Ray Tracing in Electronic Games

Transcription

1 Ray Tracing in Electronic Games Hugo Sereno Ferreira MAPi Doctoral Programme in Computer Science University of Minho, Aveiro and Porto Abstract. The past ten years in computer graphics renderization have been dominated by graphics processing units (GPUs) that use the z- buffer algorithm to compute visibility. However, ray tracing has been known to produce higher quality, physically correct output at the expense of enormous processing power. For this reason, ray tracing was systematically delegated to off-line production of photorealistic images in areas such as architecture, industrial design, film animation and special effects. Nowadays, exploiting the parallelization of current hardware, allied with research advancements in the area, ray tracing is increasingly become a viable alternative to the z-buffer algorithm in interactive applications, such as games. In a near-future time-frame, it is possible that ray tracing will become the standard rendering algorithm, implying a fundamental change in how next-generation games will be developed and in the graphics they ll be able to produce. Key words: Real-Time, Raytracing, Games, RPU, Renderization. 1 Introduction There is no doubt that much of the advancements done in recent years in the area of Computer Graphics are mainly due to the huge industry Electronic Games represent. Gamers all around the world typically tend to invest their money upon the newest, fastest machines industry can provide them, and there s no sign this tendency will slowdown as computers (and game consoles) increasingly dominate the commodity space at everyone s home. Therefore, game companies and hardware makers seriously invest in research to always have the next-generation of graphics not too far away. There are two main techniques to synthesize interactive 3D images into a computer screen, namely (a) rasterization through the use of a z-buffer and (b) ray-tracing. Their goal is to solve a large, complex integral equation which researchers have called the rendering equation, in order to generate highly realistic images (commonly known as photorealistic). Because we are interested in interactive applications, these algorithms must achieve their purpose at animationlevel frame-rates (i.e. between 30 and 60 fps). Though, while real-time rasterization has been present in commodity machines for more than a decade now, mainly through the use of discrete hardware

2 2 processing units (i.e GPUs) highly optimized for the process, the computational power required by ray-tracing was not available until very recently. This work aims to present a state-of-the-art overview concerning real-time interactive ray-tracing on current and near-future commodity computers, especially applied to electronic games. It is largely based upon previous state-of-theart reports from specialists in the area, such as [1], [2], [3] and [4]. In the first section, arguments from ray-tracing proponents specifying where this algorithm surpasses rasterization will be presented. The second part will focus on optimization techniques, both at low (machine specific) and high-level (conceptual), used to increase performance in order to reach animation-level frame-rates. Third section will be about the specific advantages ray-tracing brings to the electronic games world. Fig. 1. OpenRT real-time renders of (a) a BMW showing reflections and refractions by using glossy materials and an HDR outer sphere as a lighting source, (b) model of a car headlight with physically correct simulation of light transport, and (c) the Sponza scene showing indirect lighting and soft-shadows due to Global Illumination techniques. This level of realism and physical correctness would be otherwise impossible to achieve using typical rasterization algorithms. 2 Ray-Tracing vs Rasterization If rasterization is already proven to work, many question the advantages of dedicating effort to bring ray-tracing up to its level. Answers to this question seem to divide the research community. There are, however, some concrete forces where ray-tracing elegantly surpasses known rasterization algorithms: 2.1 Advantages of Ray Tracing Shadows, Reflections and Refractions. Achieving realistic soft shadows in scenes with multiple light sources, correct reflections and refractions is a complex problem in rasterization, and many hours of research have been dedicated to new approximations for these tasks. However, regardless of the ingenious and complex algorithms involved, none of them is able to produce the mathematically correctness and photorealism that ray tracing delivers.

