Shadow Casting with Stencil Buffer for Real-Time Rendering

Transcription

1 102 The International Arab Journal o Inormation Technology, Vol. 5, No. 4, October Shadow Casting with Stencil Buer or Real-Time Rendering Lee Weng, Daut Daman, and Mohd Rahim Faculty o Computer Science and Inormation System, Universiti Teknologi Malaysia, Malaysia Abstract: We present a new method or real-time rendering o sot shadows in dynamic scenes. Our approach is based on shadow volume algorithm which provides ast, accurate and high quality shadows. The shadow volume algorithm is used to generate hard shadows beore adding ake sot shadows onto it. Although the generated sot shadows are physically inaccurate, this method provides sot shadows that are smooth and perceptually convincing. This proposed hybrid method adds more realism to a dynamic scene which is an important actor in computer graphics. Keywords: Shadow volume, silhouette detection, rendering, depth-pass, sot shadow. Received November 14, 2006; accepted May 31, Introduction Shadows are essential elements to realistic and visually appealing images, but are diicult to compute in most display environments especially in computer games. Similar to lighting, there are increasing levels o realism possible, paid or with decreasing levels o rendering perormance. Since the introduction o shadow volume [7], shadow map [14] and ake shadows [5], there have been numerous developments done to improve shadow algorithm in real-time graphic application. Among current issues concerns real-time dynamic sot shadows and hardware improvement that improvise real-time shadow generation. The important element in shadows is the dynamic and accuracy o the hard shadow as it provides inormation and spatial cue while the sot shadow determines the type o light source. Thus in this research, we attempt to create an accurate real-time dynamic ake sot shadow, where the hard shadow will be accurate and dynamic and the sot shadow will be ake. Stencil shadow volume algorithm will be combined with plateaus sot shadow in order to create the dynamic ake sot shadow. 2. Related Work Early work on shadows bring us back to 1977 where Frank Crow irst published his paper on shadow algorithm or computer graphics [7], in which his method explicitly clip shadows geometry to the view rustums, generating perect caps where the volume crosses a clipping plane. [11] suggested the use o stencil buer in implementing crow's original algorithm which gave the algorithm the name by which it is best known today. Stencil shadows belong to the group o volumetric shadow algorithms as the shadowed volume in the scene is explicit in the algorithm. In 2000, Carmack suggested a slightly dierent approach which entails that the view rays are traced rom ininity towards the eye, stopping when encountering the pixel on the geometry that is closest to the eye [6]. This reversal o the view rays' direction has given the algorithm the name Carmack s Reverse. [13] created a hybrid algorithm that uses a aster z- pass rendering when the viewport is not shadowed and reverts to robust z-ail rendering when the viewport is shadowed. Following this, several other shadow volume improvements have also been suggested; papers [1, 2, 3] or example described how to create sot shadows using penumbra wedges rendered rom shadow volume. [9] developed a technique or highly eicient coverage calculation or spherical light sources. The technique can avoid clipping operations in the pixel shader and let the texture sampler do the clipping or ree. The only setback is the technique is limited to spherical shaped light source only. 3. Algorithm Our proposed method combines the existing stencil shadow volume algorithm with the concepts o Heckbert and Her s sot shadow technique [10] which was originally or shadow map. This research was divided into two important steps, as shown in Figure Step 1: Creating Hard Shadow - Shadow Volume A shadow volume or an object and light is the volume o space that is shadowed. That is, all points in the volume are in the shadow or that light. In

