Efficient algorithms for occlusion culling and shadows

The goal of this research is to develop more efficient techniques for computing the visibility and shadows in real-time rendering of three-dimensional scenes. Visibility algorithms determine what is visible from a camera, whereas shadow algorithms solve the same problem from the viewpoint of a light source. In rendering, a lot of computational resources are often spent on primitives that are not visible in the final image. One visibility algorithm for reducing the overhead is occlusion culling, which quickly discards the objects or primitives that are obstructed from the view by other primitives. A new method is presented for performing occlusion culling using silhouettes of meshes instead of triangles. Additionally, modifications are suggested to occlusion queries in order to reduce their computational overhead. The performance of currently available graphics hardware depends on the ordering of input primitives. A new technique, called delay streams, is proposed as a generic solution to order-dependent problems. The technique significantly reduces the pixel processing requirements by improving the efficiency of occlusion culling inside graphics hardware. Additionally, the memory requirements of order-independent transparency algorithms are reduced. A shadow map is a discretized representation of the scene geometry as seen by a light source. Typically the discretization causes difficult aliasing issues, such as jagged shadow boundaries and incorrect self-shadowing. A novel solution is presented for suppressing all types of aliasing artifacts by providing the correct sampling points for shadow maps, thus fully abandoning the previously used regular structures. Also, a simple technique is introduced for limiting the shadow map lookups to the pixels that get projected inside the shadow map. The fillrate problem of hardware-accelerated shadow volumes is greatly reduced with a new hierarchical rendering technique. The algorithm performs per-pixel shadow computations only at visible shadow boundaries, and uses lower resolution shadows for the parts of the screen that are guaranteed to be either fully lit or fully in shadow. The proposed techniques are expected to improve the rendering performance in most real-time applications that use 3D graphics, especially in computer games. More efficient algorithms for occlusion culling and shadows are important steps towards larger, more realistic virtual environments.reviewe