3 3 eye screen ray T 3 T 3 T 2 pixel T 2 T 1 T 1 z-buffer ray tracing Fig. 2. The z-buffer algorithm works by geometric projection of a triangle towards the screen, storing both pixels distance to the eye (the z value) and its color, unless a smaller distance was already stored. Conversely, ray-tracing shoots a ray into the object space, finding the closest intersection. Large data-sets. Calculating the visibility of an object is a main element of any rendering technique. Due to the nature of the z-buffer algorithm, time complexity increases linearly w.r.t the number of objects, compared to sublinear complexity of ray-tracing. While there are techniques to achieve sublinear time in rasterization, like occlusion culling, randomized culling and level-of-detail management, they add complexity and data restrictions to the implementations. Efficient Shading. Another problem due to the nature of the z-buffer lies in the fact that there occur calculations of pixel colors of objects present in the scene, regardless of being occluded by others. Because visibility is first calculated in ray-tracing disregarding shading color, only objects that are indeed visible need to have their shading routines executed. Non-Triangle Primitives. Current generation of real-time rasterization algorithms are dependent upon the scene being described as meshes of triangles. This fact implies that curved object models be tessellated, loosing their original properties in lay of approximations built with triangles. Conversely, raytracing supports higher-order objects, like curved surfaces such as NURBS in their native form without any need of tessellation, greatly simplifying models production and ultimately reducing memory requirements for model specification. Indirect Illumination. Light bounces from materials to materials, such that an entire scene can be illuminated mainly by the light bouncing through walls and mirrors. This behavior is commonly known as inter-reflections, as its simulation in rasterization with current generation algorithms [5] is awkward, complex and results in fundamentally incorrect images. Ray-tracing inherently solves this problem by casting additional rays to other objects and considering their light contribution to the illumination of any considered point.

4 4 Volume data. Volumetric data support in ray-tracing, like smoke, fog, or more generally participating media, comes naturally with the algorithm. These artifacts are modeled as semitransparent volumetric primitives with dedicated shaders. Scalability. It has been demonstrated that ray-tracing is embarrassingly parallel as long as enough bandwidth to the scene data base is provided [6], [7]. ARTICLE IN PRESS P. Shirley et al. / Computers & Graphics 32 (2008) Fig. 1. Ray tracing can robustly and naturally support next generation visual effects not easily combined with GPU graphics including depth-of-field, Fig. 3. The system presented in [8] can easily support visual effects such as depth-ofeld, motion blur, glossy and specular reflection, soft shadows, and correct refraction, motion blur, glossy and specular reflection, soft shadows, and correct refraction. More details on the system that generated of these images are available in [5]. for which very complex algorithms are needed in current-generation rasterization algorithms, often yielding incorrect and visually unpleasant results. in the number of objects while the z-buffer is linear in the number of objects. It is ray tracing s larger time constant and lack of a commodity 2.2 Current hardware Raytracing implementation Problems that makes the z-buffer a faster choice for data sets that are not huge. Ray tracing is better suited for creating shadows, reflections, and refractions because it directly simulates the physics of light. Ray oftracing complex, enables realistic thesescenes: forms of isolated visibility queries that are problematic (or impossible) for the z-buffer algorithm. Ray tracing also allows flexibility in the intersection computation for the primitive objects that allows non-polygonal primitives such as splines or curves to be represented directly. Unfortunately, computing these visual effects based on simulating light rays is computationally expensive, especially on a general-purpose CPU. The ray tracing algorithm currently requires tens to hundreds of CPUs to be interactive at full-screen resolution. Games have driven almost all desktop 3D graphics, and we believe that trend will continue. Because ray tracing is well-suited to support a quantum leap in the ability of games to support higher-order surfaces, complex models, and high quality lighting, this we technique. believe games will migrate to using it once ray tracing is fast enough. In this paper we commonly believed. Finally, we discuss what the implications of this change are for graphics and games education. Regardless of the above mentioned advantages 2. Ray of ray-tracing, versus there rasterization are still complex downsides that must be coped with in order to achieve real-time rendering In this section we review the ray tracing and z-buffer algorithms, the applications that use them, the perfor- rasterization, such applications ray-tracingdemand, suffers from and the various ways High constant time. When compared tomance an high constant time which directly invalidates such performance the gains might of sublinear be delivered. time complexity for enough small scenes. However, Current several GPUsresearchers are based assert on thethat z-buffer [1] which is a the main difference is because of thestraightforward hardware implementation algorithm. Inofhardware the rasterization algorithm. Since then, prototypes typicallyofhas specialized two framehardware buffers; one for for raycolor and one for z implementations it tracing have been constructed and shown (depth) thevalues. possibility Whileof computing achievingone interactive frame-rates with very high visual back quality buffer), output. it displays the other (the front buffer). of the color buffers (the Visual artifacts. Because there is camera When and all object of the movement colors are in computed interactive in the back buffer, the applications, visual artifacts due to discretization two buffers are in swapped pixel sampling, (the front when becomes the back and vice versa) and the new set of colors are displayed. The subpixel objects of different color moves over the pixel center, pose a main z-buffer is only used while computing the new colors in the unwanted effect. The solution relies on subpixel sampling, which has the back buffer. Computing the back buffer is a loop over all side-effect of also providing anti-aliasing at the cost of extra-rays involved in triangles: Scene access bandwidth. Because the initialize ray tracing all algorithm pixel colorsistobounded, background notcolor argue that it is feasible to make ray tracing fast enough, by CPU, but by memory access bandwidth initialize [6], all [7], pixel fastz implementations values to 1 of and that this implies that migration will take place. Because such a migration implies a basic change of algorithm, it will have major effects on both future graphics and games related courses. The main purposes of this paper are to analyze these issues and examine how courses related to graphics and games might be effected. Another reason we think ray tracing should have more of a place in computer graphics classes is that academics are well ahead of industry in ray tracing research. This enables an instructor to make the students feel like they are in the club in a way that their cohorts in industry are for all N triangles do for each pixel p that triangle might be seen through do compute color c new and depth z new if z new oz p then c p ¼ c new z p ¼ z new. The if statement is how the hidden surface elimination remains simple; overlapping polygons settle their order by means of storing the closest depth value seen so far in the triangle loop. This algorithm s runtime is proportional to

