scale space environment models and accurate architectural scene models with good shadow quality.