<p>Ambient Occlusion är en teknik för ambient ljussättning i digitala tredimensionella scener. Sådana scener ljussätts vanligtvis med en konstant mängd ambient ljus på samtliga ytor oberoende av ytornas vinkel och position gentemot olika ljuskällor i scenen. Detta ger ett platt och kalt intryck och utan vidare ljussättningstekniker är det ytterst svårt att urskönja detaljer i scenen. Ambient Occlusion åthjälper detta genom att reducera mängden ambient ljus i vissa delar av scenen. Ambient ljus är en enkel approximation av det reflekterade diffusa ljuset som antas nå nästan överallt i scenen. Genom att sänka det ambienta ljuset på punkter i scenen med tät eller täckande geometri så ger Ambient Occlusion ett intryck av att det sekundära diffusa ljuset ej når dessa platser. Pappret undersöker en äldre variant av Ambient Occlusion där mängden ambient ljus beräknas statiskt för en scen och sparas i texturer. Vidare undersöks nyare metoder där mängden ambient ljus beräknas dynamiskt på den renderade scenen i Pixel Shaders. Det senare tillvägagångssättet kallas Image Based Ambient Occlusion eller Screen Space Ambient Occlusion. Detta nya tillvägagångssätt jämförs mot den traditionella angreppsvinkeln med förberäknade texturer. Teknikerna utvärderas och jämförs mot varandra i avseende på tids- och minneskomplexitet, enkelhet och visuellt resultat utöver specifika egenskaper för de enskilda teknikerna. Arbetets resultat beskrivs i slutet av rapporten. I resultatet presenteras hur shaderteknikerna pga sina brister inte är applicerbara i alla scener.</p>

Ambient Occlusion är en teknik för ambient ljussättning i digitala tredimensionella scener. Sådana scener ljussätts vanligtvis med en konstant mängd ambient ljus på samtliga ytor oberoende av ytornas vinkel och position gentemot olika ljuskällor i scenen. Detta ger ett platt och kalt intryck och utan vidare ljussättningstekniker är det ytterst svårt att urskönja detaljer i scenen. Ambient Occlusion åthjälper detta genom att reducera mängden ambient ljus i vissa delar av scenen. Ambient ljus är en enkel approximation av det reflekterade diffusa ljuset som antas nå nästan överallt i scenen. Genom att sänka det ambienta ljuset på punkter i scenen med tät eller täckande geometri så ger Ambient Occlusion ett intryck av att det sekundära diffusa ljuset ej når dessa platser. Pappret undersöker en äldre variant av Ambient Occlusion där mängden ambient ljus beräknas statiskt för en scen och sparas i texturer. Vidare undersöks nyare metoder där mängden ambient ljus beräknas dynamiskt på den renderade scenen i Pixel Shaders. Det senare tillvägagångssättet kallas Image Based Ambient Occlusion eller Screen Space Ambient Occlusion. Detta nya tillvägagångssätt jämförs mot den traditionella angreppsvinkeln med förberäknade texturer. Teknikerna utvärderas och jämförs mot varandra i avseende på tids- och minneskomplexitet, enkelhet och visuellt resultat utöver specifika egenskaper för de enskilda teknikerna. Arbetets resultat beskrivs i slutet av rapporten. I resultatet presenteras hur shaderteknikerna pga sina brister inte är applicerbara i alla scener.

abstract: Tessellation and Screen-Space Ambient Occlusion are algorithms which have been widely-used in real-time rendering in the past decade. They aim to enhance the details of the mesh, cast better shadow effects and improve the quality of the rendered images in real time. WebGL is a web-based graphics library derived from OpenGL ES used for rendering in web applications. It is relatively new and has been rapidly evolving, this has resulted in it supporting a subset of rendering features normally supported by desktop applications. In this thesis, the research is focusing on evaluating Curved PN-Triangles tessellation with Screen Space Ambient Occlusion (SSAO), Horizon-Based Ambient Occlusion (HBAO) and Horizon-Based Ambient Occlusion Plus (HBAO+) in WebGL-based real-time application and comparing its performance to desktop based application and to discuss the capabilities, limitations and bottlenecks of WebGL 1.0. / Dissertation/Thesis / WebGL Program / OpenGL Program / Masters Thesis Computer Science 2017