5 5 ray tracing need to minimize random access to memory by using techniques such as taking advantage of coherence [6], [9] to exploit fast caching memory present in modern CPUs. Scalability beyond 16 cores, such as Intel s Terascale research [10], will certainly need careful techniques to avoid bandwidth starvation. are caustics caused by the cons or refractions (see Figure 5). al clues, especially with transuids. These scenes would look ics illumination effects. onsidered the only robust and austic effects. In a first pass tic photons in the scenes via n the second pass it then uses compute the caustic illuminaped directly to ray tracing (see ve frame rates even on a single ith up to 25 levels of recursive headlight with highly curved eed to be traced to achieve an ple from the industrial use of ortant multiple reflections for Fig. 4. Figure These caustic 5: Caustic effects effects are caused caused by by multiple multiple reflections reflections and refractions or refrac-otions between are well different handled media. by photon mapping based on ray tracing. light bouncing This images can be rendered at approximately 15 fps on a cluster of PCs with a resolution of pixels. cost. Typically radiosity methsed to compute the indirect illu- Simulation and games demand interactivity and currently use z-buffer hardware In summary, one can quote the work presented in [4] which concludes that tex colors or light textures. Onealmost3exclusively. Games However, usingthey Ray spendtracing a great deal Projects of effort and programmer pre-computed radiance transfertime creating complicated hacks to fake lighting effects and reduce N [time al. 2002], which allows chang-constantthrough a distant environmentqualityintoorder that generated to explore bythe rayrequirements tracing. Those and industries benefitswould of rayuse tracing ray tracing by model simplification and in the end they have imagery of inferior if it were based fastgames enough. we performed a number of experiments. On the one hand we re-implemented several existing rasterization based games, efficiently compute indirect illler 1997]. In a first pass this Auto: Vice City Ray-Tracing [Rockstar Games Performance 2002]. On the other hand we including Quake 3: Arena [id Software 1999] and Grand Theft 3 Increasing rces in the scene through a ranhese light sources are used to to be available and explores this new design space. developed the game Oasen that assumes fast ray tracing hardware There are three fundamental operations required by typical implementations of ray-tracing: hadow computations. This apimplementation using fast rayray Traversal. Intersecting a ray with the spatial data structure that encapsulates 3.1 the QuakeRT scene objects. et al. 2003] that already runs in Object Intersection. Intersecting the ray with the primitive objects contained in the In 2004, selected a student data structure project leaf [Schmittler nodes. et al. 2004a] was realized Shading. Computing the illumination and color of the pixel based on the in- which implemented a complete graphics engine using the OpenRT tersection API in order with to theplay primitive a simplified objectversion and the ofcollection Quake 3. of the contributions from the secondary ray segments. The goal was to fully support all graphical effects of Quake 3 as well as a collision detection system using ray tracing. Other nongraphic related algorithms like AI were simplified. Figure 6 shows some screenshots rendered on a PC cluster at 20 fps with a screen size of pixels and 4 full screen anti-aliasing.

