Hardware Accelerators for Animated Ray Tracing

Future graphics processors are likely to incorporate hardware accelerators for real-time ray tracing, in order to render increasingly complex lighting effects in interactive applications. However, ray tracing poses difficulties when drawing scenes with dynamic content, such as animated characters and objects. In dynamic scenes, the spatial datastructures used to accelerate ray tracing are invalidated on each animation frame, and need to be rapidly updated. Tree update is a complex subtask in its own right, and becomes highly expensive in complex scenes. Both ray tracing and tree update are highly memory-intensive tasks, and rendering systems are increasingly bandwidth-limited, so research on accelerator hardware has focused on architectural techniques to optimize away off-chip memory traffic. Dynamic scene support is further complicated by the recent introduction of compressed trees, which use low-precision numbers for storage and computation. Such compression reduces both the arithmetic and memory bandwidth cost of ray tracing, but adds to the complexity of tree update.This thesis proposes methods to cope with dynamic scenes in hardware-accelerated ray tracing, with focus on reducing traffic to external memory. Firstly, a hardware architecture is designed for linear bounding volume hierarchy construction, an algorithm which is a basic building block in most state-of-the-art software tree builders. The algorithm is rearranged into a streaming form which reduces traffic to one-third of software implementations of the same algorithm. Secondly, an algorithm is proposed for compressing bounding volume hierarchies in a streaming manner as they are output from a hardware builder, instead of performing compression as a postprocessing pass. As a result, with the proposed method, compression reduces the overall cost of tree update rather than increasing it. The last main contribution of this thesis is an evaluation of shallow bounding volume hierarchies, common in software ray tracing, for use in hardware pipelines. These are found to be more energy-efficient than binary hierarchies. The results in this thesis both confirm that dynamic scene support may become a bottleneck in real time ray tracing, and add to the state of the art on tree update in terms of energy-efficiency, as well as the complexity of scenes that can be handled in real time on resource-constrained platforms

Fast, effective BVH updates for dynamic ray-traced scenes using tree rotations

technical reportBounding volume hierarchies are a popular choice for ray tracing animated scenes due to the relative simplicity of refitting bounding volumes around moving geometry. However, the quality of such a refitted tree can degrade rapidly if objects in the scene deform or rearrange significantly as the animation progresses, resulting in dramatic increases in rendering times. Existing solutions involve occasional or heuristically triggered rebuilds of the BVH to reduce this effect. In this work, we describe how to efficiently extend refitting with local restructuring operations called tree rotations which can mitigate the effects that moving primitives have on BVH quality by rearranging nodes in the tree during each refit rather than triggering a full rebuild. The result is a fast, lightweight, incremental update algorithm that requires negligible memory, has minor update times and parallelizes easily, yet avoids significant degradation in tree quality or the need for rebuilding while maintaining fast rendering times. We show that our method approaches or exceeds the frame rates of other techniques and is consistently among the best options regardless of the animation scene

Faster data structures and graphics hardware techniques for high performance rendering

Computer generated imagery is used in a wide range of disciplines, each with different requirements. As an example, real-time applications such as computer games have completely different restrictions and demands than offline rendering of feature films. A game has to render quickly using only limited resources, yet present visually adequate images. Film and visual effects rendering may not have strict time requirements but are still required to render efficiently utilizing huge render systems with hundreds or even thousands of CPU cores. In real-time rendering, with limited time and hardware resources, it is always important to produce as high rendering quality as possible given the constraints available. The first paper in this thesis presents an analytical hardware model together with a feed-back system that guarantees the highest level of image quality subject to a limited time budget. As graphics processing units grow more powerful, power consumption becomes a critical issue. Smaller handheld devices have only a limited source of energy, their battery, and both small devices and high-end hardware are required to minimize energy consumption not to overheat. The second paper presents experiments and analysis which consider power usage across a range of real-time rendering algorithms and shadow algorithms executed on high-end, integrated and handheld hardware. Computing accurate reflections and refractions effects has long been considered available only in offline rendering where time isn’t a constraint. The third paper presents a hybrid approach, utilizing the speed of real-time rendering algorithms and hardware with the quality of offline methods to render high quality reflections and refractions in real-time. The fourth and fifth paper present improvements in construction time and quality of Bounding Volume Hierarchies (BVH). Building BVHs faster reduces rendering time in offline rendering and brings ray tracing a step closer towards a feasible real-time approach. Bonsai, presented in the fourth paper, constructs BVHs on CPUs faster than contemporary competing algorithms and produces BVHs of a very high quality. Following Bonsai, the fifth paper presents an algorithm that refines BVH construction by allowing triangles to be split. Although splitting triangles increases construction time, it generally allows for higher quality BVHs. The fifth paper introduces a triangle splitting BVH construction approach that builds BVHs with quality on a par with an earlier high quality splitting algorithm. However, the method presented in paper five is several times faster in construction time

