A Survey of Shadow Volume Algorithms in Computer Graphics

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2 Abstract A Survey of Shadow Volume Algorithms in Computer Graphics Hoshang Kolivand and Mohd Shahrizal Sunar Department of Computer Graphics and Multimedia Universiti Teknologi Malaysia, Johor, Malaysia This study will provide an overview of popular and famous algorithms and techniques in shadow generation based on shadow volumes. It characterizes well known techniques, describing each of them in detail, and discusses the advantages and drawbacks of each. Basic ideas, improvements, and future works of the techniques are also comprehensively summarized and analyzed in depth. Often, programmers have difficulty selecting an appropriate shadow generation algorithm based on shadow volumes that is specific to their purpose. We have classified and systemized these techniques. The main goal of this paper is to provide researchers with background on a progress of shadow volume techniques so as to make it easier for researchers to choose the method best suited to their aims. It is also hoped that our analysis will help researchers find solutions to the shortcomings of each technique. Keywords Hard shadow, Shadow mapping, Shadow rendering, Shadow volume, Soft shadow. 1. Introduction Shadows are one of the most important aspects in virtual environment, both for the spectator to detect distance relationships between objects, and to make the scene more realistic. The first researcher who worked with shadows was Leonardo Da Vinci in 1460 [1] [Figure 1]. He focused on painting and static images. There are some materials by Lambert [2], who worked with shadows; especially, he focused on the geometry underlying the shadow receiver. Shadows help to understand the relative distance of objects in a scene. Without shadows and shadow casters, it is difficult to comprehend the real size of objects when compared with other objects, which are located far away. For example, in Figure 2, on the left top side of the illustration, the distance and size of the objects are not clear but on the left down side of Figure 2, it is obvious that scissors are above the plane, and torus is on the plane. Another advantage is that shadows help the viewer to understand the geometry of a complex object. In Figure 2, in top right panel, it is not clear how many tines the fork has. However, the number of tines in the right down panel is clear. Hard shadows are kinds of shadows with a point light source. They have a sharp border outline, and they include fully shadowed regions without any soft edge. Soft shadows are another kind of shadow, which are caused by wide light sources. Soft shadows are more realistic for virtual environments. In the real world, most of light sources are wide and create soft shadows. Therefore, focusing on soft shadow will be more appropriate. Soft shadows include two parts, umbra and penumbra. An umbra is the part of the shadow region which cannot see any part of the light source, and it is fully shadowed. In this way, it is like hard shadow. The penumbra is the other part of shadow region. It can see part of light source. The part of the scene which can see the entire light source is said to be lit. Soft shadow creation is a fundamental problem in computer science, especially in computer graphics and computer generated imagery. The low speed of rendering on shadow generation is a crucial problem in the game s engine. Although current soft shadow algorithms are great achievements, they still suffer from the following drawbacks: Low speed rendering for a real time purpose Suffered from aliasing Acceptable quality for only small shadow caster Disappearing of the penumbra region when distance of shadow caster and shadow receiver is long. Unlike hard shadows shape which just depends on silhouette, a soft shadow shape depends on both shape of light source and location of occluder. Ray tracing and radiosity are other concepts, which can be used to create soft shadows. It is possible to quickly create a soft shadow by distribute ray tracing using uniform voxel structures [3]. Radiosity is another technique to create soft shadows by computing the diffuse inter reflections between different 38 IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB 2013

