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1 Camera Space Shadow Maps for Large Virtual Environments Camera Space Shadow Maps for Large Virtual Environments Ivica Kolic 1 ⋅⋅⋅ Zeljka Mihajlovic 2 Abstract This paper presents a new single-pass shadow mapping technique that achieves better 1 Introduction quality than the approaches based on perspective warping, such as perspective, light-space and A shadow is one of the most important elements for trapezoidal shadow maps. The proposed technique achieving realism in virtual environments. Over the is appropriate for real-time rendering of large years, many real-time shadow-generation virtual environments that include dynamic objects. approaches have been developed. The most well- By performing operations in camera space, this known approaches are fake and planar shadows, solution successfully handles the general and the shadow volumes (Crow 1977 ), and shadow dueling frustum cases, and produces high-quality mapping (Williams 1978 ). shadows even for extremely large scenes. This This paper primarily focuses on the shadow paper also presents a fast non-linear projection mapping technique that is recognized as the most technique for shadow map stretching that enables important shadow generation tool today because of complete utilization of the shadow map by its simplicity, generality, and predictability. The eliminating wastage. The application of stretching basic shadow mapping principle involves two results in a significant reduction in unwanted steps. In the first step, the shadow map is generated perspective aliasing, commonly found in all by storing the depth of the scene from the point of shadow mapping techniques. Technique is view of the light. In the second step, the scene is compared with other shadow mapping techniques, rendered regularly; however, for every drawn pixel, and the benefits of the proposed method are the shadow map is consulted to determine whether presented. The proposed shadow mapping that particular pixel is directly visible to the light. technique is simple and flexible enough to handle This is accomplished by comparing the distance of most of the special scenarios. An API for a generic the shadow map from the light with the distance of shadow mapping solution is presented. This API the pixel (depth) from the light. simplifies the generation of fast and high-quality In this paper, we will present a new shadow shadows. mapping approach, termed camera space shadow maps (CSSM). This approach acquires the shadow Keywords : Shadow maps, Real-time shadows, map in a simple manner by directly using a camera. Dynamic shadows, Virtual environments We will also describe a method for the maximum utilization of the shadow map to prevent wastage and significantly reduce the perspective aliasing of the shadow near the camera. The organization of the remainder of this paper is as follows: Previous work in the area of shadow mapping is summarised in Section 2, while the new CSSM approach is described in Section 3. Section 4 presents a novel pixel-accurate shadow map stretching technique. In Section 5, the experimental results are discussed, and the new CSSM approach is compared with other approaches. Details of the 1 Ivica Kolić Systemcom d.o.o. mail: [email protected] 2 Željka Mihajlović University of Zagreb, Faculty of Electrical Engineering and Computing, Croatia Unska 3, 10 000 Zagreb, Croatia mail: [email protected] 1 generic shadow mapping API (Kolic 2010 ) are pass robust solution capable of handling most of provided in Section 6, while section 7 presents the the special cases has not yet been proposed. Multi- conclusions and future work. pass approaches, on the other hand, are widely used today due to their ability to provide good-quality 2 Previous Work shadows in most of the special cases (such as dueling frustum). In these approaches, the scene is Shadow mapping techniques can be classified as split into smaller segments for which individual single-pass and multi-pass approaches, based on shadow maps are acquired and applied. The main the number of passes required to create the shadow disadvantages of these approaches are low speed, map. Single-pass approaches are fast and less high memory consumption, and discontinuity in demanding; however, in general, they cannot be shadow quality in the region where separate used for large-scale environments because it is shadow maps connect. Hence, their usage is mainly difficult to achieve good shadow map distribution reserved for scenes with only one dominant light, for both near and distant shadows. Several such as the Sun. Two popular multi-pass approaches such as perspective shadow maps approaches are cascaded shadow maps (Dimitrov (PSM) (Stamminger and Drettakis 2002 ; Kozlov 2007 ) and parallel split shadow maps (Zhang et al. 2004 ), light space shadow maps (LiSPSM) 2006 ). A detailed description of shadow mapping (Wimmer et al. 2004 ), trapezoidal shadow maps approaches can be found in Nealen (2002 ) and (TSM) (Martin and Tan 2004 , Martin 2008 ), and King (2004 ). soft shadow maps (Mo et al. 2007 ) have been developed to address this issue. However, a single- 3.1 Construction of shadow map 3 Camera space shadow maps The main principle of perspective warping-based The aim of this paper is to provide the first approaches involves treating geometry with the complete shadow mapping solution that, in a single projections of both camera and light. Thus, a pass, always produces the highest quality shadow shadow closer to the camera gets more shadow map map for any type of light/scene/camera and various space. The goal must be decided on the basis of the special cases. In theory, the highest-quality shadow manner in which regular geometry is treated by the map would be achieved if every pixel on the screen camera. The goal must be to transform shadow has exactly one corresponding texel in the shadow mapping geometry into camera space, using the map. This level of quality can only be achieved by camera matrix for shadow map warping. However, ray tracing. Hence, in this paper, a considerably it is not possible to simply transform, rotate, and simpler definition is used: the highest shadow scale geometry because this would cause loss of mapping quality is achieved when the resulting light occlusion and depth information. Depth shadow map is completely free of wastage, and information is not very relevant because it can every texel is reserved exclusively for shadow always be calculated separately in a vertex or even passing through visible space. pixel shader, but the preservation of occlusion The shadow mapping solution must also provide an information is essential. We, however, made the easy method to control distance-based shadow following observation: If shadow casting geometry distribution. This can be implemented by allowing is projected on any arbitrary plane, the limitations the specification of a distance value for which a on translation, rotation, and scaling are avoided, certain percentage of the shadow map should be and the geometry can be transformed into camera reserved. For example, the shadow within 20 m space without losing occlusion information. Once from the camera should occupy 50% of the shadow the geometry is in camera space, perspective map. This cannot be achieved using linear warping occurs automatically. transformation. Fig. 1 demonstrates a new, simple method for Existing perspective warping approaches (such as acquiring the shadow of the entire visible space TSM and LiSPSM) were designed primarily for the using only one projection plane and the camera general case, and they cannot efficiently handle itself. Projection plane P is placed such that it complex dueling frustum cases and various other touches the lower point of the viewing frustum at possible lighting conditions. Therefore, a new, the near clipping distance and the opposite point at more generic approach will be presented, in which the far clipping distance. However, today, a it is easy to perceive the configuration to be used common scenario involves rendering of enormous for special cases, such as dueling frustum, camera scenes where far clipping is not done at all. In this without far clipping and the most difficult case of case, the shadow plane should be parallel with the the large omnidirectional light located within appropriate side of the view frustum, as shown in visible space. the figure. Fig. 1 also suggests that if a light is located slightly in front of the viewer, the starting point for the plane should be the upper point of the viewing frustum at the near clipping distance. 2 • Change the FOV (Field of view) angles of the rotated camera so that the entire view frustum of the original camera can fit within the new view frustum. • Design a plane that passes through the two lower points of the near clipping plane and the two upper points of the far clipping plane, as shown in Fig. 1. If the source of light is located slightly in front of the camera, opposite points should be used and the FOV angle of the X-axis of the camera must be slightly increased, so that the plane seen from the point of view of the light covers the entire view frustum. Fig. 1. Basic principle of CSSM. The projective plane P is • Light-project shadow casting geometry on the located within the view frustum V. The case with the limited view frustum is shown on the left. When the view frustum is not constructed plane. far clipped (right), the projective plane is parallel to the side of • Use the projection matrix of the rotated camera the view frustum. to acquire the shadow map. The dueling frustum case is shown in Fig. 2. From In the dueling frustum case, CSSM is acquired in the figure on the left, it is clearly visible that a the following manner: single projection plane cannot provide good shadow projection into visible space, and hence, • Determine a point on a near clipping plane by multiple planes must be used, as shown in the applying the projection of the camera on the figure on the right.
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