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Figure 1: Left: shadow map reparametrization techniques (lightspace perspective shadow maps is used here) alone cannot guarantee subpixel accuracy (leading to perspective aliasing in the lower right corner and projection aliasing on the slope in the middle of the scene), even with a 4096 2 shadow map. Right: Queried Virtual Shadow Maps prevent both types of undersampling artifacts. Shadowing scenes by shadow mapping has long suffered from the fundamental problem of undersampling artifacts due to too low shadow map resolution, leading to so-called perspective and projection aliasing. In this paper we present a new real-time shadow mapping algorithm capable of shadowing large scenes by virtually increasing the resolution of the shadow map beyond the GPU hardware limit. We start with a brute force approach that uniformly increases the resolution of the whole shadow map. We then introduce a smarter version which greatly increases runtime performance while still being GPU-friendly. The algorithm contains an easy to use performance/quality-tradeoff parameter, making it tunable to a wide range of graphics hardware.

We evaluate several shadow map algorithms based on warping and partitioning using the maximum perspective aliasing error over the entire view frustum. With respect to our error metric, we show that a range of warping parameters corresponding to several previous warping algorithms have the same error. We also analyze several partitioning schemes to determine which produces the least maximum error using the least number of partitions. Finally, we show how warping and partitioning can be combined for interactive rendering of low error shadows in scenes with a high depth range.

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map, b) TSM with 4K×4K shadow map, and c) 1K×1K subdivided shadow map. This configuration with a small angle between the light and view directions is difficult for prior methods. Even with the largest shadow map that can be allocated on current hardware, TSMs are not able to match the quality of subdivided shadow maps for this view. We present a technique for reducing perspective aliasing error in shadow maps. From the viewpoint of the light, the scene is first split into subdivisions defined by the visible faces of the camera frustum. The frustum subdivisions may be further subdivided along their corresponding faces. We apply a separate shadow map warp to each resulting subdivision. This produces significantly less error than applying a single shadow map warp to the whole scene We layout the subdivisions in rectangular regions within a single shadow map, using the maximum error of each subdivision to assign larger regions to subdivisions with higher error. Our method runs well on commodity graphics hardware and is easy to integrate into existing shadow map systems. We are able to achieve interactive performance (8-25 fps) on a power plant model (12M triangles), a double eagle tanker model (82M triangles), and the St. Matthew model (370M triangles) running on a PC with a GeForce 7800 GTX. We observed significantly less aliasing compared to prior shadow map warping algorithms. 1

This paper presents resolution-matched shadow maps (RMSM), a modified adaptive shadow map (ASM) algorithm, that is practical for interactive rendering of dynamic scenes. Adaptive shadow maps, which build a quadtree of shadow samples to match the projected resolution of each shadow texel in eye space, offer a robust solution to projective and perspective aliasing in shadow maps. However, their use for interactive dynamic scenes is plagued by an expensive iterative edge-finding algorithm that takes a highly variable amount of time per frame and is not guaranteed to converge to a correct solution. This paper introduces a simplified algorithm that is up to ten times faster than ASMs, has more predictable performance, and delivers more accurate shadows. Our main contribution is the observation that it is more efficient to forgo the iterative refinement analysis in favor of generating all shadow texels requested by the pixels in the eye-space image. The practicality of this approach is based on the insight that, for surfaces continuously visible from the eye, adjacent eye-space pixels map to adjacent shadow texels in quadtree shadow space. This means that the number of contiguous regions of shadow texels (which can be efficiently generated with a rasterizer) is proportional to the number of continuously visible surfaces in the scene. Moreover, these regions can be coalesced to further reduce the number of render passes required to shadow an image. The secondary contribution of this paper is demonstrating the design and use of data-parallel algorithms inseparably mixed with traditional graphics programming to implement a novel interactive rendering algorithm. For the scenes described in this paper, we achieve 60–80 frames per second on static scenes and 20–60 frames per second on dynamic scenes for 5122 and 10242 images with a maximum effective shadow resolution of 32, 7682 texels. Categories and Subject Descriptors: I.3.7 [Computer Graphics]: Three-Dimensional Graphics and Realism—Color, shading, shadowing, and texture

Due to its versatility, speed and robustness, shadow mapping has always been a popular algorithm for fast hard shadow generation since its introduction in 1978, first for off-line film productions and later increasingly so in real-time graphics. So it is not surprising that recent years have seen an explosion in the number of shadow map related publications. The last survey that encompassed shadow mapping approaches, but was mainly focused on soft shadow generation, dates back to 2003 [HLHS03], while the last survey for general shadow generation dates back to 1990 [WPF90]. No survey that describes all the advances made in hard shadow map generation in recent years exists. On the other hand, shadow mapping is widely used in the game industry, in production, and in many other applications, and it is the basis of many soft shadow algorithms. Due to the abundance of articles on the topic, it has become very hard for practitioners and researchers to select a suitable shadow algorithm, and therefore many applications miss out on the latest high-quality shadow generation approaches. The goal of this survey is to rectify this situation by providing a detailed overview of this field. We provide a detailed analysis of shadow mapping errors and derive a comprehensive classification of the existing methods. We discuss the most influential algorithms, consider their benefits and shortcomings and thereby provide the readers with the means to choose the shadow algorithm best suited to their needs.