Exploiting frame coherence in real-time rendering for energy-efficient GPUs

The computation capabilities of mobile GPUs have greatly evolved in the last generations, allowing real-time rendering of realistic scenes. However, the desire for processing complex environments clashes with the battery-operated nature of smartphones, for which users expect long operating times per charge and a low-enough temperature to comfortably hold them. Consequently, improving the energy-efficiency of mobile GPUs is paramount to fulfill both performance and low-power goals. The work of the processors from within the GPU and their accesses to off-chip memory are the main sources of energy consumption in graphics workloads. Yet most of this energy is spent in redundant computations, as the frame rate required to produce animations results in a sequence of extremely similar images. The goal of this thesis is to improve the energy-efficiency of mobile GPUs by designing micro-architectural mechanisms that leverage frame coherence in order to reduce the redundant computations and memory accesses inherent in graphics applications. First, we focus on reducing redundant color computations. Mobile GPUs typically employ an architecture called Tile-Based Rendering, in which the screen is divided into tiles that are independently rendered in on-chip buffers. It is common that more than 80% of the tiles produce exactly the same output between consecutive frames. We propose Rendering Elimination (RE), a mechanism that accurately determines such occurrences by computing and storing signatures of the inputs of all the tiles in a frame. If the signatures of a tile across consecutive frames are the same, the colors computed in the preceding frame are reused, saving all computations and memory accesses associated to the rendering of the tile. We show that RE vastly outperforms related schemes found in the literature, achieving a reduction of energy consumption of 37% and execution time of 33% with minimal overheads. Next, we focus on reducing redundant computations of fragments that will eventually not be visible. In real-time rendering, objects are processed in the order they are submitted to the GPU, which usually causes that the results of previously-computed objects are overwritten by new objects that turn occlude them. Consequently, whether or not a particular object will be occluded is not known until the entire scene has been processed. Based on the fact that visibility tends to remain constant across consecutive frames, we propose Early Visibility Resolution (EVR), a mechanism that predicts visibility based on information obtained in the preceding frame. EVR first computes and stores the depth of the farthest visible point after rendering each tile. Whenever a tile is rendered in the following frame, primitives that are farther from the observer than the stored depth are predicted to be occluded, and processed after the ones predicted to be visible. Additionally, this visibility prediction scheme is used to improve Rendering Elimination’s equal tile detection capabilities by not adding primitives predicted to be occluded in the signature. With minor hardware costs, EVR is shown to provide a reduction of energy consumption of 43% and execution time of 39%. Finally, we focus on reducing computations in tiles with low spatial frequencies. GPUs produce pixel colors by sampling triangles once per pixel and performing computations on each sampling location. However, most screen regions do not include sufficient detail to require high sampling rates, leading to a significant amount of energy wasted computing the same color for neighboring pixels. Given that spatial frequencies are maintained across frames, we propose Dynamic Sampling Rate, a mechanism that analyzes the spatial frequencies of tiles and determines the best sampling rate for them, which is applied in the following frame. Results show that Dynamic Sampling Rate significantly reduces processor activity, yielding energy savings of 40% and execution time reductions of 35%.La capacitat de càlcul de les GPU mòbils ha augmentat en gran mesura en les darreres generacions, permetent el renderitzat de paisatges complexos en temps real. Nogensmenys, el desig de processar escenes cada vegada més realistes xoca amb el fet que aquests dispositius funcionen amb bateries, i els usuaris n’esperen llargues durades i una temperatura prou baixa com per a ser agafats còmodament. En conseqüència, millorar l’eficiència energètica de les GPU mòbils és essencial per a aconseguir els objectius de rendiment i baix consum. Els processadors de la GPU i els seus accessos a memòria són els principals consumidors d’energia en càrregues gràfiques, però molt d’aquest consum és malbaratat en càlculs redundants, ja que les animacions produïdes s¿aconsegueixen renderitzant una seqüència d’imatges molt similars. L’objectiu d’aquesta tesi és millorar l’eficiència energètica de les GPU mòbils mitjançant el disseny de mecanismes microarquitectònics que aprofitin la coherència entre imatges per a reduir els càlculs i accessos redundants inherents a les aplicacions gràfiques. Primerament, ens centrem en reduir càlculs redundants de colors. A les GPU mòbils, sovint s'empra una arquitectura anomenada Tile-Based Rendering, en què la pantalla es divideix en regions que es processen independentment dins del xip. És habitual que més del 80% de les regions de pantalla produeixin els mateixos colors entre imatges consecutives. Proposem Rendering Elimination (RE), un mecanisme que determina acuradament aquests casos computant una signatura de les entrades de totes les regions. Si les signatures de dues imatges són iguals, es reutilitzen els colors calculats a la imatge anterior, el que estalvia tots els càlculs i accessos a memòria de la regió. RE supera àmpliament propostes relacionades de la literatura, aconseguint una reducció del consum energètic del 37% i del temps d’execució del 33%. Seguidament, ens centrem en reduir càlculs redundants en fragments que eventualment no seran visibles. En aplicacions gràfiques, els objectes es processen en l’ordre en què son enviats a la GPU, el que sovint causa que resultats ja processats siguin sobreescrits per nous objectes que els oclouen. Per tant, no se sap si un objecte serà visible o no fins que tota l’escena ha estat processada. Fonamentats en el fet que la visibilitat tendeix a ser constant entre imatges, proposem Early Visibility Resolution (EVR), un mecanisme que prediu la visibilitat basat en informació obtinguda a la imatge anterior. EVR computa i emmagatzema la profunditat del punt visible més llunyà després de processar cada regió de pantalla. Quan es processa una regió a la imatge següent, es prediu que les primitives més llunyanes a el punt guardat seran ocloses i es processen després de les que es prediuen que seran visibles. Addicionalment, aquest esquema de predicció s’empra en millorar la detecció de regions redundants de RE al no afegir les primitives que es prediu que seran ocloses a les signatures. Amb un cost de maquinari mínim, EVR aconsegueix una millora del consum energètic del 43% i del temps d’execució del 39%. Finalment, ens centrem a reduir càlculs en regions de pantalla amb poca freqüència espacial. Les GPU actuals produeixen colors mostrejant els triangles una vegada per cada píxel i fent càlculs a cada localització mostrejada. Però la majoria de regions no tenen suficient detall per a necessitar altes freqüències de mostreig, el que implica un malbaratament d’energia en el càlcul del mateix color en píxels adjacents. Com les freqüències tendeixen a mantenir-se en el temps, proposem Dynamic Sampling Rate (DSR)¸ un mecanisme que analitza les freqüències de les regions una vegada han estat renderitzades i en determina la menor freqüència de mostreig a la que es poden processar, que s’aplica a la següent imatge..

