Stencil Shadow Volumes

Transcription

1 Helsinki University of Technology Telecommunications Software and Multimedia Laboratory T Seminar on Computer Graphics Spring 2002 Rendering of High Quality 3D-Graphics Stencil Shadow Volumes Matti Dahlbom 46540L

2 Stencil Shadow Volumes Matti Dahlbom HUT, Telecommunications Software and Multimedia Laboratory Abstract This paper discusses modern methods for realtime and non-realtime approaches to generating realistic shadows in 3D rendering systems. It describes in reasonable depth the method of Shadow Volumes, and presents some comparison among the commonly used algorithms. The paper covers essential topics concerning stencil shadow volumes, such as efficiency concerns and memory consumption during calculation of the volumes. There is also consideration about problems applying this technique in practice. 1

3 1 Introduction Creating realistic shadows has been a hard task to overcome for the entire history of real-time 3D graphics. The shadow calculation methods are inaccurate and inefficient, and usually hard to implement generically enough. Approximations and so-called fake shadows or shadow slabs have been used as a crude workaround, but in reality they have nothing to do with real shadow casting. A number of methods have been developed for this purpose one of these being Shadow Volumes. The basic idea of the method is to calculate a volume projected into the object space by the light source. Each polygon facing the light source creates a shadow volume. Each of these volumes consists of polygons formed by casting a ray from the light source through polygon vertices into the image-space. The resulting volume is then used to divide all polygons into the ones in shadow and the ones not. A sample shadow volume is demonstrated by figure 1. Figure 1: by Michael McCool: Shadow Volume of a Triangle 2

4 2 Background In 1977, Franklin J. Crow introduced a survey (see [Crow, July 1977]) distinguishing three applicable methods to the shadow generation problem known at the time: A polygon scan-line approach: shadowing polygons are projected onto a polygon being scanconverted, and the transitions to and from shadow are calculated per shadow polygon edge for each scanline. A two-pass hidden surface approach: on the first pass, the light source casting the shadow is used as the view point; polygons in shadow and polygons either partly or completely in light are determined, and the shadow polygons are added temporarily to the image space. The second pass renders the scene, using the shadow polygons to perform the shadowing. Shadow volumes approach: for each shadowing polygon P a shadow volume is created. The shadow volume is clipped against the view volume (see figure 3). Then all the polygons inside the view volume are clipped against the shadow volume, again producing shadow polygons that are used for determining the transition in and out of shadow in the rendering stage. The next year Lance Williams introduced a fourth approach (see [Williams, August 1978]), namely z-buffer shadow computation. And in 1980, Turner Whitted introduced fifth, namely the ray tracing method (see [Whitted, 1980]). The methods presented and numerous variations of them have coexisted and evolved. Currently many implementations of each exist. Not all of the approaches mentioned are fit for realtime graphics, but they have uses in numerous other contexts. While methods based on ray tracing are not suitable for practical appliance in realtime 3D rendering, they serve as an excellent tool for modelling photorealistic images of scenes in environments that do not require real-time performance. The usual tradeoff in computer graphics holds: quality comes with the expense of performance drop and increase in memory consumption. 3 Other Methods This section briefly descibes other methods available for shadow generation used in modern 3D rendering systems. 3.1 Projected Planar Shadows Projected planar shadows are shadows formed by projecting shadow polygons on visible polygons by a lightsource. The surfaces receiving the projected shadows need to be flat as the name suggests for the technique to work and be effective. Projected shadows algorithm can be rather easily changed to support also soft shadows. An implementation of the algorithm producing soft shadows using projected planar shadows is described in [Haines, 2001]. The idea is, that as the edges and vertices of 3

5 the shadowing polygon are cast onto the planar surface, the interior of the polygon is rendered similarly to sharp shadows, with the darkest color. The vertices and edges of the projected shadow polygon are then shadowed, using interpolative method to produce a soft shadow edge by drawing a gradient from darkest shadow to white when moving further from the shadow polygon. The size of the soft shadow gradient area is dependent from the distance from the casting vertex to the surface receiving the shadow. This technique is demonstrated in figure 2. This method effectively mimics the real world where there is no such thing as a point light source. Figure 2: by Eric Haines: Soft Planar Shadows 3.2 Shadow Maps Applying shadow maps involves using the depth buffer with projective texture mapping to create a shadow map for the shadowing object. This is done by transforming the scene so that light source acts as a camera; then the scene gets rendered, creating the depth buffer. The resulting depth map is then projected on the objects to be rendered. Each pixel to be drawn is compared against the corresponding value in the projected map do decide whether the pixel is in shadow or not. Shadow mapping is easily extended to handle soft shadows. This method is discussed in more detail in [McReynolds, 1996]. Williams [Williams, August 1978] suggested the following steps be taken: acquire depth image from light source construct the viewpoint image 4