3 Figure 1: Left: Study of shadows by Leonardo da Vinci. Right: Shadow construction by Lambert. parts of the scene [4]. Radiosity is geometrical and needs more complex calculation; therefore, it has been used for simple surfaces. The increase in the number of techniques in shadow generation encouraged us to prepare an article that includes current famous techniques and investigation about them in details. In this paper, most improvements on shadow volume algorithms will be described in detail. It is believed that sufficient information about each could inspire researchers to find solutions for each of the weakness listed above and help researchers choose the best algorithms for their purposes. It is also hoped that after reading this paper, the researcher will be aware of the latest and most widely used techniques, and will have the best information on each of them. 2. Previous Survey The number of researchers who have worked with shadows is vast but in this section, some relevant papers with our approach are referenced. A well written but older paper on shadows is a survey of shadows by Woo et al. [5]. They focused on the complexity of algorithms. Storage usage, runtime complexity during actual rendering, and pre processing runtime complexity were the main factors that they focused on. They also presented information on different types of ray tracing and radiosity approaches to create soft shadows. The paper also included information about shadows from transparent objects. Haines and Möller [6] proposed an excellent survey on real time shadows. Another good survey on real time soft shadow generation can be found in [7]. In this paper, real time soft shadow algorithms are described and evaluated. After the introduction of some shadow concepts, important issues in computing soft shadows are investigated. Composition sampling using several light Figure 2: Left two panels: Comparison between a scene with shadow and without shadow to understand distance and size of objects, Right two panels: Comparison between a scene with shadow and without shadow to understand the geometry of complex objects. sources, occluders, and fake shadows were explained. Approximation of soft shadow is another topic that readers can find more information about it in this paper. The authors also mentioned that all soft shadow techniques suffer from approximation. This paper is very beneficial, but it is out of date and there is nothing about the latest algorithms. The strongest point of our paper is that it focuses on the most current and widely used algorithms. Cruz [8] prepared a survey on shadow maps. At first, he introduced the previous algorithms and then presented some of their advantages and drawbacks. A main shortcoming of this paper is lack of comparison between the discussed algorithms. Liu et al. [9] prepared a survey of shadow rendering with attention to projection shadows and shadow volumes. To date, Daniel Scherzer et al. [10] published the last survey on shadow entitled A Survey of Real Time Hard Shadow Mapping Methods. In this survey, the authors focused on shadow mapping and especially on shadow mapping errors. It is an overview of Casting Shadows in Real Time [11]. The main strengths of this research compared to previous surveys include the four following points: First, it includes all progress of shadow volume algorithms and describes them in detail focus on current algorithms. Second, we have tried to reveal all advantages and shortcoming of each technique. Third, hierarchy of developments of each algorithm based on existing drawback is expressed. Finally, the current work proposes some advices to remove the drawback and improve the shortcoming of each algorithm. IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB

4 3. Real time Shadow Techniques After producing hard shadows in computer games, many graphic researchers have shifted their focus to soft shadows. Nowadays, soft shadows are one of the most important effects in computer games. Shadow mapping is a convenient way of generating soft shadows, because it has aliasing. In order to omit aliasing, most of the proposed algorithm created soft outlines automatically. There are a lot of soft shadow algorithms for flat surfaces. Drettakis et al. [12] proposed an algorithm for wide light sources. They used shadow projection for a wide light source. Heckbert and Herf [13] proposed one of the famous algorithms on soft shadows. They proposed an algorithm for simulating a soft shadow for a wide light source. They could generate soft shadows with complex occluders on arbitrary objects. The amazing aspect of their algorithm was that it created soft shadows independent of the size of the light source. The generation of soft shadow was done by sampling the hard shadows. This algorithm needed a high number of samples. This technique s biggest problem was in the rendering of the attenuation maps. Gooch et al. [14] are other researchers who have worked on soft shadows. They used simple sampling with different colors to create soft shadows. Haines [15] was one of the first researchers who focus on a geometric approach to create soft shadows. In this method, a plane is a receiver, and an attenuation map produces soft shadows. The attenuation map is generated using modeling texture, which was created by converting the occluder s edges into a volume. To create soft shadows with respect to the light source, occluder, and shadow receiver, various techniques are used. A famous technique considers several narrow light sources or several occluders and creates separate hard shadows. The soft shadow is then created by combining these hard shadows. The quality of shadow depends on the number of samples. Another technique is fake shadow or physically exact. In this case, shadows are created from a wide light source. In this case, shadows are created by extending the shadow pattern outward from the center of the light source [16 19]. Third technique is approximating the penumbra region. Several different methods use this technique but the three principle ones are: 1) Extend the umbra region toward the outside of shadow and compute the penumbra region. 