Exploiting frame coherence in real-time rendering for energy-efficient GPUs

The computation capabilities of mobile GPUs have greatly evolved in the last generations, allowing real-time rendering of realistic scenes. However, the desire for processing complex environments clashes with the battery-operated nature of smartphones, for which users expect long operating times per charge and a low-enough temperature to comfortably hold them. Consequently, improving the energy-efficiency of mobile GPUs is paramount to fulfill both performance and low-power goals. The work of the processors from within the GPU and their accesses to off-chip memory are the main sources of energy consumption in graphics workloads. Yet most of this energy is spent in redundant computations, as the frame rate required to produce animations results in a sequence of extremely similar images. The goal of this thesis is to improve the energy-efficiency of mobile GPUs by designing micro-architectural mechanisms that leverage frame coherence in order to reduce the redundant computations and memory accesses inherent in graphics applications. First, we focus on reducing redundant color computations. Mobile GPUs typically employ an architecture called Tile-Based Rendering, in which the screen is divided into tiles that are independently rendered in on-chip buffers. It is common that more than 80% of the tiles produce exactly the same output between consecutive frames. We propose Rendering Elimination (RE), a mechanism that accurately determines such occurrences by computing and storing signatures of the inputs of all the tiles in a frame. If the signatures of a tile across consecutive frames are the same, the colors computed in the preceding frame are reused, saving all computations and memory accesses associated to the rendering of the tile. We show that RE vastly outperforms related schemes found in the literature, achieving a reduction of energy consumption of 37% and execution time of 33% with minimal overheads. Next, we focus on reducing redundant computations of fragments that will eventually not be visible. In real-time rendering, objects are processed in the order they are submitted to the GPU, which usually causes that the results of previously-computed objects are overwritten by new objects that turn occlude them. Consequently, whether or not a particular object will be occluded is not known until the entire scene has been processed. Based on the fact that visibility tends to remain constant across consecutive frames, we propose Early Visibility Resolution (EVR), a mechanism that predicts visibility based on information obtained in the preceding frame. EVR first computes and stores the depth of the farthest visible point after rendering each tile. Whenever a tile is rendered in the following frame, primitives that are farther from the observer than the stored depth are predicted to be occluded, and processed after the ones predicted to be visible. Additionally, this visibility prediction scheme is used to improve Rendering Elimination’s equal tile detection capabilities by not adding primitives predicted to be occluded in the signature. With minor hardware costs, EVR is shown to provide a reduction of energy consumption of 43% and execution time of 39%. Finally, we focus on reducing computations in tiles with low spatial frequencies. GPUs produce pixel colors by sampling triangles once per pixel and performing computations on each sampling location. However, most screen regions do not include sufficient detail to require high sampling rates, leading to a significant amount of energy wasted computing the same color for neighboring pixels. Given that spatial frequencies are maintained across frames, we propose Dynamic Sampling Rate, a mechanism that analyzes the spatial frequencies of tiles and determines the best sampling rate for them, which is applied in the following frame. Results show that Dynamic Sampling Rate significantly reduces processor activity, yielding energy savings of 40% and execution time reductions of 35%.La capacitat de càlcul de les GPU mòbils ha augmentat en gran mesura en les darreres generacions, permetent el renderitzat de paisatges complexos en temps real. Nogensmenys, el desig de processar escenes cada vegada més realistes xoca amb el fet que aquests dispositius funcionen amb bateries, i els usuaris n’esperen llargues durades i una temperatura prou baixa com per a ser agafats còmodament. En conseqüència, millorar l’eficiència energètica de les GPU mòbils és essencial per a aconseguir els objectius de rendiment i baix consum. Els processadors de la GPU i els seus accessos a memòria són els principals consumidors d’energia en càrregues gràfiques, però molt d’aquest consum és malbaratat en càlculs redundants, ja que les animacions produïdes s¿aconsegueixen renderitzant una seqüència d’imatges molt similars. L’objectiu d’aquesta tesi és millorar l’eficiència energètica de les GPU mòbils mitjançant el disseny de mecanismes microarquitectònics que aprofitin la coherència entre imatges per a reduir els càlculs i accessos redundants inherents a les aplicacions gràfiques. Primerament, ens centrem en reduir càlculs redundants de colors. A les GPU mòbils, sovint s'empra una arquitectura anomenada Tile-Based Rendering, en què la pantalla es divideix en regions que es processen independentment dins del xip. És habitual que més del 80% de les regions de pantalla produeixin els mateixos colors entre imatges consecutives. Proposem Rendering Elimination (RE), un mecanisme que determina acuradament aquests casos computant una signatura de les entrades de totes les regions. Si les signatures de dues imatges són iguals, es reutilitzen els colors calculats a la imatge anterior, el que estalvia tots els càlculs i accessos a memòria de la regió. RE supera àmpliament propostes relacionades de la literatura, aconseguint una reducció del consum energètic del 37% i del temps d’execució del 33%. Seguidament, ens centrem en reduir càlculs redundants en fragments que eventualment no seran visibles. En aplicacions gràfiques, els objectes es processen en l’ordre en què son enviats a la GPU, el que sovint causa que resultats ja processats siguin sobreescrits per nous objectes que els oclouen. Per tant, no se sap si un objecte serà visible o no fins que tota l’escena ha estat processada. Fonamentats en el fet que la visibilitat tendeix a ser constant entre imatges, proposem Early Visibility Resolution (EVR), un mecanisme que prediu la visibilitat basat en informació obtinguda a la imatge anterior. EVR computa i emmagatzema la profunditat del punt visible més llunyà després de processar cada regió de pantalla. Quan es processa una regió a la imatge següent, es prediu que les primitives més llunyanes a el punt guardat seran ocloses i es processen després de les que es prediuen que seran visibles. Addicionalment, aquest esquema de predicció s’empra en millorar la detecció de regions redundants de RE al no afegir les primitives que es prediu que seran ocloses a les signatures. Amb un cost de maquinari mínim, EVR aconsegueix una millora del consum energètic del 43% i del temps d’execució del 39%. Finalment, ens centrem a reduir càlculs en regions de pantalla amb poca freqüència espacial. Les GPU actuals produeixen colors mostrejant els triangles una vegada per cada píxel i fent càlculs a cada localització mostrejada. Però la majoria de regions no tenen suficient detall per a necessitar altes freqüències de mostreig, el que implica un malbaratament d’energia en el càlcul del mateix color en píxels adjacents. Com les freqüències tendeixen a mantenir-se en el temps, proposem Dynamic Sampling Rate (DSR)¸ un mecanisme que analitza les freqüències de les regions una vegada han estat renderitzades i en determina la menor freqüència de mostreig a la que es poden processar, que s’aplica a la següent imatge...Postprint (published version