2 103 The International Arab Journal o Inormation Technology, Vol. 5, No. 4, October generating shadow volume, the irst step is to create the shadow volume using the silhouette edges o shadowing object/ occluder as seen by the light source. The edges are then extruded away rom the light as shown in Figure 2. should not depend on any particular light source. Make sure depth buer is written. B. Starting with a light source, clear the stencil buer and calculate the silhouette o all the occluders with respect to the light source. C. Extrude the silhouette away rom the light source to an ininite distance to orm the shadow. D. Render the shadow volumes using the depth-pass. E. Using the updated stencil buer, do a lighting pass to shade (make it a tone darker) the ragments that corresponds to non-zero stencil values. F. Repeat step 2 to 5 or all the lights in the scene. Figure 1. Research methodology. Figure 3. Shadow volume clipping with view volume. Figure 2. Silhouette edge. Next, as shown in Figure 3, the shadow volume is clipped to the view/ camera volume, and orms the polygons that bound the shadow volume. The inal result is a set o shadow volume boundary polygons with all points within the shadow volume are in the shadow. Along a ray rom the eye, we can track the shadow state by looking at intersections with shadow volume boundaries ollowing the rules below (assume the eye is not in shadow): Each time the ray crosses a ront acing shadow polygon, add one to a counter. Each time the ray crosses a back-acing shadow polygon, subtract one rom a counter. Places where the counter is zero are lit, others are shadowed. The algorithm to implement stencil shadow volumes is summarized as the ollowing [12]: A. Render all the objects using only ambient lighting and any other surace-shading attribute. Rendering From the above list o steps, it should be quite obvious that having more lights leads to having more passes, which can increase rame rate intensity. Thus, we have to be very selective when deciding which lights should be used or casting shadows Silhouette Determination As stated beore, the very irst step to construct a shadow volume is to determine the silhouette o the occluder. The stencil shadow algorithm requires that the occluders be closed triangle meshes. This means that every edge in the model must only be shared by 2 triangles thus disallowing any holes that would expose the interior o the model. There are many ways to calculate the silhouette edges and every single one o these methods are CPU cycles hungry. Edge connectivity inormation must be precomputed so that we can determine a mesh s silhouette or shadow volume rendering. The method used here can be explained by using an array o N vertices V 1, V 2..,V N and an array o M triangle aces F 1, F 2,.., F M. Each triangle aces simply indicate which three vertices it uses by storing three integer indexes i 1, i 2 and i 3. An index i p precedes an index i q i the number p immediately precedes the number q in

3 Shadow Casting with Stencil Buer or Real-Time Rendering 104 the cyclic chain The indexes i 1, i 2 and i 3 are ordered such that the positions o the vertices V i 1, V i 2 and V i 3 to which they reer are wound counterclockwise about the triangles normal vector. Suppose that two triangles share an edge with endpoints o vertices V a and V b. The consistent winding rule enorces the property that or one o the triangle aces, the index reerring to V a precedes the index reerring to V b and that or the other triangle and the index reerring to V b precedes the index reerring to V a. With this the edges o a triangle mesh can be identiied by making a single pass through the triangle ace list. For any triangle having vertex indexes i 1, i 2 and i 3, create an edge record or every instance in which i 1 i 2, i 2 i 3, and i 3 i 1 and store the index o the current triangle ace in the edge record. Once all the edges are identiied, make a second pass through the triangle ace list to ind the second triangle that shares each edge. This is done by locating triangles or which i 1 i 2, i 2 i 3, or i 3 i 1 and matching it to an edge having the same vertex indexes that has not yet been supplied with a second triangle index. The general concept o the explanation above can be summarized in the ollowing pseudo code: A.or each triangle ace (A) in the object/ model B. or each edge in A C. i we don t know this edges triangle ace (neighbors) yet D.or each triangle ace (B) in the object/ model except A E. or each edge in B F. i A s edge is the same as B s edge, then they are neighboring each other on that edge, set the neighbor property or each triangle ace A and B, then move onto next edge in A. With the edge list o a triangle mesh/ ace, the silhouette is determined by substituting the light position with the plane equation. A triangle ace that is visible to light source will have a value o plane equation>0 otherwise the triangle ace is not visible rom light source. The silhouette is equal to the set o edges shared by one triangle ace that is visible by the light and one triangle ace that is not visible by the light. This is done by going through all the triangle aces and i it is visible, the edges are