Dynamic Shadows and Lighting for Walkthrus of Large Models

Problem Description

Dynamic shadows and lighting are important visual cues that aid in the understanding of spatial relationships between objects. Historically, dynamic lighting in walkthru applications has been very simple, usually consisting of a single directional light source that does not cast shadows. We would like add dynamic shadows and lighting to walkthrus of large models while maintaining interactive frame rates.

Previous Work

Many techniques have been proposed for generating shadows. Woo et al. [5] is an older paper that provides a survey of shadow algorithms. Recent advances in graphics hardware has made it possible to calculate shadows at interactive framerates for scenes of moderate complexity. Two approaches that have been used with graphics hardware are shadow volumes [3] and shadow maps [4]. A shadow volume is the region of space consisting of all points which are occluded to a light source. By counting  the intersections with shadow volumes along the line-of-sight towards a point in the scene it can be determined whether the point lies in shadow or not. Shadow maps are depth maps created from the position of the light source. To determine whether a point is in shadow, it is projected into the light space in which the shadow map was generated. The depth of the point is compared to the depth stored in the shadow map. If it the point is farther away, it lies in shadow. Improvements to shadow maps have been proposed that increase the resolution adaptively [5] and calculate the shadow map in post-perspective space to improve sample distribution [6].

At its heart, shadow determination is a visibility problem. There has been a lot of research in visibility algorithms. Cohen-Or et al. [1] is a recent survey of visibility techniques.

Approach

Recent advances in graphics hardware have increased the rendering speed and programmibility of the graphics pipeline. I would like to explore methods for leveraging the power of graphics hardware in order to calculate dynamic shadows and lighting. My target model will be the 13 million triangle power plant model. I would like to provide at least one dynamic light that moves with the viewer, or can be moved independently of the viewer. A larger number of static lights would also be desirable.

One interesting idea would take advantage of path coherence by using depth billboards to accelerate the construction of shadow maps. The idea is to cache the depth values calculated for regions of distant geometry in textures that are updated incrementally according to an error metric as the viewpoint changes. These textures can be warped to account for small changes in viewpoint and composited quickly to generate a new shadow map. This is similar in spirit to the technique used by Shade et al. [2] for visible geometry.

References


[1]    D. Cohen-Or, Y. Chrysanthou, and C. T. Silva. A Survey of Visibility for Walkthrough Applications. Proceedings of EUROGRAPHICS '00, Course Notes, 2000.

[2]    J. Shade, D. Lischinski, D. Salesin, T. DeRose, J. Snyder, Hierarchical Image Caching for Accelerated Walkthroughs of Complex Environments. SIGGRAPH 1996, pp. 75-82.

[3]    C. Everitt and M. J. Kilgard. Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering. SIGGRAPH 2002 Course Notes, Course #31, 2002

[4]     M. Segal.  Fast Shadows and Lighting Effects Using Texture Mapping. SIGGRAPH 92, 1992

[5]    A. Woo, P. Poulin, and A. Fournier. A Survey of Shadow Algorithms. IEEE Computer Graphics and Applications, 10(6):13-32, 1990.

[6]     R. Fernando, S. Fernandez, K. Bala, and D. Greenberg. Adaptive Shadow Maps. SIGGRAPH 2001, pp. 387-390, 2001.

[7]     M. Stamminger and G. Drettakis. Perspective Shadow Maps. SIGGRAPH 2002, pp 557-562, 2002