Optimization techniques for computationally expensive rendering algorithms

Realistic rendering in computer graphics simulates the interactions of light and surfaces. While many accurate models for surface reflection and lighting, including solid surfaces and participating media have been described; most of them rely on intensive computation. Common practices such as adding constraints and assumptions can increase performance. However, they may compromise the quality of the resulting images or the variety of phenomena that can be accurately represented. In this thesis, we will focus on rendering methods that require high amounts of computational resources. Our intention is to consider several conceptually different approaches capable of reducing these requirements with only limited implications in the quality of the results. The first part of this work will study rendering of time-­¿varying participating media. Examples of this type of matter are smoke, optically thick gases and any material that, unlike the vacuum, scatters and absorbs the light that travels through it. We will focus on a subset of algorithms that approximate realistic illumination using images of real world scenes. Starting from the traditional ray marching algorithm, we will suggest and implement different optimizations that will allow performing the computation at interactive frame rates. This thesis will also analyze two different aspects of the generation of anti-­¿aliased images. One targeted to the rendering of screen-­¿space anti-­¿aliased images and the reduction of the artifacts generated in rasterized lines and edges. We expect to describe an implementation that, working as a post process, it is efficient enough to be added to existing rendering pipelines with reduced performance impact. A third method will take advantage of the limitations of the human visual system (HVS) to reduce the resources required to render temporally antialiased images. While film and digital cameras naturally produce motion blur, rendering pipelines need to explicitly simulate it. This process is known to be one of the most important burdens for every rendering pipeline. Motivated by this, we plan to run a series of psychophysical experiments targeted at identifying groups of motion-­¿blurred images that are perceptually equivalent. A possible outcome is the proposal of criteria that may lead to reductions of the rendering budgets

