Summary 
Rendering and visualizing large 3D synthetic models is a crucial component of many engineering disciplines and is becoming increasingly more important for simulations, gaming, and education. Although rendering hardware continues to improve, the desire to render even larger models continues to increase. Historically, large models could only be rendered on highly specialized computers. However, today's PC are an attractive platform for interactive rendering as well. In the following work, we investigate several approaches to rendering acceleration of large 3D models. 
 Hybrid Simplification: Combining Polygon and Point Rendering.
Multiresolution hierarchies of polygons and more recently of points are familiar and useful tools for achieving interactive rendering rates. We present an algorithm for tightly integrating the two into a single hierarchical data structure. The tradeoff between rendering portions of a model with points or with polygons is made automatically. We apply a bottomup simplification process involving not only polygon simplification operations, but point replacement and point simplification operations as well. Given one or more surface meshes, our algorithm produces a hybrid hierarchy comprising both polygon and point primitives. This hierarchy may be optimized according to the relative performance characteristics of these primitive types on the intended rendering platform. We also provide a range of aggressiveness for performing point replacement operations. The most conservative approach produces a hierarchy that is better than a purely polygonal hierarchy in some places, and roughly equal in others. A less conservative approach can trade reduced complexity at the far viewing ranges for some increased complexity at the near viewing ranges. We demonstrate our approach on a number of input models, achieving primitive counts that are 1.3 to 4.7 times smaller than those of triangleonly simplification. 
 Massive Model Rendering.
We described a system for interactively rendering very complex 3D models by tightly integrating several very different simplification methods. We select a subset of the model as preferred viewpoints and partition the space into virtual cells. Each cell contains near geometry, rendered using levels of detail and visibility culling, and far geometry, rendered as a textured depth mesh. Our system automatically balances the screenspace errors resulting from geometric simplification with those from textureddepthmesh distortion. We describe our prefetching and data management schemes, both crucial for models significantly larger than available system memory. Our system accelerated walkthroughs of a 13 million triangle model of a large coalfired power plant and of a 1.7 million triangle architectural model. We demonstrate the walkthrough of a 1.3 GB power plant model with only a 140 MB cache footprint. 
 Guaranteed Frame Rate Rendering.
We presented a preprocessing algorithm and runtime system for rendering 3D geometric models at a guaranteed frame rate. Our approach trades off space for frame rate by using images to replace distant geometry. The preprocessing algorithm automatically chooses a subset of the model to display as an image so as to render no more than a specified number of geometric primitives. We also summarize an optimized layereddepthimage warper to display images surrounded by geometry at run time. We have applied our method to accelerate the interactive walkthrough of several complex models ranging up to 2 million geometric primitives. Our results, both empirical and theoretical, indicate we can reduce the geometric complexity by approximately an order of magnitude using a practical amount of storage (by today's standards). 
 ImageBased Acceleration for Architectural Walkthroughs.
We have developed several methods to accelerate rendering of architectural walkthroughs. In these papers, we improve upon a cells and portals framework by using images to replace geometry visible through portals (doors and windows) in a threedimensional model. We present several solutions: images texturemapped onto the portal, 3D warping of single reference images, and 3D warping of layered depth images. We have achieved significant speedups and present the results of applying the techniques to large architectural models. 
 Rendering Complex Models using Geometry Warping and Textures.
We investigated techniques for providing smooth transitions when simplifying large, static geometric models with texturebased representations (or impostors). Traditionally, textures have been used to replace complex geometry, for example the books on a shelf or the complex foliage on a tree. Using textures in a more general manner is a relatively new area of research. The key idea is that 2D image textures can be used to temporarily represent 3D geometry. Rendering speed is increased if a replacement texture can be used for several frames, since textures can be rendered quickly and independently of model complexity. Because a texture is only correct from a single viewpoint, visual discontinuities, such as misalignment with adjacent geometry, begin to appear as the texture is used from other viewpoints. Previous approaches have controlled these errors by recreating the textures often or providing a large set of precomputed textures. We have improved upon these methods by: (a) providing a geometry warping algorithm to maintain continuous imagery across borders between geometry and sampled textures at all times; (b) providing an approach for smooth dynamic transitions between geometry and texture. Thus, geometry and texturebased representations can be switched without sudden jumps (as is the case with many current texturing techniques). All the computations for the transitions can be done a priori without the need to change the textures each frame thereafter. 
Publications 
Hybrid Simplification: Combining Multiresolution Polygon And Point Rendering. J. Cohen, D. Aliaga, W. Zhang, Proceedings of IEEE Visualization, pp. 140147, Oct., 2001.
Automatic Image Placement to Provide a Guaranteed Frame Rate. D. Aliaga, A. Lastra, Proceedings of ACM SIGGRAPH, 307316, 1999.
MMR: An Interactive Massive Model Rendering System Using Geometric and ImageBased Acceleration. D. Aliaga, J. Cohen, A. Wilson, E. Baker, H. Zhang, C. Erikson, K. Hoff, T. Hudson, W. Stuerzlinger, R. Bastos, M. Whitton, F. Brooks, D. Manocha, Proceedings of ACM Symposium on Interactive 3D Graphics, 199206, 1999.
Efficient Warping for Architectural Walkthroughs using Layered Depth Images. V. Popescu, A. Lastra, D. Aliaga, M. Neto, Proceedings of IEEE Visualization, 211215, 1998.
Images for Accelerating Architectural Walkthroughs, M. Rafferty, D. Aliaga, V. Popescu, A. Lastra, IEEE Computer Graphics and Applications (CG&A), November/December, 1998.
Smooth Transitions in TextureBased Simplification. D. Aliaga, A. Lastra, Computer & Graphics, 22:1, 7181, 1998.
3D Image Warping in Architectural Walkthroughs. Matthew Rafferty, Daniel G. Aliaga, Anselmo Lastra, Proceedings of Virtual Reality Annual International Symposium, 228233, 1998.
Virtual Backdrops. D. Aliaga, A. Lastra, ACM SIGGRAPH Technical Sketch, 1997.
Architectural Walkthroughs using Portal Textures. D. Aliaga, A. Lastra, Proceedings of IEEE Visualization, 355362, 1997.
Visualization of Complex Models using Dynamic Texturebased Simplification. D. Aliaga, Proceedings of IEEE Visualization, 101106, 1996. 