6 do transformation from view space to light source space draw based on comparison of depth value to one in shadow depth buffer William s method is often referred to as the Z-Buffer shadowing. Pros for shadow maps is the simplicity of the algorithm, while cons are possibly incorrect self-shadowing and aliasing effects due to limited resolution of the shadow map. Similar, more modern methods applying shadow maps have been published, one being Hansong Zhang s Forward Shadow Mapping ([Zhang, 1998]). A more accurate and widely used technique is deep shadow mapping. Instead of storing single depth information at each pixel, a fractional visibility value is stored for a pixel at all depth levels needed. This approach is described in [Lokovic & Veach, August 2000]. 3.3 Global Illumination The radiosity method is the most commonly known and used method of global illumination. It is based on thermal engineering and models of radiation emission and reflection. The algorithm can be crudely simplified as such: break environment into finite area patches and assume each is a Lambertian (diffuse) emitter and reflector find all interactions between patches exchange energy between patches until in equilibrium The biggest advantage of the radiosity method is that since diffuse lighting of a surface is view-independent, the lighting can be precalculated. Radiosity method may be used to create highly realistic yet static scenes. The radiosity method is described in [Sillion & Puech, 1994]. To complete all the aspects of the scene lighting, methods such as Monte Carlo Raytracing [McCabe, 1999] or Photon Mapping [Jensen, 2001] must be used. 5

7 4 Method This section considers the shadow volume method, covering the basic idea of the approach as well as the use of more advanced algorithms to improve quality and efficiency of shadow volumes. 4.1 Basics In a few words, the shadow volume technique for one light source can be described as follows: Each polygon that is facing the light source, generates a shadow volume. Shadow planes forming the frustum are created from each edge of the shadowing polygon. The shadow planes are then clipped against the view volume. Resulting polygons (called shadow polygons by Crow, see [Crow, July 1977]) are added to the scene s polygon data structure as invisible polygons. During the scan-conversion process, the shadow polygons mark transitions into or out of shadow. Figure 3 demonstrates the clipping of the shadow volume against the view frustum. The use of silhouettes is described in section 4.2. Figure 3: by Dave Salvator: Shadow Volume Intersecting View Volume 6

8 4.2 Using Silhouettes Calculating the shadow volumes for every single polygon in an object takes up a lot of time and most of the work is in vain. The algorithm is vastly improved by finding the contour edges of the whole object at a time, making up the silhoutte of the object. The use of a silhouette for creating the shadow volume for an object is demonstrated by figure 3. Using the silhoutte edges of the object for creating a single shadow volume for the entire object is fully accurate and much less memory consuming because of the huge amount of shadow polygons saved. Also the scan-line renderer performs faster because of less intersections with the shadow polygons. The silhouette must be recalculated each time a light source moves proportional to the object for which the silhouette is calculated, or the shape of the object changes. The change in position or orientation of the camera does not force recalculation of the silhouette. 4.3 Stencil Buffering It is imperative to correctly find the parts of the objects that lie in a shadow. For each pixel in the scene, a count is kept about entering and leaving the shadow. When a shadow volume is entered by crossing its front-facing boundary shadow polygon, the count is increased. When a shadow volume is exited by crossing its back-facing boundary shadow polygon, the count is decreased. When done, the pixels with a nonzero count are found to lie in shadow. The values are stored in an optimized buffer, called the stencil buffer. 5 Problems Shadow volumes require intensive calculation and therefore there are issues requiring serious consideration for optimizations when implementing shadowing for a scene. Also allowing complex object structures poses a problem one must handle in order to produce results with no artifacts and flaws in shadow rendering. 5.1 Open Models Using open polyhedras (ones with missing faces) is a problem for the Crow s original algorithm, since the missing faces are not there to increase the depth count. Because no such object exists in the real world, the problem can be avoided by using closed models. Despite of the fact that uses for an open model even in computer graphics are scarce, this issue has to be dealt with. The updated version of the Crow algorithm by P. Bergeron handles open models correctly by taking use of countour edges and altering the depth count, described in [Bergeron, September 1986]. 5.2 Multiple Light Sources For multiple light sources, the algorithm must be extended slightly to accommodate the possibility of one light source lighting up an area shadowed by another. For each light 7