checked. I at the edge, there is no neighboring triangle ace or the neighboring triangle ace is not visible to light source then this edge is a silhouette and it cast shadow. It is important to note that silhouette determination is one o the two most expensive operations in stencil shadow volume implementation. The other operation is the shadow volume rendering passes to update the stencil buer. These two areas are the prime candidates or aggressive optimizations Shadow Volume Construction Once the set o an object s silhouette edges is determined with respect to a light source, each edge must be extruded away rom the light source s position to orm the object s shadow volume. For a point light source, which was implemented in this research prototype, the extrusion o the silhouette edges consist a set o quads (which can also be substituted with triangle strips). The quads are constructed rom the two vertices belonging to an edge and two additional vertices corresponding to the extrusion o the same edge to ininity (a large value) by using homogeneous coordinates. Shadow volume is extruded to ininity in order to avoid light source being too close to an occluder. I this happens, the situation shown in Figure 4 will occur where a inite shadow volume extrusion ails to cover all the shadow receivers in a scene. However, it is not compulsory to extrude the silhouette edges to ininity i it is ensured that the situation in Figure 4 can be avoided. In practical cases, a large value would normally be more than adequate. Figure 4. Finite shadow volume ails to shadow other objects. The extrusion distance is the distance the vertices o the bottom cap o the shadow volume are extruded away rom the light. The approach implemented in this research prototype is brute orce approach that simply draws the extrusion polygon to ininity (a large value) and the shadow volume is just clipped against all the polygon it encounters (reer Figure 5 or illustration). The procedure o silhouette determination and getting the triangle aces that is visible to light source will provide a triangle aces that are situated at the edge o the silhouette, a triangle ace with no neighboring triangle ace or the neighboring triangle ace is not visible to light source (reer Figure 6) which will be called silhouette triangle ace rom this point on. With a single silhouette triangle ace (marked with black and white lines in Figure 6), edge test are done

4 105 The International Arab Journal o Inormation Technology, Vol. 5, No. 4, October so that the edge o the silhouette (the black line edge) can be obtained. That single edge provides two vertices which will be used to extrude another two vertices that will be generated ater this. Ater obtaining the two vertices, the extrusion to ininity will be done using homogeneous coordinates. Here a basic scaling transormation is used to extrude the shadow volume by using the two vertices o the silhouette edge. This will produce two new vertices which are projected along the vector between the light source and the irst silhouette edge. It is scaled to INFINITY (the variable used in the implemented prototype or ininity value) which has been set to a very large value (reer Figure 7). Ininity Occluder/ object Shadow volume Shadow at d, distances Figure 5. Extrusion option. Light source Depth-pass is also commonly known as z-pass. Let's assume that the objects had already been rendered onto the rame buer prior to the above stenciling operations. This means that the depth buer would have been set with the correct values or depth testing or z-testing. Reerring to Figure 8, the two letmost ray originating rom the eye position does not hit any part o the shadow volume (in grey), hence the resultant stencil values is 0, which means that the ragment represented by this two rays are not in shadow. The 3 rd ray rom the let, when the ront ace o the shadow volume is rendered, the depth test would pass and the stencil value would be incremented to 1. However when the back ace o the shadow volume is rendered, the depth test would ail since the back ace o the shadow volume is behind the occluder. Thus the stencil value or the ragment represented by this ray remains at 1. This means that the ragment is in shadow since its stencil value is non-zero. Silhouette edge with two vertices New vertices To INFINITY Light source Silhouette triangle ace Shadow volume Figure 7. Extruding to INFINITY by producing two new vertices. Figure 6. The edge or casting shadow volume. The scaling transormations used to produce the new vertices are shown below; x ' = x + ( x x ) s (1) y ' = y + ( y y ) s (2) z ' = z + ( z z ) s (3) Vertices P (x, y, z ) is a new vertex, L(x, y, z) is the light source location and O(x, y, z) is the vertex rom the silhouette edge. With two vertices rom the silhouette edge, using the above equation, two new vertices are produced and this orm the quadrilateral (in research prototype implementation triangle strips are used) needed to create shadow volume. Ater creating the shadow volume the next process is to render it so that the hard shadow is visible, which will be explained in the next section Rendering Shadow Volume with Depth Pass x z y Figure 8. Depth-pass. Ater determining the object s silhouette with respect to a light source and constructing a shadow volume by extruding the silhouette edges away rom the light source, inally the shadow volume is ready to be rendered into stencil buer using depth-pass technique. The rame buer is cleared and an ambient rendering pass is perormed to initialize the depth buer. Lighting is disabled because there will be no rendering to the colour buer except the stencil buer. Then the stencil buer is cleared, initialized and conigured so that it always passes (which is why it is called depth-pass technique). The depth test is conigured to always pass only when ragment depth values are less or equal than those already in the depth

5 Shadow Casting with Stencil Buer or Real-Time Rendering 106 buer. The drawing will only be done into the stencil buer, write to colour buer and depth buer are disabled so that shadow volume don t appear as solid objects in the depth buer. Shadow volume aces which were constructed as described earlier are rendered using dierent stencil operations depending on whether they are acing towards or away rom the camera. It is rendered in two passes, irst pass increase the stencil buer with ront aces (casting shadow) and the second pass decrease the stencil buer with back aces ( turning o the shadow between the object and any other suraces). Once shadow volumes have been rendered or all objects that could potentially cast shadows into the visible region o the scene, it will cause all the areas that are in the shadow volume to have a non-zero stencil value while all those areas in the light area remain zero. Lighting pass is perormed to illuminate suraces where the stencil value remain zero, re-enable write to the colour buer, change the depth test to pass only when ragment depth values are equal or less to those in the depth buer and conigure the stencil test to pass when the value in stencil buer is not equal to zero. Finally, a blend drawn over the whole screen will cast a shadow which is how the hard shadow is produced. This ulils the irst objective o generating accurate hard shadow. The technique is known as depth-pass technique since it manipulates the stencil values only when the depth test passes, and below is the general overview o the algorithm; A.Render ront ace o shadow volume. I depth test passes, increment stencil value, else does nothing. Disable draw to rame and depth buer. B. Render back ace o shadow volume. I depth test passes, decrement stencil value, else does nothing. Disable draw to rame and depth buer Step 2: Adding Fake Sot Shadows Adding ake sot shadows to existing shadows generated by shadow volume will increase the realism o the shadow in a 3D scene. Our proposed method in implementing sot shadows in shadow volume was developed originally rom the concept rom [10] along with the well-known stencil shadow volume. The algorithm by [10] had been previously implemented in shadow map algorithm where the result proved to be quite convincing even though it is not a conventional method. The algorithm was implemented at interactive rates by exploiting graphics workstation hardware. Since hardware have become increasingly aster and more aordable, nowadays this technique is easible to be implemented on desktop computers with an adequate graphic card. Our method was developed by methodically addressing the undamental limitations o the conventional stenciled shadow volume approach towards sot shadow and also the approach taken by previous researchers. The work low is shown in Figure 9 ollowed by brie description o each step. Get the original hard shadow geometrics Generate new sample o hard shadows with new blending o gradient colour Blend/average the sample o new hard shadows with original hard shadow Figure 9. Work low or adding sot shadows Get the Original Hard Shadow Geometrics The very irst step in generating the sot shadows is to obtain the geometrics o the hard shadows generated earlier. This were done by saving all the generated coordinates o the vertices while rendering shadow volume in depth-pass