Reducing redundancy of real time computer graphics in mobile systems

The goal of this thesis is to propose novel and effective techniques to eliminate redundant computations that waste energy and are performed in real-time computer graphics applications, with special focus on mobile GPU micro-architecture. Improving the energy-efficiency of CPU/GPU systems is not only key to enlarge their battery life, but also allows to increase their performance because, to avoid overheating above thermal limits, SoCs tend to be throttled when the load is high for a large period of time. Prior studies pointed out that the CPU and especially the GPU are the principal energy consumers in the graphics subsystem, being the off-chip main memory accesses and the processors inside the GPU the primary energy consumers of the graphics subsystem. First, we focus on reducing redundant fragment processing computations by means of improving the culling of hidden surfaces. During real-time graphics rendering, objects are processed by the GPU in the order they are submitted by the CPU, and occluded surfaces are often processed even though they will end up not being part of the final image. When the GPU realizes that an object or part of it is not going to be visible, all activity required to compute its color and store it has already been performed. We propose a novel architectural technique for mobile GPUs, Visibility Rendering Order (VRO), which reorders objects front-to-back entirely in hardware to maximize the culling effectiveness of the GPU and minimize overshading, hence reducing execution time and energy consumption. VRO exploits the fact that the objects in graphics animated applications tend to keep its relative depth order across consecutive frames (temporal coherence) to provide the feeling of smooth transition. VRO keeps visibility information of a frame, and uses it to reorder the objects of the following frame. VRO just requires adding a small hardware to capture the visibility information and use it later to guide the rendering of the following frame. Moreover, VRO works in parallel with the graphics pipeline, so negligible performance overheads are incurred. We illustrate the benefits of VRO using various unmodified commercial 3D applications for which VRO achieves 27% speed-up and 14.8% energy reduction on average. Then, we focus on avoiding redundant computations related to CPU Collision Detection (CD). Graphics applications such as 3D games represent a large percentage of downloaded applications for mobile devices and the trend is towards more complex and realistic scenes with accurate 3D physics simulations. CD is one of the most important algorithms in any physics kernel since it identifies the contact points between the objects of a scene and determines when they collide. However, real-time accurate CD is very expensive in terms of energy consumption. We propose Render Based Collision Detection (RBCD), a novel energy-efficient high-fidelity CD scheme that leverages some intermediate results of the rendering pipeline to perform CD, so that redundant tasks are done just once. Comparing RBCD with a conventional CD completely executed in the CPU, we show that its execution time is reduced by almost three orders of magnitude (600x speedup), because most of the CD task of our model comes for free by reusing the image rendering intermediate results. Although not necessarily, such a dramatic time improvement may result in better frames per second if physics simulation stays in the critical path. However, the most important advantage of our technique is the enormous energy savings that result from eliminating a long and costly CPU computation and converting it into a few simple operations executed by a specialized hardware within the GPU. Our results show that the energy consumed by CD is reduced on average by a factor of 448x (i.e., by 99.8\%). These dramatic benefits are accompanied by a higher fidelity CD analysis (i.e., with finer granularity), which