ArborX: A Performance Portable Geometric Search Library

Searching for geometric objects that are close in space is a fundamental component of many applications. The performance of search algorithms comes to the forefront as the size of a problem increases both in terms of total object count as well as in the total number of search queries performed. Scientific applications requiring modern leadership-class supercomputers also pose an additional requirement of performance portability, i.e. being able to efficiently utilize a variety of hardware architectures. In this paper, we introduce a new open-source C++ search library, ArborX, which we have designed for modern supercomputing architectures. We examine scalable search algorithms with a focus on performance, including a highly efficient parallel bounding volume hierarchy implementation, and propose a flexible interface making it easy to integrate with existing applications. We demonstrate the performance portability of ArborX on multi-core CPUs and GPUs, and compare it to the state-of-the-art libraries such as Boost.Geometry.Index and nanoflann

Algorithmic patterns for H\mathcal{H}-matrices on many-core processors

In this work, we consider the reformulation of hierarchical ($\mathcal{H}$) matrix algorithms for many-core processors with a model implementation on graphics processing units (GPUs). $\mathcal{H}$ matrices approximate specific dense matrices, e.g., from discretized integral equations or kernel ridge regression, leading to log-linear time complexity in dense matrix-vector products. The parallelization of $\mathcal{H}$ matrix operations on many-core processors is difficult due to the complex nature of the underlying algorithms. While previous algorithmic advances for many-core hardware focused on accelerating existing $\mathcal{H}$ matrix CPU implementations by many-core processors, we here aim at totally relying on that processor type. As main contribution, we introduce the necessary parallel algorithmic patterns allowing to map the full $\mathcal{H}$ matrix construction and the fast matrix-vector product to many-core hardware. Here, crucial ingredients are space filling curves, parallel tree traversal and batching of linear algebra operations. The resulting model GPU implementation hmglib is the, to the best of the authors knowledge, first entirely GPU-based Open Source $\mathcal{H}$ matrix library of this kind. We conclude this work by an in-depth performance analysis and a comparative performance study against a standard $\mathcal{H}$ matrix library, highlighting profound speedups of our many-core parallel approach

A sparse octree gravitational N-body code that runs entirely on the GPU processor

We present parallel algorithms for constructing and traversing sparse octrees on graphics processing units (GPUs). The algorithms are based on parallel-scan and sort methods. To test the performance and feasibility, we implemented them in CUDA in the form of a gravitational tree-code which completely runs on the GPU.(The code is publicly available at: http://castle.strw.leidenuniv.nl/software.html) The tree construction and traverse algorithms are portable to many-core devices which have support for CUDA or OpenCL programming languages. The gravitational tree-code outperforms tuned CPU code during the tree-construction and shows a performance improvement of more than a factor 20 overall, resulting in a processing rate of more than 2.8 million particles per second.Comment: Accepted version. Published in Journal of Computational Physics. 35 pages, 12 figures, single colum

Visibility-Based Optimizations for Image Synthesis

Katedra počítačové grafiky a interakce

Ray tracing of dynamic scenes

In the last decade ray tracing performance reached interactive frame rates for nontrivial scenes, which roused the desire to also ray trace dynamic scenes. Changing the geometry of a scene, however, invalidates the precomputed auxiliary data-structures needed to accelerate ray tracing. In this thesis we review and discuss several approaches to deal with the challenge of ray tracing dynamic scenes. In particular we present the motion decomposition approach that avoids the invalidation of acceleration structures due to changing geometry. To this end, the animated scene is analyzed in a preprocessing step to split it into coherently moving parts. Because the relative movement of the primitives within each part is small it can be handled by special, pre-built kd-trees. Motion decomposition enables ray tracing of predefined animations and skinned meshed at interactive frame rates. Our second main contribution is the streamed binning approach. It approximates the evaluation of the cost function that governs the construction of optimized kd-trees and BVHs. As a result, construction speed especially for BVHs can be increased by one order of magnitude while still maintaining their high quality for ray tracing.Im letzten Jahrzehnt wurden interaktive Bildwiederholraten bei dem Raytracen von nicht trivialen Szenen erreicht. Dies hat den Wunsch geweckt, auch sich verändernde Szenen mit Raytracing darstellen zu können. Allerdings werden die vorberechneten Datenstrukturen, welche für die Beschleunigung von Raytracing gebraucht werden, durch Veränderungen an der Geometrie einer Szene unbrauchbar gemacht. In dieser Dissertation untersuchen und diskutieren wir mehrere Lösungsansätze für das Problem der Darstellung von sich verändernden Szenen mittels Raytracings. Insbesondere stellen wir den Motion Decomposition Ansatz vor, welcher die bisher nötige Neuberechnung der Beschleunigungsdatenstrukturen aufgrund von Geometrieänderungen zu einem großen Teil vermeidet. Dazu wird in einem Vorberechnungsschritt die animierte Szene untersucht und diese in sich ähnlich bewegende Teile zerlegt. Da dadurch die relative Bewegung der Primitiven der Teilszenen zueinander sehr klein ist kann sie durch spezielle, vorberechnete kd-Bäume toleriert werden. Motion Decomposition ermöglicht das Raytracen von vordefinierte Animationen und Skinned Meshes mit interaktiven Bildwiederholraten. Unser zweiten Hauptbeitrag ist der Streamed Binning Ansatz. Dabei wird die Kostenfunktion, welche die Konstruktion von für Raytracing optimierten kd-Bäumen und BVHs steuert, näherungsweise ausgewertet, wobei deren Qualität kaum beeinträchtigt wird. Im Ergebnis wird insbesondere die Zeit für den Aufbau von BVHs um eine Größenordnung reduziert