<p>Ultrasound (US) volume data is noisy, so traditional methods for direct volume rendering (DVR) are less appropriate. Improved methods or new techniques are required. There are furthermore a high performance requirement and limited pre-processing to be considered in order for it to be used interactively, since the volume data might be time-varying.</p><p>There exist numerous techniques for improving visual perception of volume rendering, and while some perform well and produce a visually enhanced result, many are designed and compared for use with medical data that has a high signal-to-noise ratio. This master thesis describe and compare recent methods for DVR illumination, in the form of ambient occlusion or direct/indirect lighting from an external light source. New designs and modifications are introduced for efficiently and effectively enhancing the visual quality of DVR with US data. Furthermore, this thesis addresses the issue of how clipping is performed during rendering and for the different illumination techniques, which is commonly used in ultrasound visualization.</p><p>This diploma work was conducted at Siemens Corporate Research in Princeton, NJ where the partially open source framework XIP is developed. The framework was extended further to include modern methods for DVR illumination that are described in detail within this thesis. Finally, presented results show that several methods can be used to visually enhance the visualization within highly interactive frame-rates.</p>

Ultrasound (US) volume data is noisy, so traditional methods for direct volume rendering (DVR) are less appropriate. Improved methods or new techniques are required. There are furthermore a high performance requirement and limited pre-processing to be considered in order for it to be used interactively, since the volume data might be time-varying. There exist numerous techniques for improving visual perception of volume rendering, and while some perform well and produce a visually enhanced result, many are designed and compared for use with medical data that has a high signal-to-noise ratio. This master thesis describe and compare recent methods for DVR illumination, in the form of ambient occlusion or direct/indirect lighting from an external light source. New designs and modifications are introduced for efficiently and effectively enhancing the visual quality of DVR with US data. Furthermore, this thesis addresses the issue of how clipping is performed during rendering and for the different illumination techniques, which is commonly used in ultrasound visualization. This diploma work was conducted at Siemens Corporate Research in Princeton, NJ where the partially open source framework XIP is developed. The framework was extended further to include modern methods for DVR illumination that are described in detail within this thesis. Finally, presented results show that several methods can be used to visually enhance the visualization within highly interactive frame-rates.

Context. In this project a comparison between two screen space ambientocclusion techniques are presented. The techniques are Scalable AO (SAO)and Multiresolution SSAO (MSSAO) since they both are techniques thatuse mipmaps to accelerate their calculations. Objectives. The aim is to see how big the difference is between the resultsof these two techniques and a golden reference that is an object space raytraced texture that is created with mental ray in Maya and how long timethe computation takes. Methods. The comparisons between the AO textures that these techniquesproduce and the golden references are performed using Structural SimilarityIndex (SSIM) and Perceptual Image Difference (PDIFF). Results. On the lowest resolution, both techniques execute in about thesame time on average, except that SAO with the shortest distance is faster.The only effect caused by the shorter distance, in this case, is that moresamples are taken in higher resolution mipmap levels than when longerdistances are used. The MSSAO achieved a better SSIM value meaningthat MSSAO is more similar to the golden reference than SAO. As theresolution increases the SSIM value between both techniques become moresimilar with SAO getting a better value and MSSAO getting slightly worse,while the execution time for MSSAO has larger increases than SAO. Conclusions. It is concluded that MSSAO is better than SAO in lowerresolution while SAO is better in larger resolution. I would recommendthat SAO is used for indoor scenes where there are not many small geometryparts close to each other that should occlude each other. MSSAO shouldbe used for outdoor scenes with a lot of vegetation which has many smallgeometry parts close to each other that should occlude. At higher resolution,MSSAO takes longer computational time as compared with SAO, while atlower resolution the computational time is similar.