improves the quality and realism of the application.El objetivo de esta tesis es proponer técnicas efectivas y originales para eliminar computaciones inútiles que aparecen en aplicaciones gráficas, con especial énfasis en micro-arquitectura de GPUs. Mejorar la eficiencia energética de los sistemas CPU/GPU no es solo clave para alargar la vida de la batería, sino también incrementar su rendimiento. Estudios previos han apuntado que la CPU y especialmente la GPU son los principales consumidores de energía en el sub-sistema gráfico, siendo los accesos a memoria off-chip y los procesadores dentro de la GPU los principales consumidores de energía del sub-sistema gráfico. Primero, nos hemos centrado en reducir computaciones redundantes de la fase de fragment processing mediante la mejora en la eliminación de superficies ocultas. Durante el renderizado de gráficos en tiempo real, los objetos son procesados por la GPU en el orden en el que son enviados por la CPU, y las superficies ocultas son a menudo procesadas incluso si no no acaban formando parte de la imagen final. Cuando la GPU averigua que el objeto o parte de él no es visible, toda la actividad requerida para computar su color y guardarlo ha sido realizada. Proponemos una técnica arquitectónica original para GPUs móviles, Visibility Rendering Order (VRO), la cual reordena los objetos de delante hacia atrás por completo en hardware para maximizar la efectividad del culling de la GPU y así minimizar el overshading, y por lo tanto reducir el tiempo de ejecución y el consumo de energía. VRO explota el hecho de que los objetos de las aplicaciones gráficas animadas tienden a mantener su orden relativo en profundidad a través de frames consecutivos (coherencia temporal) para proveer animaciones con transiciones suaves. Dado que las relaciones de orden en profundidad entre objetos son testeadas en la GPU, VRO introduce costes mínimos en energía. Solo requiere añadir una pequeña unidad hardware para capturar la información de visibilidad. Además, VRO trabaja en paralelo con el pipeline gráfico, por lo que introduce costes insignificantes en tiempo. Ilustramos los beneficios de VRO usango varias aplicaciones 3D comerciales para las cuales VRO consigue un 27% de speed-up y un 14.8% de reducción de energía en media. En segundo lugar, evitamos computaciones redundantes relacionadas con la Detección de Colisiones (CD) en la CPU. Las aplicaciones gráficas animadas como los juegos 3D representan un alto porcentaje de las aplicaciones descargadas en dispositivos móviles y la tendencia es hacia escenas más complejas y realistas con simulaciones físicas 3D precisas. La CD es uno de los algoritmos más importantes entre los kernel de físicas dado que identifica los puntos de contacto entre los objetos de una escena. Sin embargo, una CD en tiempo real y precisa es muy costosa en términos de consumo energético. Proponemos Render Based Collision Detection (RBCD), una técnica energéticamente eficiente y preciso de CD que utiliza resultados intermedios del rendering pipeline para realizar la CD. Comparando RBCD con una CD convencional completamente ejecutada en la CPU, mostramos que el tiempo de ejecución es reducido casi tres órdenes de magnitud (600x speedup), porque la mayoría de la CD de nuestro modelo reusa resultados intermedios del renderizado de la imagen. Aunque no es así necesariamente, esta espectacular en tiempo puede resultar en mejores frames por segundo si la simulación de físicas está en el camino crítico. Sin embargo, la ventaja más importante de nuestra técnica es el enorme ahorro de energía que resulta de eliminar las largas y costosas computaciones en la CPU, sustituyéndolas por unas pocas operaciones ejecutadas en un hardware especializado dentro de la GPU. Nuestros resultados muestran que la energía consumida por la CD es reducidad en media por un factor de 448x. Estos dramáticos beneficios vienen acompañados de una mayor fidelidad en la CD (i.e. con granularidad más fina)Postprint (published version