6 are caustics caused by the cons or refractions (see Figure 5). al clues, especially with transuids. These scenes would look ics illumination effects. onsidered the only robust and austic effects. In a first pass6 tic photons in the scenes via n the second pass it then uses compute the caustic illuminaped directly to ray tracing (see ve frame rates even on a single In 2004, a student project [Schmittler et al. 2004a] was realized which implemented a complete graphics engine using the OpenRT API in order to play a simplified version of Quake 3. The goal was to fully support all graphical effects of Quake 3 as well as a collision detection system using ray tracing. Other nongraphic related algorithms like AI were simplified. Figure 6 shows some screenshots rendered on a PC cluster at 20 fps with a screen size of pixels and 4 full screen anti-aliasing. ave very recently also been imare [Ernst et al. 2005; Dachse approaches are quite limited. of light and completely ignore se of the difficulty to compute uited to compute the visibility g algorithms. Its close match physically-based computation ty when combining geometric ay manner. Finally, ray tracx models efficiently and can be Fig. 5. irect illumination that is likely Figure Port of6: Quake Various 3 toscreenshots the OpenRTofEngine. QuakeRT Fromfeaturing top to bottom, several shadingwith effects detailed likemodel complex geometry colored without shadows using from bump-mapping, many objects, (b) mul- presence of left to right: (a) walls ames due to its significant role multiple detailed objects, (c) multiple reflective spheres and (d) spot lights. tiple reflective spheres, and spot lights. Efforts in achieving real-time ray tracing focus on improving the performance 44 of the above mentioned steps. The first one, Ray Traversal, is of extreme importance when calculating the visibility of objects, since it can discard huge quantities of information based on intersecting rays with simple bounding geometries, instead of testing the intersection with every object present in the scene world. The use of spatial data-structures has been a common strategy in off-line rendering, since the typical time for constructing represent a small fraction of the final rendering time. But interactive applications like games, poses new difficulties regarding spatial structures, mainly because of the scene s dynamic nature, which can potentially imply it to be updated (or reconstructed) every frame. The constant time to build and update these structures become then an important factor when balancing the final slice of rendering time. The second operation, Object Intersection, returns the exact intersection point with every object contained in the leaf nodes of an intersected boundary geometry. The final operation, Shading, uses a mathematical model to output the color of the pixel at the point of intersection of the ray with the primitive object. In classic ray tracing, this model would only take into account direct illumination from visible light sources, reflection and refraction rays. But ray tracing with Global Illumination methods are deeply recursive, since it also takes into account the light contribution of other objects in the scene, significantly raising

7 7 the number of fired rays, and requiring the re-execution of the above mentioned steps for each secondary ray. 3.1 Low-level Optimizations Low-level optimizations focus on taking advantage of hardware-level facilities without any significant change in the ray tracing engine. The most common researched (and implemented) optimizations are the following: SIMD. Single-Instruction, Multiple-Data instructions, such as Intel s SSE and AMD s 3DNow, allow the execution of the same floating point instruction in parallel on several data values. In ray-tracing, they have been used to simultaneously intersect a packet of four rays 1 with a single object, yielding experimental speed-ups ranging between 3.5 and 3.7. Multi-threading. Industry s answer to the increasing difficulty of achieving higher performance due to natural physics limitations is to focus on higher number of processing units within a single chip. Nowadays, virtually every commodity processor is double-core, with quad and eight-core becoming the standard in near-future. Because of the inherently scalability characteristic of ray-tracing, and the platforms ability to automatically balance threads between processors, multi-threading is an easy technique to take advantage of the multi-core paradigm of CPUs [6]. Raytracing Processing Unit (RPU). Similarly to what has happened with rasterization, the implementation of dedicated hardware specialized in performing ray tracing tasks can improve performance by orders of magnitude. Prototypes done with FPGA chips, even when operating at clock rates 40 times slower and memory bandwidth 30 times less of a typical CPU architecture, are able to yield between 50% and 160% increases in performance [11]. Programmable Pipelines in GPUs. While the hardware for an industriallevel RPU is not here for commodity computers, some researchers tried to use the vertex and fragments programmers available in current-generation GPUs as a general programmable stream processor. The work presented in [12] shows that a GPU-based implementation of ray-tracer is feasible, though the performance increase is not what was expected, mainly due to an architecture mismatch between the recursive nature of the algorithm and the feed-forward design of GPUs. Other low-level optimizations include inlining, expanding variables to vectors, memory management, cache line alignment and usage of const keyword in C/C++ compilers. More information can be found in [3]. 1 This is due to SSE and 3DNow instructions operating over 4 different values at once.

