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Figure 1: Images produced by renderers which use the open source Embree ray tracing kernels. These scenes are computationally challenging due to complex geometry and spatially incoherent secondary rays. From left to right: The White Room model by Jay Hardy rendered in Autodesk RapidRT, a car model rendered in the Embree path tracer, a scene from the DreamWorks Animation movie "Peabody & Sherman" rendered with a prototype path tracer, and the Imperial Crown of Austria model by Martin Lubich rendered in the Embree path tracer. AbstractWe 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 stateof-the-art methods on CPUs and GPUs.

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.

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Figure 1: Example subdivision surface scenes rendered with diffuse path tracing (up to 8 bounces, 7 − 12 secondary rays/primary ray).Right: the Courtyard scene (66K patches after feature-adaptive subdivision) is adaptively-tessellated into 1.4M triangles from scratch per frame, and ray traced with over 90M rays per second (including shading) on a high-end Intel R Xeon R processor system using our efficient lazy-build caching scheme. Left: four Barbarians embedded in the Sponza Atrium scene (426K patches) and adaptively-tessellated into 11M triangles are ray traced with 40M rays per second. A 60MB lazy-build cache allows for rendering this scene with over 91% of the performance of an unbounded memory cache. Compared to ray tracing a pre-tessellated version, the memory consumption is reduced by 6 − 7×.

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