Interactive ray tracing of massive and deformable models

Ray tracing is a fundamental algorithm used for many applications such as computer graphics, geometric simulation, collision detection and line-of-sight computation. Even though the performance of ray tracing algorithms scales with the model complexity, the high memory requirements and the use of static hierarchical structures pose problems with massive models and dynamic data-sets. We present several approaches to address these problems based on new acceleration structures and traversal algorithms. We introduce a compact representation for storing the model and hierarchy while ray tracing triangle meshes that can reduce the memory footprint by up to 80%, while maintaining high performance. As a result, can ray trace massive models with hundreds of millions of triangles on workstations with a few gigabytes of memory. We also show how to use bounding volume hierarchies for ray tracing complex models with interactive performance. In order to handle dynamic scenes, we use refitting algorithms and also present highly-parallel GPU-based algorithms to reconstruct the hierarchies. In practice, our method can construct hierarchies for models with hundreds of thousands of triangles at interactive speeds. Finally, we demonstrate several applications that are enabled by these algorithms. Using deformable BVH and fast data parallel techniques, we introduce a geometric sound propagation algorithm that can run on complex deformable scenes interactively and orders of magnitude faster than comparable previous approaches. In addition, we also use these hierarchical algorithms for fast collision detection between deformable models and GPU rendering of shadows on massive models by employing our compact representations for hybrid ray tracing and rasterization

Scalable Realtime Rendering and Interaction with Digital Surface Models of Landscapes and Cities

Interactive, realistic rendering of landscapes and cities differs substantially from classical terrain rendering. Due to the sheer size and detail of the data which need to be processed, realtime rendering (i.e. more than 25 images per second) is only feasible with level of detail (LOD) models. Even the design and implementation of efficient, automatic LOD generation is ambitious for such out-of-core datasets considering the large number of scales that are covered in a single view and the necessity to maintain screen-space accuracy for realistic representation. Moreover, users want to interact with the model based on semantic information which needs to be linked to the LOD model. In this thesis I present LOD schemes for the efficient rendering of 2.5d digital surface models (DSMs) and 3d point-clouds, a method for the automatic derivation of city models from raw DSMs, and an approach allowing semantic interaction with complex LOD models. The hierarchical LOD model for digital surface models is based on a quadtree of precomputed, simplified triangle mesh approximations. The rendering of the proposed model is proved to allow real-time rendering of very large and complex models with pixel-accurate details. Moreover, the necessary preprocessing is scalable and fast. For 3d point clouds, I introduce an LOD scheme based on an octree of hybrid plane-polygon representations. For each LOD, the algorithm detects planar regions in an adequately subsampled point cloud and models them as textured rectangles. The rendering of the resulting hybrid model is an order of magnitude faster than comparable point-based LOD schemes. To automatically derive a city model from a DSM, I propose a constrained mesh simplification. Apart from the geometric distance between simplified and original model, it evaluates constraints based on detected planar structures and their mutual topological relations. The resulting models are much less complex than the original DSM but still represent the characteristic building structures faithfully. Finally, I present a method to combine semantic information with complex geometric models. My approach links the semantic entities to the geometric entities on-the-fly via coarser proxy geometries which carry the semantic information. Thus, semantic information can be layered on top of complex LOD models without an explicit attribution step. All findings are supported by experimental results which demonstrate the practical applicability and efficiency of the methods

Visibility rendering order: Improving energy efficiency on mobile GPUs through frame coherence