8 8 Figure 9: Forest scene with 365,000 plants and a total number of 1.5 billion triangles. Note the detailed shadows on leaves and the smooth illumination on the trunks due to integrating illumination from the entire sky. Fig. 6. This forest scene is composed with 365,000 plants, summing up to approximately 1.5 billions of triangles. The soft-shadows and smooth illumination upon leaves and trunks are due to integrating illumination from the entire sky. ago [Reinhard et al. 2000; Lext and Akenine-Möller 2001; Wald et al. 2003b] but they are limited to a small subset of animation types (rigid body only) or they are not fast enough to deliver interactive rendering performance. 3.2 High-level Optimizations The difficulty in dynamic ray tracing lies in maintaining spatial index structures that organize all scene geometry in order to significantly reduce the number of expensive ray intersection calculations. With their help typically only a few (< 5) primitives need to be tested for intersection with a given ray even if the scene contains billions of them. High-level optimizations require some new conceptual techniques to accelerate ray-tracing performance. The following two concerns represent the most important advancements done to increase the performance 6 HardwareofSupport ray tracing for Ray implementa- Tracing Kd-trees are widely used for achieving realtime ray tracing performancetions: However, their spatial subdivision approach made it difficult and costly to update it after changes to geometry. Fortunately, the situation improved drastically this year. By exploiting coherence with packets of many rays, competitive realtime ray tracing performance was also demonstrated with grids [Wald et al. 2006b] as well as with BVHs [Wald et al. 2006a]. Additionally, two methods were proposed that allow for very fast updates to kd-trees. Firstly, a hybrid b-kd-tree acceleration structure combines the fast update properties of the BVH with the high ray tracing performance of the kd-tree and is already implemented in hardware [Woop et al. 2006]. And secondly, fuzzy kd-trees together with a motion decomposition approach avoid the costly rebuilt of kd-trees almost completely at least for some types of animation [Günther et al. 2006]. All these novel techniques provide realtime ray tracing performance for much more general dynamics than was ever possible before. The rendering of predefined animations, skinned meshes, and even more flexible deformations of the scene geometry are now supported by ray tracing. Thus ray tracing is at least competitive with rasterization in rendering dynamic worlds created by today s games. Towards Fully Dynamic Game Environments Traditionally games rely on precomputations to gain speed, e.g. they use pre-computed radiosity solutions [Cohen and Wallace 1993] stored in light maps or pre-computed radiance transfer (PRT) [Kautz et al. 2002; Sloan et al. 2002]. Pre-computation is also necessary to allow for visibility culling, reducing the number of polygons sent to rasterization hardware by e.g. potential visible sets (PVS) [Airey et al. 1990], binary space partition (BSP)-trees [Fuchs et al. 1980], or portals [Luebke and Georges 1995]. Thus most games are essential a walkthrough application with moving objects: Really changing the geometry such as destroying (parts of) buildings or digging holes into the landscape is seldom possible. The only exception we know of is Red Faction with the GeoMod engine [THQ Inc. 2001] which allows for dynamic changes of the world, e.g. blasting holes in walls. Because ray tracing computes visibility and simulates lighting on the fly the pre-computed data structures needed for rasterization are unnecessary. Thus dynamic ray tracing would most likely allow for simulation-based games with fully dynamic environments as sketched above, leading to a new level of immersion and game experience. Based on the presentations in the previous sections we conclude that ray tracing is able to play an important role for future games. However, the most important question that still needs to be answered is that of suitable hardware support for achieving acceptable performance levels. Cache Coherence. Works such as [6] and [7] clearly show that a ray tracer is not bound by CPU speed, but bound by bandwidth access to scene data base. Minimization of random access to memory maximizes cache hit and boosts memory-bounded calculations. By exploiting coherence throughout all the ray tracing algorithm [7], [13], [6] one can take advantage of processor-level cache improving access time by a factor of 10. Ray Traversal. Interactive ray tracing differs from its static counterpart in the fact that objects move around the world. Nonetheless, this fact poses a great difficulty in achieving animation-like frame-rates because of the needed overhead in maintaining spatial index structures. These structures are critical to increase ray tracing performance since they reduce the number of expensive ray intersection calculations from billions of potential objects in the whole to very few (typically less than five) primitives. But if the cost of generating these data-structures (such as kd-trees) were negligible in static scenes, 6.1 Multicore Architectures updates performed upon these structures as the scene specification evolves over time, becomes a significant fraction of the involved total cost. Fortunately, recent advancements have been done in this area, namely [14] and [15] which allow much more general dynamics than was ever possible before. An exhaustive state-of-the-art report about Ray Traversal techniques can be found in [1]. 4 Creating Games for Raytracing Engines As a rough estimate for the following discussion we assume that at least 300 million ray per second ( pixels, 30 fps, 10 rays per pixel) are required to achieve a minimally necessary performance level. This is comparable to the performance of a low end graphics card for scenes with significant shadow and reflection computations. Based on experience, high-quality anti-aliasing may increase this number by about a factor of two if using an adaptive super-sampling strategy equivalent to 8 samples per pixel. In the following we will explore how different hardware approaches may be able to provide adequate computational power. Based on the performance data given in Section 1 we assume in the following a currently achievable ray tracing performance of about 10 million rays per second on a single CPU for static scenes and half of that for dynamic scenes. In other words we will need an improvement in performance in software by a factor of 30 to 60, respectively, or 60 to 120 with anti-aliasing. For a long time CPUs have mainly tried to increase single-thread performance by driving clock rates up. However, this required longer pipelines, much larger caches, and complex technologies such large reorder buffers, advanced branch prediction, and others to prevent problems due to the increasing cost of pipeline stalls. Also, these techniques all occupy precious die area that cannot be used for the main purpose of computing. In contrast, many applications from computer graphics and other disciplines operate in a highly data parallel fashion. They are able to distribute their computations over an almost arbitrary number 47 The work presented in [16], shows some of the characteristics that a game developed with ray-tracing in mind can take advantage from: Open Space Worlds. Creation of games that operate in a fully open space world, without using any level-of-detail mechanisms, clipping-planes, or fog