2) Shrink the umbra region and compute the penumbra region toward the inside shadow. 3) Combine both previous methods to create high quality soft shadow [7]. Fourth technique creates the umbra and penumbra using illumination. The regions which are completely hidden from all light sources are the umbra, and it should be dark. Regions which are not completely hidden from a light source create the penumbra and it should be partially illuminated [20]. The last technique can be used to create shadows from different objects. Obviously, the shadow generated from the combined shadows of several objects will be larger than the shadow created from a collection of individual shadows. The boarder of a combination shadows can be a curve line [21]. Shadowing of special objects is another method used to create soft shadows. In this method, shadows can be created for very complex objects such as hair, fur, and smoke. Deep shadow is convenient for these kinds of objects [22]. There are many techniques used to improve soft shadows but two of them are popular and widely used, shadow volume and shadow mapping. 4. Shadow Volume Algorithm Frank Crow [23] introduced the main idea of shadow volume and published his ray casting paper based on shadow volume algorithms. The method explicitly links shadow geometry to the view frustum. He introduced three types of shadows. This classification is helpful to the researcher as it provides many algorithms for shadow generation. Silhouette detection is most important part of shadow volume. Many researchers have tried to improve this more expensive part of shadow volume, but no one has been completely successful [24-32]. Z pass algorithm is an algorithm used for counting the shadows where there is more than one occluder. The first count is done according to view position. Whenever a ray from the light source enters the volume, the shadow count increases by 1. Whenever the ray exits from a shadow volume, the shadow count deceases by 1. If the final shadow count is 0, then the visible pixel is lit, if the final shadow count is positive then the visible pixel count is in shadow. Heidmann [33] published a paper based on shadow volume using stencil buffers, which remains the main shadow volume algorithm. He completed the shadow volume algorithm of Crow [23] and implemented it using stencil buffers for generating shadows on arbitrary objects. This well known algorithm can create shadow volume using stencil buffers and depth buffers. Z pass algorithm is used when the point of view is located out of the shadow 40 IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB 2013

5 and Z fail algorithm is used where the point of view is located in the shadow. Stencil buffer is another buffer that is discussed in these papers to create real time shadows. This kind of buffer is used to limit the area that requires shadow. In other words, stencil buffers can help to limit the scene of rendering. One of the convenient usages of the stencil buffers is where it combines with Z buffer. Stencil values can be automatically increased or decreased for every pixel when depth test is failed or passed. One of the crucial problems of shadow volume using stencil buffers occurs when the near plane of viewer s frustum intersect with one or more shadow volume planes. One of the solutions that McReynolds proposed is called capping [34]. The summary of the shadow volume algorithm is as follows: Step 1: Clear color buffer and z buffer Step 2: Render the scene with only ambient and emissive lighting Step 3: For all lights, clear stencil buffer, disable writing to color buffer and z buffer, set z buffer test to less than 0 (zero) Step 4: Render all front facing shadow mesh triangles generated by l; increase the stencil value when the z test passes Step 5: Render all back facing shadow mesh triangles generated by l; decrease the stencil value when the z test passes Step 6: Re enable writing to color buffer, set z buffer test to equal, set stencil test to pass when value is 0, and enable additive blending Step 7: Render the scene with only diffuse and secular lighting. After the first loop, which fills the z buffer, the whole loop process is repeated for every one of the other light sources in the scene. Since the shadowed areas resulting from one light source are completely independent of those caused by other light sources, stencil buffers must be cleared before repeating the loop process for subsequent light sources. Additive blending ensures that the sum of light contributions from each light source plus the ambient and emissive light is achieved. McCool [35] presented a new algorithm which combined shadow volume and shadow mapping that it will be discussed in the next section. He used depth mapping to recognize the silhouette in order to reduce the polygon count. Although McCool s technique is suitable for complex scenes, it has yet to overcome the aliasing problem. 