During real-time graphics rendering, objects are processed by the GPU in the order they are submitted by the CPU, and occluded surfaces are often processed even though they will end up not being part of the final image, thus wasting precious time and energy. To help discard occluded surfaces, most current GPUs include an Early-Depth test before the fragment processing stage. However, to be effective it requires that opaque objects are processed in a front-to-back order. Depth sorting and other occlusion culling techniques at the object level incur overheads that are only offset for applications having substantial depth and/or fragment shading complexity, which is often not the case in mobile workloads. We propose a novel architectural technique for mobile GPUs, Visibility Rendering Order (VRO), which reorders objects front-to-back entirely in hardware by exploiting the fact that the objects in graphics animated applications tend to keep its relative depth order across consecutive frames (temporal coherence). Since order relationships are already tested by the Depth Test, VRO incurs minimal energy overheads because it just requires adding a small hardware to capture that information and use it later to guide the rendering of the following frame. Moreover, unlike other approaches, this unit works in parallel with the graphics pipeline without any performance overhead. We illustrate the benefits of VRO using various unmodified commercial 3D applications for which VRO achieves 27% speed-up and 14.8% energy reduction on average over a state-of-the-art mobile GPU.Peer ReviewedPostprint (author's final draft

Physiological system modelling

Computer graphics has a major impact in our day-to-day life. It is used in diverse areas such as displaying the results of engineering and scientific computations and visualization, producing television commercials and feature films, simulation and analysis of real world problems, computer aided design, graphical user interfaces that increases the communication bandwidth between humans and machines, etc Scientific visualization is a well-established method for analysis of data, originating from scientific computations, simulations or measurements. The development and implementation of the 3Dgen software was developed by the author using OpenGL and C language was presented in this report 3Dgen was used to visualize threedimensional cylindrical models such as pipes and also for limited usage in virtual endoscopy. Using the developed software a model was created using the centreline data input by the user or from the output of some other program, stored in a normal text file. The model was constructed by drawing surface polygons between two adjacent centreline points. The software allows the user to view the internal and external surfaces of the model. The software was designed in such a way that it runs in more than one operating systems with minimal installation procedures Since the size of the software is very small it can be stored in a 1 44 Megabyte floppy diskette. Depending on the processing speed of the PC the software can generate models of any length and size Compared to other packages, 3Dgen has minimal input procedures was able to generate models with smooth bends. It has both modelling and virtual exploration features. For models with sharp bends the software generates an overshoot

Interactive global illumination on the CPU

Computing realistic physically-based global illumination in real-time remains one of the major goals in the fields of rendering and visualisation; one that has not yet been achieved due to its inherent computational complexity. This thesis focuses on CPU-based interactive global illumination approaches with an aim to develop generalisable hardware-agnostic algorithms. Interactive ray tracing is reliant on spatial and cache coherency to achieve interactive rates which conflicts with needs of global illumination solutions which require a large number of incoherent secondary rays to be computed. Methods that reduce the total number of rays that need to be processed, such as Selective rendering, were investigated to determine how best they can be utilised. The impact that selective rendering has on interactive ray tracing was analysed and quantified and two novel global illumination algorithms were developed, with the structured methodology used presented as a framework. Adaptive Inter- leaved Sampling, is a generalisable approach that combines interleaved sampling with an adaptive approach, which uses efficient component-specific adaptive guidance methods to drive the computation. Results of up to 11 frames per second were demonstrated for multiple components including participating media. Temporal Instant Caching, is a caching scheme for accelerating the computation of diffuse interreflections to interactive rates. This approach achieved frame rates exceeding 9 frames per second for the majority of scenes. Validation of the results for both approaches showed little perceptual difference when comparing against a gold-standard path-traced image. Further research into caching led to the development of a new wait-free data access control mechanism for sharing the irradiance cache among multiple rendering threads on a shared memory parallel system. By not serialising accesses to the shared data structure the irradiance values were shared among all the threads without any overhead or contention, when reading and writing simultaneously. This new approach achieved efficiencies between 77% and 92% for 8 threads when calculating static images and animations. This work demonstrates that, due to the flexibility of the CPU, CPU-based algorithms remain a valid and competitive choice for achieving global illumination interactively, and an alternative to the generally brute-force GPU-centric algorithms