technique so that no recalculation is needed, saving ample computation cost Generate Samples o Hard Shadows Next is to generate samples o hard shadows with the blending o gradient colours. This can be done by drawing the same shadow volume geometrics obtained earlier only that the size o the shadow volume polygon is scaled to be slightly bigger than the original shadow volume, as shown in Figure 10. The number o samples that needs to be generated depends on how high the required quality o the sot shadows beore blending it with gradient o reduced shadow colours. Figure 10. Samples o new hard shadows generated. While generating these samples doesn t aect much o the processing time but rendering it again with depth-pass and stencil buer test does involve a

6 107 The International Arab Journal o Inormation Technology, Vol. 5, No. 4, October lot o processing. Thus the number o samples generated must be taken into consideration so that it won t slow down the rame rate. Although the more samples generated will produce a higher quality o sot shadows, this will also in turn increase CPU consumption Blending Samples o Hard Shadows Ater the samples were generated, the last step is to blend or average out the samples together with the original shadow volume. This process is done by stacking the samples on top o one another, starting with the lightest sample to the darkest sample available and inally to the original shadow volume. The illustration is shown in Figure 11. Stacking Figure 11. Stacking o the sample and original shadow volume. The order o the stacking must be done rom the less dark to the darkest so that the produced sot shadow will create a penumbrae eect to the edges o the original shadow. As shown in Figure 12, the dierence can be seen when compared this with the original shadow volume with only the hard shadow. Original hard shadow Sample hard shadow 4. Results Gap actor Figure 13. Length and gap actor. As one o the main goals or this research was to test the applicability o sot shadow in a true 3D environment, the prototype implementation o our method and testing stage were done using complex, high polygon models. The tests perormed are the accuracy or resemblance test, ollowed by quality test and lastly real-time test. Side by side visual comparisons with reerence examples were used to determine the accuracy o the produced shadow on how ar it resembles real lie shadow. This comparison approach was also applied to measure the quality o the produced ake sot shadows. Speed test were perormed by observing the rame rate and comparing it to reerence examples. The running time o the algorithms depends on actors such as number o polygons, desired quality, screen resolution, graphics hardware and also the speed o the CPU Accuracy or Resemblance Test Length actor The shadows generated by the prototype are guaranteed true, real and accurate as it uses the model or shadow caster geometric to produce the shadow. Moreover there is no model simpliication done to optimize rendering. By providing the graphic images o the implemented shadow below, it is shown that the shadow in this research prototype is accurate and urthermore resembles the model/ shadow caster, as shown in Figure 14. Figure 12. Comparison o original shadow volume and new sot shadow volume. The creation o sot shadow using this technique accepts two parameters that can dierentiate the quality o the sot shadow produced. The irst parameter is the length actor which determines how ar the penumbrae or the sot shadow will be extended to and the second parameter is the gap actor which determines the gap between the samples o the hard shadows produced (reer Figure 13). The number o samples the hard shadow produced depends on the length and gap actor which is equal to length divided by gap. The produced new sot shadow will have a more realistic and convincing eect compared to the original algorithm, thus ulilling the second objective o this research Quality Test Figure 14. Resemblance test. The test involved rendering a cube with 8 vertices and 12 aces o triangle polygons using dierent values o length and gap actors as parameters. Images rom test results were captured and the number o polygon