5. Geometry based Algorithms Tomas Akenine [36] worked on soft shadows using a penumbra wedge. His algorithm is based on shadow volume which replaces a quadrilateral shadow with a penumbra wedge. The difference between shadow volume and his algorithm is that when a ray from the point of view enters the wedge, stencil buffer increases and when the ray exits the wedge, stencil buffer decreases. Everitt and Kilgard [37] proposed an algorithm of shadow volume to make it more robust when the point of view is located inside of shadow. Haller et al. [38] introduced a modified real time shadow volume algorithm that can be used in augmented reality applications. Fauerby and Kjaer [39] and then Assersson and Moller [40] introduced a new technique for generating soft shadows from a spherical light source. Lengyel enhanced their algorithm with different upgrades to filtrate and got a precise detection on penumbra fragments in 2005 [41]. Chan [42] is another researcher who worked on soft shadows. His technique was called smoothies. Smoothies are silhouette based, which, also describes soft shadow wedges. This method is geometrical and as a result, it can be accelerated with graphics hardware. Chan s method focuses on the outer penumbra, and the occluders projecting the umbra. Weng s theory [43] on soft shadows was entitled Real Time Shadow Casting using Fake Soft Shadow Volume with Stencil Buffer which is based on shadow volume. Recently, Billeter et al. [44] published a paper called Real Time Volumetric Shadows using Polygonal Light Volumes. They introduced a more appropriate method of computing the effects of scatterings in homogeneous contributor media for real time purposes rather than using the old ray marching based algorithms which most researchers were using. A polygonal mesh was used to enclose the volume of space that was directly illuminated by a light source. The base of the algorithm is Max s algorithm [45]. Max s algorithm generates shadow volume from a shadow map using the GPU rasterize for computing the lit segments. The algorithm sharpens the shadow map resolution and it is fast enough for low resolution shadow maps but not convenient when the shadow map resolution reach to 4086 by Recently, Chen et al. [46] proposed a new real time algorithm for computing volumetric shadows. The algorithm is GPU based in single scattering media. Evaluation of the integral scattering over the intersections of shadow map with view point rays is necessary to express a 2D height field. The viewpoint ray s travel through the shadow map is greatly enhanced with the application of an epipolar correction which allows the visible segments to be found but only in 1D height filed. A 1D min max mipmap is an acceleration structure of the algorithm which allows it to find lit segments in parallel epipolar segments. IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB

6 Baran et al. [47] proposed an algorithm similar to the one proposed by Chan et al. [46]. They use an epipolar correction to reduce the scattering integration. Although it is fast and has been an excellent worst case upper bound, an incremental traversal of multiple view point rays is required in a particular order. This makes the algorithm s full parallelism provided by a GPU difficult to utilize. Special hardware and lack of support by all Graphics cards are other shortcomings of this method. 5.1 Alleviating Geometric Overhead As mentioned earlier, shadow volumes are geometrically based and as a result, they need more calculations. Silhouette detection is one of the most expensive components of shadow volume. To reduce the geometric overhead, some techniques have proposed accelerating shadow volume. Culling and clamping are widely used and beneficial techniques Culling Culling is a technique used to omit the parts of the scene which do not contribute to the rendering Figure 3a. It is practical for wide and complex environments. Lloyd et al. [48] were the first researchers to use culling and called CC Shadow Volumes. In a scene, each object can be a shadow caster or a shadow receiver. Shadow volume culling tries to eliminate each shadow caster located inside the boarders of other shadow casters, or they do not contribution to the final shadow generated. This process is done in two steps. First, identify potential shadow receivers (PSR). In this step, PSR may include some objects, which are visible to the camera. In the next step, potential shadow caster (PSC) must be identified. A PSC is an object, which can be seen from the point of view of the light source [Figure 3b] Clamping Figure 4 (left: theory of shadow volume) illustrates that the volume continues infinitely. As a result, its projection will cover a huge number of pixels in the stencil buffer. To alleviate this consummation, clamped volumes must be computed so that they will more tightly surround a PSR [Figure 3c]. Two types of clamping were proposed by Lloyd et al. [48]. The Continuous Shadow Clamp (CSC) accurately outlines the shadow receivers. It can overestimate the size of shadow volume when a small part of the receiver is in the shadow. Another technique is Discrete Shadow Clamping (DSC) which clamps only occupied regions, but the region s outline will be only as precise as the region discretization. DSC may be combined with CSC to improve clamping (CV5 in [Figure 3d]). 5.2 Soft Shadow Volume Algorithms Akenine Möller and Assarsson [40] proposed a new geometrical base for soft shadow generation using a penumbra wedge. A penumbra wedge is constructed based on improved shadow volume [36]. The first proposed approach suggested encapsulating the penumbra region for each silhouette edge [49]. In this section soft shadow, using shadow volume is introduced. There are five phases used to generate soft shadows. Briefly, they are: 1. Render the whole scene with specular and diffuse then put into the frame buffer 2. Compute the visibility mask and put into V buffer 3. Create a hard shadow quad using the visibility mask with ambient lighting for silhouette edges and put into V buffer to make a soft shadow umbra region 4. Create a wedge using silhouette edges for the penumbra region 5. Determine what percentage of the light source can be seen for a pixel in the penumbra with respect to (a) (b) (c) (d) Figure 3: (a) Normal, PSC and PSR are in the scene. (b) Culling: PSC = {O 1, O 5} and PSR = {O 1, O 2, O 3, O 5 }. (c) Clamping: CSC: Shadow volumes SVi are clamped. (d) Clamping: DSC. 42 IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB 2013

7 the corresponding silhouette. Use this percentage in a V buffer to produce the amount of penumbra. For a pixel in the umbra region, this percentage will be 0; for a pixel in lit area, the percentage will be 1 and for each pixel in the remaining regions, the penumbra will be between 0 and Wedge Construction To construct the wedge Figure 5 for a silhouette edge requires that all the vertices must first be lifted to the same height from the light center (lc) (in the nearest vertex respect to lc) and the same for back and front of quads. Two planes are constructed containing the silhouette edge tangential to the light source. Front and back planes must be created based on the pass from silhouette vertex and tangential to the light source. Figure 4: Left: Theory of shadow volume, Right: Result of shadow volume using stencil buffer. The space between left, Right, front, and back planes is the final wedge Computing Soft Shadow After constructing the wedge, a soft shadow must be computed. To do this requires that the actual shadow intensity be computed. First, an arbitrary point must be chosen for a reference point such as light center. Next, a triangle is created with each edge meeting at the point. It is possible to treat the edges which have same orientations, independently and combine the results. As the only areas inside the lit regions are required, only those edges needed to be clamped. There is a cost associated with this method. On the other hand, it is much easier if one sums opposite sides of the triangle [Figure 6]. The main drawback of this technique is that it is necessary to combine occluders one by one in situations where the shadow information of the occluders is not the same to avoid using other expensive methods [49]. A strong umbra is a result of this combination. Although this method is precise, in case of non overlapping silhouettes, it has not yet been used due to its high cost rendering. Several improvements on volume soft shadow algorithms have been proposed. Lengyel [41] proposed a method to optimize penumbra wedge rendering. Assarsson [49] suggested splitting the light source into small parts and rendering each part separately to enhance the (a) (b) (c) (d) Figure 5: Wedge construction steps. Figure 6: Computing the occluded area for an occluder at a receiver point. Assuming each silhouette edge treats are not the same. IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB

8 quality. Forest et al. [50] recommended splitting the light source into four patches and compute the occluding contributions during each pass for all four patches in an effort to improve quality and speed. Forest et al. [51] proposed a real time algorithm based on soft shadow volume which focused on GPU. They did not use a hierarchical structure. They determine the view samples with the light silhouette edge instead of penumbra wedge. The algorithm is suitable when the number of light samples is low. However, it is not fully robust, but it is appropriate for use by shadow volumes in situation with more than one sample position and for selecting appropriate soft shadow values. Johnson et al. [52] proposed a silhouette based technique using a combination of geometry based and image based applications. An irregular shadow map is used to identify umbra contribution. Then, the penumbra wedge for each silhouette edge is generated, which is then projected into the light space. Affected screen space samples are tested for each projected wedge. Using inverse cosine function made this method faster than the inverse tangent values used by conventional techniques [50]. The algorithm is closer to shadow maps compared to shadow volumes. 6. Discussion 6.1 Image based Algorithms Shadow mapping requires an extension to be implemented in current graphics hardware. It uses GL_ARB_ SHADOW for the comparison between depth values. A problem with shadow mapping sampling occurs when surfaces are too close together. Offsetting the Z value using a small bias is a solution to this problem [5,7,53,54]. Shadow mapping is sometimes unable to project a scene in single shadow map, especially when the light source has been cut off at too large of an angle. In this case, a single light source can be converted to a combination of light sources, but this solution requires several shadow maps and consequently increases the time of rendering. 6.2 Geometrical based Algorithms Shadow volume is the most accurate technique used to create shadows on other objects. A Sharp outline of shadow volume would make it more precise. Geometrical base of shadow volume made it quite different from aliasing. Shadow volume has some advantages as well as some critical limitations. Advantages: Precise shadowing is the main advantage of this technique. There is no aliasing in shadow volume. On the contrary, shadow mapping and shadow volume support omni directional light sources. Shadow volume also supports self shadowing as well as shadow maps. Shortcomings: Sub pixel addressing is one of the drawbacks with which algorithms cannot work with [33]. Graphic card are filtered during the rendering. A large number of polygons are another problem. In this situation, each triangle must create three quadrilaterals that will need to be rendered in the stencil buffer. Although silhouette detection can solve this problem, it is one of the more expensive phases in shadow volumes. It increases the cost of rendering when there is a complex scene. Curve surfaces must be split into a small mesh. This will reduce the speed of rendering. In other words, the speed of rendering is directly related to numbering of polygon edges. A BSP tree is a solution that was proposed by Batagelo and Junior [55]. McCool [35] tried to reduce the cost of silhouette detection by computing a shadow map, and then extracting the discrete shadow map. The problem with his method was that it required rendering back the Z buffer from GPU to CPU. This transfer increased the cost of rendering. Brabec and Seidel [56] proposed a programmable graphics hardware based technique to compute the silhouette. Although their method is hardware based, it still suffers from reading back the depth value to CPU. 7. Conclusion and Future Works In this paper, we have tried to discuss about some famous shadow algorithms. Most importantly, object based and image based algorithms were discussed. An overview on each of them was presented. Identifying the main advantages of each algorithm was a prominent characteristic of this paper. The shortcomings of the earlier algorithms are the main reason new algorithms were devised. Most importantly, how each algorithm works was discussed. The figures illustrate the properties, advantages, and drawbacks of the algorithms. Finally, it was suggested that it would be beneficial to know which algorithm is best to use in different situations. Shadow volume is convenient when the precise shadowing is needed and when rendering cost is not a concern. Anti aliasing is another reason that may encourage a researcher to use shadow volume. On the other hand, the huge amount of calculation for an occluder is a major reason shadow volume is not used for large environments. Special hardware requirements are a drawback to using geometrical based shadow algorithms. Image based algorithms are convenient when the cost of rendering is an important consideration. 44 IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB 2013

