Sven Woop

Embree: a kernel framework for efficient CPU ray tracing

We describe Embree, an open source ray tracing framework for x86 CPUs. Embree is explicitly designed to achieve high performance in professional rendering environments in which complex geometry and incoherent ray distributions are common. Embree consists of a set of low-level kernels that maximize utilization of modern CPU architectures, and an API which enables these kernels to be used in existing renderers with minimal programmer effort. In this paper, we describe the design goals and software architecture of Embree, and show that for secondary rays in particular, the performance of Embree is competitive with (and often higher than) existing state-of-the-art methods on CPUs and GPUs.

Combining Single and Packet-Ray Tracing for Arbitrary Ray Distributions on the Intel MIC Architecture

Wide-SIMD hardware is power and area efficient, but it is challenging to efficiently map ray tracing algorithms to such hardware especially when the rays are incoherent. The two most commonly used schemes are either packet tracing, or relying on a separate traversal stack for each SIMD lane. Both work great for coherent rays, but suffer when rays are incoherent: The former experiences a dramatic loss of SIMD utilization once rays diverge; the latter requires a large local storage, and generates multiple incoherent streams of memory accesses that present challenges for the memory system. In this paper, we introduce a single-ray tracing scheme for incoherent rays that uses just one traversal stack on 16-wide SIMD hardware. It uses a bounding-volume hierarchy with a branching factor of four as the acceleration structure, exploits four-wide SIMD in each box and primitive intersection test, and uses 16-wide SIMD by always performing four such node or primitive tests in parallel. We then extend this scheme to a hybrid tracing scheme that automatically adapts to varying ray coherence by starting out with a 16-wide packet scheme and switching to the new single-ray scheme as soon as rays diverge. We show that on the Intel Many Integrated Core architecture this hybrid scheme consistently, and over a wide range of scenes and ray distributions, outperforms both packet and single-ray tracing.

Embree ray tracing kernels for CPUs and the Xeon Phi architecture

Modern CPUs achieve high computational throughput by implementing increasingly wide SIMD vector units (such as 8-wide AVX or 16-wide SIMD for the Xeon Phi instructions). Achieving optimal performance on these architectures requires leveraging these wide SIMD vector units effectively. We present Embree [Ernst and Woop 2011], an open source ray tracing library developed to show performance-focused graphics programmers how to take full advantage of multiple cores and wide SIMD units in the context of ray tracing. Embree features spatial acceleration structures and traversal algorithms that are optimized for CPUs and the Intel Xeon Phi architecture. In particular, Embree supports hybrid ray packet/single ray traversal algorithms---optimized for both CPUs and Xeon Phi---that are designed to handle both coherent and incoherent workloads efficiently [Benthin et al. 2012]. While a first version of Embree originally focused only on single ray traversal on SSE- or AVX-enabled CPUs, this talk specifically covers the upcoming Embree 2.0 release that explicitly also supports the Xeon Phi architecture, adds support for packet tracing, two level hierarchies, partial scene updates, dynamic content, and virtual intersectors for user defined primitives.

Exploiting local orientation similarity for efficient ray traversal of hair and fur

Hair and fur typically consist of a large number of thin, curved, and densely packed strands which are difficult to ray trace efficiently. A tight fitting spatial data structure, such as a bounding volume hierarchy (BVH), is needed to quickly determine which hair a ray hits. However, the large number of hairs can yield a BVH with a large memory footprint (particularly when hairs are pre-tessellated), and curved or diagonal hairs cannot be tightly bounded within axis aligned bounding boxes. In this paper, we describe an approach to ray tracing hair and fur with improved efficiency, by combining parametrically defined hairs with a BVH that uses both axis-aligned and oriented bounding boxes. This BVH exploits similarity in the orientation of neighboring hairs to increase ray culling efficiency compared to purely axis-aligned BVHs. Our approach achieves about 2x the performance of ray tracing pre-tessellated hair models, while requiring significantly less memory.