7 Shadow Casting with Stencil Buer or Real-Time Rendering 108 triangles produced was recorded so that comparison and analysis can be done to evaluate the quality o the sot shadow produced. Figure 15 illustrate the result. Figure 15. Test results using cube with dierent length and gap. The test results achieved a rame per second rate o 60, which can be deemed as a good run time rate. The captured images also show that with increased number o rendered polygon triangles, along with smaller gap and greater length parameters will produce a better quality o sot shadow. Although these parameters provided a good blending o the shadow with the environment, some drawbacks were also ound. The bigger the gap is the more aliasing eect will be visible to the image, as shown in the images with 1.0 gap actor in Figure 15. Greater length also causes more changes to the geometric shape o the shadow. Thus it is concluded rom this test that or this model or shadow caster the appropriate value or parameter length should be between 2 to 3, as any dierent value will cause dramatic changes in geometric shape. Moreover, the optimized gap actor value or this cube should be anywhere between 0.3 to 0.5, as the produced sot shadow will not be visible to the human eyes i any dierent values were used, beside causing aliasing eect. This also applies to model or shadow caster other than cubes which was not shown here. We also conduct tests by visually comparing the shadow images rendered by our research prototype in comparison to other existing algorithm. We choose the shadow volume algorithm [1], known as Approximate Sot Shadow on Arbitrary Suraces Using Penumbra Wedge to be our reerence example. The chosen model or shadow caster was a cube and sphere; this was done without any other model in the scene except the wall and loor. The result showed that our prototype were able to render at about 60 FPS which was about 6 time aster than Ul and Tomas algorithm although the quality o our produced sot shadow is undeniably lesser. However, the sot shadows produced were still convincingly realistic on top o being able to render at real-time speed. The comparison is shown in Figure 16. Figure 16. Comparison o sot shadow generated Real-Time Test One way to obtain a ast sot shadow algorithm is to utilize graphics hardware. According to Moore s Law, the speed o graphics hardware has so ar more than doubled every 12 months, compare to CPUs which double approximately every 18 months. As shadow volume requires a lot o computations especially in silhouette edge determination and twopass rendering, it is very high consuming or both CPU and GPU processor. Here, the shadow volume algorithm developed by Ul and Tomas will again be used as a reerence example to compare its speed with our produced algorithm. Both algorithms will be tested against three dierent hardware systems to determine the best system available to the public. Our proposed method was tested using the same model or shadow caster that was used in Ul and Tomas implementation. The quality o the sot shadow produced by both algorithms was identically leveled with parameters set to 3.0 or length and 0.3 or gap. Identical resolution at 800x600 with 32 bit colours was also set as the environment or both algorithms. This is to ensure that the same amounts o polygon are used to render the sot shadow so that a precise comparison can be done. Two graphs were produced at the end o the test shown here as Figure 17 and 18. Figure 17 shows the result o the test using our proposed method where system 2 is shown to be the best system, ollowed by system 3 and system 1.

8 109 The International Arab Journal o Inormation Technology, Vol. 5, No. 4, October the number o triangle polygon count increased three times. However, in terms o speed this test has shown that our proposed method perormed much better than Ul and Tomas sot shadow algorithm as it has at least matches the FPS or even aster, which at one point it was eleven times aster. Figure 17. FPS vs. model polygon count or 3 dierent systems using ake sot shadow volume with stencil buer. From here it could be concluded that system 1 requires a much better graphics hardware and system as it can only achieve 10 FPS with 216 polygons and will dropped even urther to 3 FPS i the numbers o polygon exceed 500. System 2 only have 1.5 GHz CPU and 768 MB RAM, much lower than system 3 which have a 2.4 GHz CPU and 1 GB RAM but yet the graphic card used in system 2 was way too powerul and was able to render sot shadow at almost 640 FPS or 200 polygons. However, it also gradually goes down to FPS ater the number o polygon exceeds 500 as the object geometric become complex. System 3 is a mid range system which is able to render sot shadow at 60 FPS or polygons below 250. Ater 500 o triangle polygons the FPS remain static at 15 FPS even though the number o polygons reached Figure 18. FPS vs. model polygon count or 3 dierent systems with approximate sot shadows on arbitrary suraces using penumbra wedges algorithm. Figure 18 above is the result o the test using Ul and Tomas