9 As we have shown, none of the algorithms are suitable for all situations nor can they solve all the problems simultaneously. We hope this review can help researchers choose the best algorithm to meet their goals. It is some years that researchers have focused on shadow maps and have forgotten shadow volumes. We hope that this research could reveal some new ideas to improve current shadow volume algorithms and techniques for researchers who are interested in shadow volumes. References 1. P.H. Rautenbach, An empirically devived system for high speed shadow rendering, University Pretoria, J. H. Lambert, "Die freye perspektive," 1774, E. Lafortune, and Y. Willems, Bidirectional path tracing, Compugraphics 93, pp , C. Goral, K. Torrance, D. Greenberg, and B. Battaile, Modeling the interaction of light between diffuse surfaces, Computer Graphics, Vol. 18, no. 3, pp , A. Woo, P. Poulin, and A. Fournier, A survey of shadow algorithms, Computer Graphics and Applications, IEEE, pp , E. Haines, and T. Mller, Real time shadows, Game Developers Conference, J. Hasenfratz, M. Lapierre, N. Holzschuch, and F. Sillion, A survey of real time soft shadows algorithms, Computer Graphics Forum, Vol. 22, no. 4, pp , D.D. Cruz, Shadow maps: A survey, Universidade CCTC, Braga Portugal, N. Liu, and M. Pang, A survey of shadow rendering algorithms: Projection shadows and shadow volumes, Second International Workshop on Computer Science and Engineering, pp , D. Scherzer, M. Wimmer, and W. Purgathofer, A survey of real time hard shadow mapping methods, Computer Graphics Forum, Vol. 30, no. 1, pp , E. Eisemann, U. Assarsson, M. Schwarz, and M. Wimmer, Casting shadows in real time, ACM SIGGRAPH ASIA 2009 Courses, Drettakis, George, and W. Fiume, A fast shadow algorithm for area light sources using back projection, SIGGRAPH 94 Proceedings, pp , P. Heckbert, and M. Herf, Simulating soft shadows with graphics hardware, Carnegie Mellon University: Technical Report CMU CS, pp , B. Gooch, P. Sloan, A. Gooch, P. Shirley, and R. Riesenfeld, Interactive technical illustration, Proceedings 1999 Symposium on Interactive 3D Graphics, pp. 31 8, E. Haines, Soft planar shadows using plateaus, Journal of Graphics Tools, Vol. 6, no. 1, pp , C. Soler, and F. Sillion, Fast calculation of soft shadow textures using convolution, In Computer Graphics (SIGGRAPH 1998), pp , W. Heidrich, S. Brabec, and H.P. Seide, Soft shadow maps for linear lights high quality, In Rendering Techniques 2000 (11 th Eurographics Workshop on Rendering), pp , S. Laine, Split plane shadow volumes, In Proceedings of Graphics Hardware, Eurographics Association, pp , M. Tinghuai, Y. Qiaoqiao, L. Wenjie, G. Donghai, and L. Sungyoung, Grid Task Scheduling: Algorithm Review, Vol. 28, no.2, pp , S. Morein, Ati radeon hyperz technology, In Graphics Hardware Workshop, pp , H. Kolivand, and M.S. Sunar, To combine silhouette detection and stencil buffer for generating real time shadow, International Journal of Computer Graphic, Vol. 2, no. 1, pp. 1 8, T. Lokovic, and E. Veach, Deep shadow maps, In Proceedings of the 27 th Annual Conference on Computer Graphics and Interactive Techniques, pp , F. Crow, Shadow algorithms for computer graphics, Computer Graphics, Vol. 11, no. 2, pp , T. Isenberg, B. Freudenberg, N. Halper, S. Schlechtweg, and T. Strothotte, A developers guide to silhouette algorithms for polygonal models, IEEE CG and A, pp , F. Benichou, and G. Elber, Output sensitive extraction of silhouettes from polygonal geometry, In Proc. of Pacific Graphics, pp. 60 9, J. Buchanan, and M. Sousa, The edge buffer, a data structure for easy silhouette rendering, In Proc. of NPAR 2000, pp , S. Jung, S. Kwon, and K. Kim, Real time silhouette extraction using hierarchical face clusters, Dept. of Computer Engineering, pp. 1 8, H. Kolivand, M.S. Sunar, A. Amirshakarami, and Z. Ranjbar, Real Time Volume Shadow using Visible Non Visible Algorithm, Journal of Computer Science, Vol. 7, no.7, pp , F. Benichou, and G. Elber, Output sensitive extraction of silhouettes from polygonal geometry, In Proc. of Pacific Graphics 1999, pp. 60 9, Oct J. Buchanan, and M. Sousa, The edge buffer, A data structure for easy silhouette rendering, In Proc. of NPAR 2000, pp , Jun H. Kolivand, and M.S. Sunar, Silhouette Detection for Real Time Shadow in Virtual Environments, The International Journal of Virtual Reality, Vol. 10, no.3, pp , H. Kolivand, M.S. Sunar, and A. Rehman, Extended edge silhouette detection algorithm to create real time shadow volume, International Journal of Academic Research, Vol. 4, no.3, pp , T. Heidmann, Real shadows real time, IRIS Universe, Vol. 11, pp , McReynolds, T. David, Blythe, B. Grantham, and S. Nelson, Advanced graphics programming techniques using opengl, Course notes at SIGGRAPH 99, pp , M. McCool, Shadow volume reconstruction from depth maps, ACM Transactions on Graphics, pp. 1 25, T. Akenine Moller, and U. Assarsson, Approximate soft shadows on arbitrary surfaces using penumbrawedges, Thirteenth Eurographics Workshop on Rendering, C. Everitt, and M. Kilgard, Practical and robust stenciled shadow volumes for hardware accelerated rendering, available from: [Last accessed on 2010]. 