In computer graphics, lighting is an important area. To simulate shadows from area light sources, indirect lighting and shadows from indirect light, a class of algorithms commonly known as global illumination algorithms can be used. Ambient occlusion is an approximation to global illumination that can emulate shadows from area light sources and shadows from indirect light, giving very soft shadows. For real-time applications, ambient occlusion can be precomputed and stored in maps or per vertex. However, that can only be done with good results if the geometry is static. Therefore, a number of methods that can handle more or less dynamic scenes have been introduced in the recent years. In this thesis, a collection of ambient occlusion methods for dynamic objects and procedural environments will be described. The main contribution is the introduction of a novel method that handles ambient occlusion for procedural environments. Another contribution is a description of an implementation of Screen Space Ambient Occlusion (SSAO). SSAO is an algorithm that calculates approximate ambient occlusion in real-time by using the depths of surrounding pixels. It handles completely dynamic scenes with good performance. The method for procedural environments handles the scenario where a number of building blocks are procedurally assembled at run-time. The idea is to precompute an ambient occlusion map for each building block where the self-occlusion is stored. In addition, an ambient occlusion grid is precomputed for each block to accommodate the inter-block occlusion. At run-time, after the building blocks have been assembled, the ambient occlusion from the grids is blended with the ambient occlusion from the maps to generate new maps, valid for the procedural environment. Following that, the environment can be rendered with high quality ambient occlusion at almost no cost, in the same fashion as for a static environment where the ambient occlusion maps can be completely precomputed.

[pt] Oclusão ambiente é uma técnica barata de simular a iluminação ambiente indireta de forma realista. Nesta dissertação, propõe-se elaborar um método de oclusão ambiente produzindo resultados de boa qualidade em tempo real. Através da utilização de um eficiente método de voxelização, cria-se uma descrição volumétrica da geometria em uma grade regular. Durante a renderização da cena, o hemisfério em torno de cada ponto visível é amostrado por diversos cones, cada um equivalendo ao lançamento de múltiplos raios. O volume de cada cone é amostrado por uma série de esferas e estas são utilizadas para determinar o volume do cone obstruído pela geometria da cena. Ao acumular os resultados de cada esfera em cada cone, e de cada cone no hemisfério, a oclusão ambiente causada pela geometria em cada ponto visível pelo observador é estimada. Esta estratégia mostra-se adequada, pois a interseção de cada esfera com a cena voxelizada pode ser feita de forma muito eficiente. / [en] Ambient occlusion is a low-cost technique to simulate indirect ambient illumination in a realistic way. In this dissertation, we propose an ambient occlusion method that achieves good quality results in real time. Using an efficient voxelization algorithm, we create a volumetric description of the geometry in a regular grid. During the scene rendering, the hemisphere around each visible point is sampled by several cones, each one corresponding to the trace of multiple rays. The volume of each cone is sampled by a serie of spheres, which are used to compute the cone s volume obstructed by the scene geometry. The occlusion caused by the geometry at each visible point is estimated by accumulating each sphere result in the cones and each cone result in the hemisphere. We found this approach to be adequate, since the intersection of each sphere with the voxelized scene can be performed in a very efficient manner.

[pt] A interpretação tridimensional de conjuntos densos de linhas exige o uso de modelos de iluminação mais elaborados. A oclusão ambiente é uma técnica utilizada para simular, de forma realista e barata, a iluminação ambiente indireta. Este trabalho apresenta um novo algoritmo para renderização de linhas com oclusão de ambiente. O algoritmo proposto é baseado na voxelização da cena e no cálculo do volume ocupado do hemisfério associado a cada ponto do linha. Propõe-se uma adaptação no algoritmo de voxelização de cenas 3D formada por sólidos para o tratamento correto da cena formada por linhas. Assim, uma descrição volumétrica da geometria é criada em um buffer de textura. O hemisfério em torno de cada ponto visível é amostrado por diversos pontos, e para cada amostra se acessa um prisma, cujo volume ocupado é calculados a partir da voxelização. Ao acumular os resultados de cada amostra, estima-se a oclusão de ambiente causada pela geometria em cada ponto visível pelo observador. Esta estratégia mostra-se adequada, pois tem como resultado imagens com alta qualidade em tempo real para cenas com grande complexidade. / [en] The three-dimensional understanding of dense line sets requires the use of more sophisticated lighting models. Ambient occlusion is a technique used to simulate realistically and efficiently, the indirect ambient lighting. This paper presents a new algorithm for rendering lines with ambient occlusion. The proposed algorithm is based on the voxelization of the scene and on the computation of occlusionin the hemisphere associated to each visible point. It is proposed an adaptation of the voxelization algorithm of 3D scenes made up of solids to the correct treatment of the scene formed by lines. Thus, a volumetric geometry description is created in a texture buffer. The hemisphere around every visible point is sampled by several points, and for each sample is generated a prism, which occluded volume is calculated from the voxelization. By accumulating the results of each sample, the estimated ambient occlusion caused by the geometry at each point visible to the observer is computed. This strategy proved to be appropriate, resulting in high-quality images in real time for complex scenes.