sot shadow algorithm where in comparison to the graph in Figure 17, shows a much slower rate o FPS. From the graph it is shown that the maximum FPS is not very high but as the number o triangle polygons increase, the decreasing o FPS is not as low as the statistic shown in Figure 17. The graph had shown that it only decreased almost two to three times in FPS as 5. Conclusions and Future Work In this paper we have shown a method to generate an accurate hard shadow volume and adding ake sot shadows onto it. Among clear beneits is that the method produces high quality sot shadows at high speed. This method can be urther improved and be implemented using programmable graphics hardware to achieve real-time perormance. An important aspect yet to be incorporated is how to extend the eect o shadows onto other suraces and objects in the scene other than planar/ surace. Thus this area is something we would like to explore in our uture work in attempts to urther improve eiciency. Problems concerning shadow volumes that intersect at near or ar clipping plane were not discussed in this paper as the solution were already presented in [8]. Reerences [1] Akenine-Moller T. and Assarsson U., Approximate Sot Shadows on Arbitrary Suraces Using Penumbra Wedges, in Proceedings o the 13 th Eurographics Workshop on Rendering, Eurographics Association, pp , [2] Assarsson U. and Akenine-Moller T., A Geometry-Based Sot Shadow Volume Algorithm Using Graphics Hardware, ACM Transactions on Graphics, vol. 22, no. 3, pp , [3] Assarsson U., Dougherty M., Mounier M., and Akenine-Möller T., An Optimized Sot Shadow Volume Algorithm with Real-Time Perormance, in Proceedings o ACM SIGGRAPH/ EUROGRAPHICS Conerence on Graphics Hardware, Eurographics Association, pp , [4] Blinn J. and Corner J., A Trip Down the Graphics Pipeline, Morgan Kaumann Publishers, San Francisco, [5] Blinn J., Me and My (Fake) Shadow, IEEE Computer Graphics and Applications, vol. 8, no. 1, pp , [6] Carmack J., John carmack on shadow volumes, object/ robustshadow - volumes.html, [7] Crow F., Shadow Algorithms or Computer Graphics, ACM SIGGRAPH Computer

9 Shadow Casting with Stencil Buer or Real-Time Rendering 110 Graphics (SIGGRAPH 77), vol. 11, no. 3, pp , [8] Everitt C. and Killgard M., Practical and Robust Stenciled Shadow Volumes or Hardware- Accelerated Rendering, Technical Report, NVIDIA Cooperation, [9] Fauerby K. and Kjaer C., Real-Time Sot Shadows in a Game Engine, Master s Thesis, [10] Heckbert P. and Her M., Simulating Sot Shadows with Graphics Hardware, Technical Report CMU-CS , Carnegie Mellon University, January [11] Heidmann T., Real Shadows, Real Time, Iris Universe, vol. 18, pp , Silicon Graphics Inc., 1991 [12] Kwoon H., The Theory o Stencil Shadow Volumes, [13] Lengyel E., The Mechanics o Robust Stencil Shadows, Gamasutra, com/eatures/ /lengyel_01.htm, [14] Williams L., Casting Curved Shadows on Curved Suraces, Computer Graphics, vol. 12, no. 3, Mohd Rahim received his Diploma in computer science 1997, BSc computer science 1999 orm University Teknologi Malaysia and MSc in GIS and visualization in 2002 rom University Teknologi Malaysia. Now he is going to complete the PhD (spatial modelling) in University Putra Malaysia. He also is lecturer in Department o Computer Graphic and Multimedia, Faculty o Computer Science and Inormation System o University Teknologi Malaysia. He has over 8 years o experience in the ield o computer graphics and visualization and GIS data model. He has also been actively involved in many research projects related to GIS and visualization, GIS data management, computer graphics and has published more than 70 publications. Currently his research was on going in spatial, temporal and spatiotemporal data management applications, 3D data visualization, mobile computing or computer graphics application. Lee Weng received his BSc orm Universiti Teknologi Malaysia in computer science with majoring computer graphics. He also receives MSc rom University Teknologi, Malaysia, in computer science with research project in computer games. He is currently a senior computer graphics engineer at international company which is Seagate Penang, Malaysia. Daut Daman received his BSc orm University Sains Malaysia and MSc rom University o Cranield, United Kingdom. He is currently an associate proessor in the Faculty o Computer Science and Inormation System o Universiti Teknologi Malaysia. He has over 27 years o experience in the ield o computer graphics and visualization. He has also been actively involved in many research projects related to computer graphics and visualization and has published more than 120 publications.