38. M. Haller, S. Drab, and W. Hartmann, A real time shadow approach for an augmented reality application using shadow volumes, In Proceedings of VRST 03, pp , K. Fauerby, and C. Kjaer, Real Time Soft Shadows in a Game Engine, Master s Thesis, U. Assarsson, and T. Akenine Moller, A geometry based soft shadow volume algorithm using graphics hardware, ACM Transactions on Graphics (TOG), Vol. 22, no. 3, pp , E. Lengyel, "Mathematics for 3d game programming and computer graphics," Stamford, Connecticut, U.S: Cengage Learning; E. Chan, Efficient shadow algorithms on graphics hardware, Master s thesis, Massachusetts Institute of Technology, L.K. Weng, Real Time Shadow Casting using Fake Soft Shadow Volume with Stencil Buffer, Master Thesis, M. Billeter, E. Sintorn, and U. Assarsson, Real time volumetric shadows using polygonal light volumes, High Performance Graphics, pp , N.L. Max, Atmospheric illumination and shadows, In Computer Graphics (Proc. SIGGRAPH 86, New York, NY, USA), ACM, IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB

10 pp , J. Chen, I. Baran, F. Durand, and W. Jarosz, Real time volumetric shadows using 1d min max mipmaps, Proceedings of the 2011 ACM SIGGRAPH, I. Baran, J. Chen, J. Ragan Kelley, J. Durand, and F. Aad Lehtinen, A hierarchical volumetric shadow algorithm for single scattering, In ACM Transactions on Graphics (Proc. Of SIGGRAPH Asia) (2010, to appear), Vol. 29, no. 6, pp.178, B. Lloyd, J. Wendt, N.K. Govindaraju, and D. Manocha, Cc shadow volumes, In Rendering Techniques (Proceedings of the Eurographics Symposium on Rendering), Springer Computer Science. Eurographics, Eurographics Association, U. Assarsson, M. Dougherty, Michaeland Mounier, and T. Akenine Moller, An optimized soft shadow volume algorithm with real time performance, In Graphics Hardware, pp , V. Forest, L. Barthe, and M. Paulin, Accurate shadows by depth complexity sampling, Computer Graphics Forum (Proceedings of Eurographics 2008), Vol. 27, no. 2, pp , V. Forest, L. Barthe, and M. Paulin, Realistic soft shadows by penumbra wedges blending, In Proceedings of Graphics Hardware 2006, pp. 3947, G.S. Johnson, W.A. Hunt, W.R. Hux, Allen and Mark, S. Burns, A. Christopher, and Junkins, Soft irregular shadow mapping: Fast, high quality, robust soft shadows, In Proceedings of ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games 2009, pp , B. Lloyd, Logarithmic perspective shadow maps, PhD thesis, University of North Carolina at Chapel Hill, H. Kolivand, and M.S. Sunar, Shadow mapping or shadow volume?, International Journal of New Computer Architectures and their Applications, Vol. 1, no. 2, pp , R. Raskar, and M. Cohen, Image precision silhouette edges, In: Spencer SN, editor, Proceedings of the conference on the 1999 symposium on interactive 3D graphics. New York: ACM Press; pp , J. Northrup, and L. Markosian, Artistic silhouettes: A hybrid approach, In Proceedings of NPAR 2000, pp. 71 8, AUTHORS Hoshang Kolivand is now pursuing Ph.D in UTM ViCubelab under the supervision of Dr Mohd Shahrizal Bin Sunar. His research interests include Computer Graphics and Virtual Environment. He received the M.S degree in Applied Mathematics and computer from Amirkabir University, Iran, in 1999; B.S degree in Computer Science and Mathematic from Islamic Azad University, Iran in Previously, he was a lecturer in Shahid Beheshti University Iran. He has published enormous articles in international journals and conferences as well as national journals, conference proceedings, and technical papers including article in a book. Hoshang Kolivand is an active reviewer of some conferences and international journals. He had published many books in object oriented programming and one in mathematics. shahinkey@yahoo.com Mohd Shahrizal Sunar obtained his PhD from National University of Malaysia in His major field of study is real-time and interactive computer graphics and virtual environment. He received his MSc in Computer Graphics and Virtual Environment (2001) from The University of Hull, UK and BSc degree in Computer Science majoring in Computer Graphics (1999) from Universiti Teknologi Malaysia. He served as academic member at Computer Graphics and Multimedia Department, Faculty of Computer Science and Information System, Universiti Teknologi Malaysia since Since 2009, he had been given responsiblity to lead the department. The current research program that he lead are Driving Simulator, Augmented Reality, Natural Interaction and Creative Content Technology. He had published numerous articles in international as well as national journals, conference proceedings and technical papers including article in magazines. Dr. Shahrizal is an active professional member of ACM SIGGRAPH. He is also a member Malaysian Society of Mathematics and Science. shahrizal@utm.my DOI: / ; Paper No. TR 351_12; Copyright 2013 by the IETE 46 IETE TECHNICAL REVIEW Vol 30 ISSUE 1 JAN-FEB 2013