Recently, new hardware capabilities in GPUs has opened the possibility of ray tracing in real-time at interactive framerates. These new capabilities can be used for a range of ray tracing techniques - the focus of this thesis is on ray traced ambient occlusion (RTAO). This thesis evaluates real-time ray RTAO by comparing it with ground- truth ambient occlusion (GTAO), a state-of-the-art screen space ambient occlusion (SSAO) method. A contribution by this thesis is that the evaluation is made in scenarios that includes animated objects, both rigid-body animations and skinning animations. This approach has some advantages: it can emphasise visual artefacts that arise due to objects moving and animating. Furthermore, it makes the performance tests better approximate real-world applications such as video games and interactive visualisations. This is particularly true for RTAO, which gets more expensive as the number of objects in a scene increases and have additional costs from managing the ray tracing acceleration structures. The ambient occlusion methods are evaluated in terms of image quality and performance. Image quality is assessed using structural similarity index (SSIM) and through visual inspection. The performance is assessed by measuring computation time, in milliseconds. This thesis shows that the image quality of RTAOis a substantial improvement over GTAO, being close to offline rendering quality. The primary visual issue with RTAO is visible noise - especially noticeable around the contours of moving objects. Nevertheless, GTAO is very competitive due to its performance, the computation time for all GTAO tests were below one ms per frame. At 1080p full-resolution GTAO was computed in 0.3883 ms on a RTX 3070 GPU. In contrast, the computation time of RTAO at 1080p and two samples per pixels were 2.253 ms. The cost of updating and rebuilding ray tracing acceleration structures were also noteworthy. Overall, the results indicate that hardware accelerated ray tracing can be used for significant improvements in image quality but adoption of this technique is not trivial due to performance concerns. / Med hårdvaruaccelererad strålspårning på grafikkort som introducerades nyligen möjliggjordes flera strålspårningsbaserade tekniker för rendering i realtid. Detta examensarbete undersöker en sådan teknik - strålspårad ambient ocklusion (engelska: ray traced ambient occlusion (RTAO)). RTAO undersöks och utvärderas för användning i realtidsapplikationer genom en jämförelse med en ambient-ocklusionsmetod som beräknas i bildrummet (screen space ambient occlusion (SSAO)) kallad ground-truth ambient occlusion (GTAO). Detta examensarbete bidrar genom att utvärdera metoderna i testscenarion som inkluderar animerade objekt. Detta medför ett antal fördelar: utvärderingen kan betona visuella artefakter som kan uppstå när objekt rör sig och animeras. Vidare gör det att prestandatesterna kan inkludera kostnader som tillkommer när scener innehåller animerade objekt - detta är särskilt betydande för RTAO som blir dyrare att beräkna när antalet objekt stiger samt har ytterligare kostnader för att uppdatera datastrukturer som används för att accelerera strålspårningen. På så vis närmar sig testscenarion en bred kategori av applikationer som använder rendering i realtid, exempelvis spel och interaktiva visualiseringar. Utvärderingen sker på uppnådd bildkvalité samt metodernas prestanda. Bildkvalitén utvärderas genom structural similarity index (SSIM) samt visuellinspektion. Prestandan utvärderas genom att mäta beräkningstiden i millisekunder. Resultaten visar att RTAOs bildkvalité är tydligt överlägsen GTAO och närmar sig de resultat som uppnås genom förrendering. Det primära problemet med RTAOs bildkvalité är förekomsten av visuellt brus. Detta är extra tydligt runt konturerna på de objekt som är animerade och förflyttar sig. Hursomhelst är GTAO attraktivt då denna metod kan beräknas betydligt snabbare än RTAO. Samtliga GTAOs prestandatester visade på beräkningstider under en millisekund. Vid en upplösning på 1080p med två prov per pixel (samples per pixel) var beräkningstiden för RTAO 2,253 ms. Kostnaden för att uppdatera datastrukturerna för strålspårningen visade sig också vara betydlig i många tester. Sammantaget indikerar resultaten att hårdvaruaccelererad strålspårning kan resultera i en signifikant förbättring av bildkvalité men att det kan innebära en kostnad som kräver betänklighet.