One of the most demanding challenges for real-time shadow algorithms is their application to large-scale, polygon-rich and dynamic environments. In this paper, we discuss the major problems encountered in applying shadow maps to such an environment and provide practical and robust solutions to the appearing problems. We tackle projection aliasing with the aid of an eye space blur. We compare the major biasing methods to remove incorrect self-shadowing of polygons. Finally we are providing some advancements to the recently published light space perspective shadow mapping method to resolve projection aliasing problems.

This research focuses on developing efficient algorithms for computing shadows in computer-generated images. A distinctive feature of the shadow algorithms presented in this thesis is that they produce correct, physically-based results, instead of giving approximations whose quality is often hard to ensure or evaluate. Light sources that are modeled as points without any spatial extent produce hard shadows with sharp boundaries. Shadow mapping is a traditional method for rendering such shadows. A shadow map is a depth buffer computed from the scene, using a point light source as the viewpoint. The finite resolution of the shadow map requires that its contents are resampled when determining the shadows on visible surfaces. This causes various artifacts such as incorrect self-shadowing and jagged shadow boundaries. A novel method is presented that avoids the resampling step, and provides exact shadows for every point visible in the image. The shadow volume algorithm is another commonly used algorithm for real-time rendering of hard shadows. This algorithm gives exact results and does not suffer from any resampling problems, but it tends to consume a lot of fillrate, which leads to performance problems. This thesis presents a new technique for locally choosing between two previous shadow volume algorithms with different performance characteristics. A simple criterion for making the local choices is shown to yield better performance than using either of the algorithms alone. Light sources with nonzero spatial extent give rise to soft shadows with smooth boundaries. A novel method is presented that transposes the classical processing order for soft shadow computation in offline rendering. Instead of casting shadow rays, the algorithm first conceptually collects every ray that would need to be cast, and then processes the shadow-casting primitives one by one, hierarchically finding the rays that are blocked. Another new soft shadow algorithm takes a different point of view into computing the shadows. Only the silhouettes of the shadow casters are used for determining the shadows, and an unintrusive execution model makes the algorithm practical for production use in offline rendering. The proposed techniques accelerate the computing of physically-based shadows in real-time and offline rendering. These improvements make it possible to use correct, physically-based shadows in a broad range of scenes that previous methods cannot handle efficiently enough. This thesis consists of an overview and of the following 5 publications: 1. T. Aila and S. Laine. Alias-Free Shadow Maps. In Rendering Techniques 2004 (Eurographics Symposium on Rendering), pages 161-166. Eurographics Association, 2004. 2. S. Laine and T. Aila. Hierarchical Penumbra Casting. Computer Graphics Forum, 24 (3): 313-322, 2005. 3. S. Laine. Split-Plane Shadow Volumes. In Graphics Hardware 2005 (Eurographics Symposium Proceedings), pages 23-32. Eurographics Association, 2005. 4. S. Laine, T. Aila, U. Assarsson, J. Lehtinen and T. Akenine-Möller. Soft Shadow Volumes for Ray Tracing. ACM Transactions on Graphics, 24 (3): 1156-1165, 2005. 5. J. Lehtinen, S. Laine and T. Aila. An Improved Physically-Based Soft Shadow Volume Algorithm. Computer Graphics Forum, 25 (3): 303-312, 2006.

Figure 1: Shadows comparison with image resolution at 800×600. Solving aliasing artifacts is an essential problem in shadow mapping approaches. Many works have been proposed, however, most of them focused on removing the texel-level aliasing that results from the limited resolution of shadow maps. Little work has been done to solve the pixel-level shadow aliasing that is produced by the rasterization on the screen plane. In this paper, we propose a fast, sub-pixel antialiased shadowing algorithm to solve the pixel aliasing problem. Our work is based on the alias-free shadow maps, which is capable of computing accurate per-pixel shadow, and only incurs little cost to extend to sub-pixel accuracy. Instead of direct supersampling the screen space, we take facets to approximate pixels in shadow testing. The shadowed area of one facet is rapidly evaluated by projecting blocker geometry onto a supersampled 2D occlusion mask with bitmasks fusion. It provides a sub-pixel occlusion sampling so as to capture fine shadow details and features. Furthermore, we introduce the silhouette mask map that limits visibility evaluation to pixels only on the silhouette, which greatly reduces the computation cost. Our algorithm runs entirely on the GPU, achieving real-time performance and is an order of magnitude faster than the brute-force supersampling method to produce comparable 32 × antialiased shadows.