Efficient ray tracing of subdivision surfaces using tessellation caching

A common way to ray trace subdivision surfaces is by constructing and traversing spatial hierarchies on top of tessellated input primitives. Unfortunately, tessellating surfaces requires a substantial amount of memory storage, and involves significant construction and memory I/O costs. In this paper, we propose a lazy-build caching scheme to efficiently handle these problems while also exploiting the capabilities of todays many-core architectures. To this end, we lazily tessellate patches only when necessary, and utilize adaptive subdivision to efficiently evaluate the underlying surface representation. The core idea of our approach is a shared lazy evaluation cache, which triggers and maintains the surface tessellation. We combine our caching scheme with SIMD-optimized subdivision primitive evaluation and fast hierarchy construction over the tessellated surface. This allows us to achieve high ray tracing performance in complex scenes, outperforming the state of the art while requiring only a fraction of the memory. In addition, our method stays within a fixed memory budget regardless of the tessellation level, which is essential for many applications such as movie production rendering. Beyond the results of this paper, we have integrated our method into Embree, an open source ray tracing framework, thus making interactive ray tracing of subdivision surfaces publicly available.

Ray Tracing with Shared-Plane Bounding Volume Hierarchies

Abstract We present a representation of an axis-aligned bounding volume hierarchy (BVH) for ray tracing that removes redundant bounding information from the data structure. The observation that two children of a binary BVH node share at least six planes with their parent node leads to two possible optimizations: (a) the size of the data structure can be optimized by storing six floats instead of 12 for the bounds of the two children, and (b) the box-intersection code can be optimized by avoiding the reintersection with shared planes. These optimizations reduce memory consumption of the hierarchy by 37.5 percent with only a marginal impact on ray-shooting performance.

Improved two-level BVHs using partial re-braiding

We propose a novel approach for improving the quality of two-level BVHs (i.e., a two-level data structure that uses a top-level BVH built over second-level object BVHs). After building an individual, high-quality BVH for each object, our new top-level BVH build approach selectively re-braids (opens and merges) object BVHs during the build process to reduce overlap and improve SAH quality. We demonstrate that compared to the two main state-of-the-art techniques---brute-force re-construction of a single, flat BVH; and building a traditional two-level BVH over objects, respectively---the proposed approach achieves build times significantly faster than the former, while simultaneously yielding traversal performance that is much higher than the latter.

STBVH: a spatial-temporal BVH for efficient multi-segment motion blur

We present the STBVH, a new approach for rendering multi-segment motion blur using a bounding volume hierarchy (BVH) that stores both spatial linearly interpolated bounds and temporal bounds. The approach is designed for different number of time steps per mesh or object. While separating the individual meshes using standard partitioning techniques, it performs temporal splits for locations with large or curved motion inside the meshes. Our approach uses a modified motion blur surface area heuristic (MBSAH) that calculates probabilities in the presence of spatial-temporal bounds and works on linear motion segments of primitives rather than on full motion curves. We show that our approach is able to handle challenging scenes with varying degrees of motion blur per mesh, using significantly less memory and having competitive rendering performance compared to building separate linear motion blur BVHs per global time segment.

Embree ray tracing kernels: overview and new features

Embree is an open source ray tracing library consisting of high-performance kernels optimized for modern CPUs with increasingly wide SIMD units. Since its original release, it has become the state-of-the-art for professional CPU-based rendering applications. In the first half of this talk, we will give a brief overview of the Embree framework and how to use it. In the second half, we will present recent improvements and features introduced since the initial publication of the system. These additions include new geometry types commonly used in production renderers (quads, subdivision surfaces, and hair), improved motion blur support, and ray streams that can be traversed more efficiently than single rays and ray packets.

Compressed-leaf bounding volume hierarchies

We propose and evaluate what we call Compressed-Leaf Bounding Volume Hierarchies (CLBVH), which strike a balance between compressed and non-compressed BVH layouts. Our CLBVH layout introduces dedicated compressed multi-leaf nodes where most effective at reducing memory use, and uses regular BVH nodes for inner nodes and small, isolated leaves. We show that when implemented within the Embree ray tracing framework, this approach achieves roughly the same memory savings as Embrees compressed BVH layout, while maintaining almost the full performance of its fastest non-compressed BVH.