9 9 to artificially reduce scene complexity. This would bring unprecedented realism, specially in games with seamless worlds like World of Warcraft. Using large spheres to model worlds as planets could also be used to limit the ray-traversal step for the boundary structures. Realistic Sky and Weather. Ability to simulate daytime skylight and weather conditions, using participating media as volumetric objects to model rain, fog, clouds, etc. Reuse of rays. Though ray-tracing is basically a visibility calculation algorithm, it can be reused for physics engines, acoustics modeling and collision detection, taking advantage of the existing algorithms and data-structures. High Number of Light Sources. The Oasen game efficiently handled huge numbers of light sources by exploiting the restricted range of each light and organizing them in a way such that it is possible to locate light sources that contribute to an object s illumination. 4.1 Platform Homogenization Fig. 7. Screenshots of the Oasen game: (a) sunset showing distant objects and light conditions simulation, (b) water showing correct refraction of inside objects, caustics and reflection of the sky and (c) visualization of plants, using the same highly-detailed models for the near as well as far trees. Besides the above mentioned advantages, advocates also assert that development for ray-tracing engines could homogenize platform development, specially regarding handheld devices. This is mainly due to the linear time complexity regarding output image size. For example, if a particular ray tracing implementation (software and/or hardware) can yield an average of 60fps at a resolution of 1280x800, then it will be able to deliver up to four times more frames at half the resolution (i.e. 640x400). Because current handheld devices, such as Sony s PSP and Nintendo s Gameboy, have resolutions rounding 400x272, then a processor with only a fraction of its desktop equivalent will be required to power them. In conclusion, disregarding potential memory requirements, the same game used on desktop hardware could, in theory, be easily ported to an handheld device without any significant change in the underlying architecture.