9 source, the shadow volumes must be regenerated. When rendering the scene for each light source, instead replacing pixel values of the unshadowed areas (for that light), a blending function is applied to add the current light s contribution to the lighting of the pixel. The number of light sources actively producing shadows is in direct proportion to the required rendering time. 5.3 Eye in Shadow, Capping When the camera lies inside the shadow volume, the algorithm does not hold; there is no front-facing shadow polygon (one that causes transition into shadow when rendering) in front of the camera, and therefore we might never meet an shadow polygon when rendering a polygon inside the same shadow volume. We need something to detect the situation of the camera being inside the volume. A workaround to this one is simple; we cap the shadow volume. This means closing the semi-infinite volume in such a way that the far end normally reaching out to infinity is cut and a closing polygon is added. The shadow volume needs to be culled in front of the camera also, to produce a plane of entrance into the shadow. Although the idea is simple and straightforward, an efficient implementation might not be. 5.4 Efficiency The number of shadow polygons for a scene can be enormous, especially if silhouttes are not being used for the generation of the volumes. One efficient way to go lessen the burden generated by the shadow polygons is occlusion culling. It means reducing the shadow volume s complexity when possible. This is the case when a large, simple object receives shadow from a smaller, more complex object. If the larger object totally blocks the shadow volume of the smaller object, the smaller object s shadow volume may be discarded and the larger one s be used from there on. 5.5 Memory Consumption Due to the creation of the shadow volumes and their shadow volumes, the memory strain may become huge. Since each triangle (used here as a special case of a polygon) has 3 edges, the resulting shadow volume has 3 shadow polygons. Each light source generates its own shadow volume; therefore a scene with 3 lightsources, 10 shadowing objects consisting of 500 triangles each would generate approximately 3 * 10 * 250 * 3 = shadow polygons, assuming that 50% of the polygons of an object are facing each light. Also the stencil buffer takes up space, depending on the resolution and buffer depth used. For a example scene presented above, a 8bit buffer might easily overflow due to the huge amount of shadow transitions caused by the shadow polygons. As discussed in section 4.2, using silhouettes of objects instead of the polygons separetely would simplify the shadow volumes significantly, decreasing also the memory consumption. In practice, silhouettes are always used to obtain reasonable performance. 8

10 5.6 Producing Soft Shadows The algorithm as explained above produces shadows with sharp edges, often called hard shadows. The method may be used to create soft shadows also by runtime replacing one light source by multiple virtual light sources with lesser intensity, and variating their position slightly. The resultant shadow appears as one with soft edges. This method naturally decreases efficiency somewhat, since the algorithm needs to be applied more than once for a light source. 6 Conclusion Shadow volumes are a noteworthy choice when selecting shadowing algorithm in a real-time 3D graphics application. The algorithm is easy to understand and it is rather straightforward to implement in a crude form. The optimizations and special case handlings such as multiple lights and eye-in-shadow can be added based on the needs later on. Since the algorithm is well separated from the scanline rendering, it is easy to implement to an existing system using simple or no shadowing at all. It already has many flexible implementations in OpenGL and will most likely set the standard of real-time shadowing in modern games and modelling applications. 9

11 References [Bergeron, September 1986] Bergeron Philippe. September Shadow Algorithms for Computer Graphics. Computer Graphics (Proceedings of SIGGRAPH 78), 12(2), [Crow, July 1977] Crow Franklin. July Shadow Algorithms for Computer Graphics. Computer Graphics (Proceedings of SIGGRAPH 77), 11(2), Held in San Jose, California. [Haines, 2001] Haines Eric Soft planar shadows using plateaus. Journal of Graphics Tools, 6(1), [Jensen, 2001] Jensen Henrik Wann Realistic Image Synthesis Using Photon Mapping. Natick, MA: A. K. Peters. [Lokovic & Veach, August 2000] Lokovic Tom, & Veach Eric. August Deep Shadow Maps. SIGGRAPH 2000 Proceedings. [McCabe, 1999] McCabe Hugh Markov Chain Monte Carlo Methods for Global Illumination. Tech. rept. TCD-CS Department of Computer Sciene, Trinity College, Dublin, Ireland. Available from [McReynolds, 1996] McReynolds Tom Shadow Maps. Tech. rept. Silicon Graphics. SIGGRAPH 96 Course. [Sillion & Puech, 1994] Sillion Francois, & Puech Claude Radiosity and Global Illumination. San Francisco: Morgan Kaufmann Publishers. ISBN [Whitted, 1980] Whitted Turner An Improved Illumination Model for Shaded Display. Communications of the ACM, 23(6). [Williams, August 1978] Williams Lance. August Casting Curved Shadows on Curved Surfaces. Computer Graphics (SIGGRAPH 78 Conference Proceedings), 12, [Zhang, 1998] Zhang Hansong Forward Shadow Mapping. Proceedings of the 9th EuroGraphics Rendering Workshop. Held in Vienna, Austria, June 29-July 1. 10