10 Standard API Industry knows how much the OpenGL API [17], and more recently, DirectX, has done to boost game deployment on the PC architecture. Such an interface separates the concerns between the underlying graphical engine and the highlevel game platform, allowing developers to concentrate in content delegating the rendering task to highly-optimized software and/or hardware. However, these interfaces are highly based upon triangle rasterization algorithms, making them unsuitable for ray-tracing purposes. Understanding the necessity of having an underlying platform for game development using ray tracing, the OpenRT initiative [18] understood this well, by building an API syntactically similar to OpenGL. More information about the usage of OpenRT in games can be found in [2], [18] and its webpage. Future graphics processors, would their be RPU based graphics-cards, GPU implementations of the ray-tracing algorithm, or simply CPUs capable of handling the required processing power, will undoubtedly benefit from the standardization of an interface very similar to the OpenRT API for the very same reasons OpenGL and DirectX were, and still are, a success. Fig. 8. Screenshots of Ray City, a port of GTA Vice City, showing outdoors rendering. Figure 7: Example screenshots of Ray City. 4.3 Impact on Graphics and Gaming Courses Some features of this engine are realistic glass with reflection and refraction, An interesting correct perspective mirrors, as been per-pixel raised in shadows, [4] where the colored authorslights, analyzefog- ging, and Bézier patches with high tessellation. All of these effects the impact that achievement of real-time ray tracing will have on academic graphics and gaming courses. Here are their points summarized, some of them already are mentioned simplein to previous implement sections: with rudimentary ray tracing techniques. Additionally, two different light sources are supported: point lights Special Effects. Several techniques used to simulate transparency and reflection will fall-back into natural illumination computations. for ordinary lightning and spot lights to simulate a flashlight. Other effects Perspective which transform can be defined and homogeneous with the Quake coordinates. 3 shader Perspective languagedoes are alsonot supported. need to berather explicitly then simulated using the since original it comescene naturally geometry with the some ray of the tracing walls algorithm. and floors have been replaced by highly tessellated geometry using static displacement mapping yielding approximately one million triangles e.g. for the Temple of Retribution map. The game engine was written from scratch and supports player and bot movement including shooting and jumping as well as many special effects like jump-pads and teleporters. The complete rendering engine was implemented by a single student within five months. Furthermore, the code complexity to support all these effects is very low. All effects were implemented Figure 8: Some e Oasen featuring se on a PC cluster wit Quake 3 project us These experiments liminary show th indeed simple to c selecting the correc

11 11 Higher-order Surfaces. The notion that every model should be expressed in triangles will disappear, eliminating the need for complex tessellation algorithms, decreasing memory consumption and increasing detail. Volumetric effects. Smoke, Fog, Fire and every kind of participating media can be modeled as semi-transparent volumetric primitives with dedicated illumination models, eliminating the need for common tricks and hacks. Beyond visualization. Using the capabilities of visibility calculation beyond graphics visualization can leverage foundational frameworks to support collision detection, physics simulation and acoustics modeling. Partial redraw. The notion of temporal coherence, redrawing only sections that changed from the previous frame, will become much more important than simple primitive count. 5 Conclusions This work slightly drafted the advantages of ray tracing over common rasterization algorithms based on the z-buffer. It was given an overview about research problems solved in recent years that are making interactive ray tracing a reality with near-future technology. These advancements are taking interest from the gaming industry as well as from hardware makers. It was also suggested that the use of ray tracing in games would bring several advantages to game developers and users beyond better graphics. While there is no consensus among all researchers that ray tracing is the future of 3D rendering, there is no doubt that the visual quality and mathematical correctness achieved by this algorithm has not yet been nearly surpassed by rasterization. The growing complexity involved in rasterizing several special effects such as soft-shadows, refractions, reflections, participating media and indirect lighting are elegantly and inherently handled by ray tracing. The only problem with it is pure performance. Since exploiting this area in commercial application like games is a relatively new reality (though an old goal among computer scientists), it is very probable that novel techniques will arise in near-future. The fact that processors seem to be moving toward a multi-core philosophy, the inherent parallelism of ray tracing seem to be an excellent candidate to take advantage from this new technology. With some hardware makers highly interested in achieving this goal, and several users becoming enthusiastic by the notion of achieving highly-appealing graphics in future games, it would probably not be long before this technology arrives to our computers. 6 Acknowledgments The images used in this work were borrowed from various sources on the Internet, namely the OpenRT webpage, and papers from the bibliography.

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