Ray Coherence Research Articles

NANOSCALE CHARACTERIZATION OF CR, CU, AL AND NI METALLIC MAGNETRON NANOFILM MULTILAYERS ON SITALL

Results of nanoscale study (by atomic force microscopy and X−ray diffraction) of single−, two− and three−layered Cr, Cu, Al and Ni metallic nanofilms formed on a ceramic sital substrate on MVU TM−Magna T magnetron equipment (NIITM, Zelenograd) have been reported. The growth rates and the structure of the nanofilms were determined while varying of power/current ratio from 200/0.7 to 800/2 Wt/A and magnetron sputtering time from 30 to 360s at an operating pressure of 0.5 Pa Ar. The criterion for optimization quality based on the minimum roughness was as follows: Ra = min{Rai} and/or Rq → min{Rqi} (i is the number of varies modes used). The mean roughness Ra and RRMS = Rq have been determined from the scan of the vertical profile (resolution 20 pm) of the atomic force microscopic image. We found that the nanofilm–forming nanocluster structure size for the modes when Ra and Rq were the smallest had a close–to–Gaussian grain size distribution. The film growth rates have been determined based on the atomic force images of the nanofilm structure in the form of either a single step or steps obtained at different time intervals. The mode and parameters of magnetron sputtering and the composition of the Cr, Cu, Al and Ni targets affect the size of clusters which form the surface of the metallic nanofilms. X−ray phase and structural analyses have been carried out in order to determine the texture and the change in the distances between the lattice planes. The correctness of the optimization criterion correlating the nanolayer deposition parameters and their quality has been corroborated by the coincidence of the magnetron sputtering modes which provided for the lowest roughness and the smallest average size of the X−ray coherence region as using the Debye− Scherrer equation.

A rendering framework for multiscale views of 3D models

Images that seamlessly combine views at different levels of detail are appealing. However, creating such multiscale images is not a trivial task, and most such illustrations are handcrafted by skilled artists. This paper presents a framework for direct multiscale rendering of geometric and volumetric models. The basis of our approach is a set of non-linearly bent camera rays that smoothly cast through multiple scales. We show that by properly setting up a sequence of conventional pinhole cameras to capture features of interest at different scales, along with image masks specifying the regions of interest for each scale on the projection plane, our rendering framework can generate non-linear sampling rays that smoothly project objects in a scene at multiple levels of detail onto a single image. We address two important issues with non-linear camera projection. First, our streamline-based ray generation algorithm avoids undesired camera ray intersections, which often result in unexpected images. Second, in order to maintain camera ray coherence and preserve aesthetic quality, we create an interpolated 3D field that defines the contribution of each pinhole camera for determining ray orientations. The resulting multiscale camera has three main applications: (1) presenting hierarchical structure in a compact and continuous manner, (2) achieving focus+context visualization, and (3) creating fascinating and artistic images.

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.

Review: Kd‐tree Traversal Algorithms for Ray Tracing

Abstract In this paper we review the traversal algorithms for kd‐trees for ray tracing. Ordinary traversal algorithms such as sequential, recursive, and those with neighbour‐links have different limitations, which led to several new developments within the last decade. We describe algorithms exploiting ray coherence and algorithms designed with specific hardware architecture limitations such as memory latency and consumption in mind. We also discuss the robustness of traversal algorithms as one issue that has been neglected in previous research.

Cache-oblivious ray reordering

We present a cache-oblivious ray reordering method for ray tracing. Many global illumination methods such as path tracing and photon mapping use ray tracing and generate lots of rays to simulate various realistic visual effects. However, these rays tend to be very incoherent and show lower cache utilizations during ray tracing of models. In order to address this problem and improve the ray coherence, we propose a novel Hit Point Heuristic (HPH) to compute a coherent ordering of rays. The HPH uses the hit points between rays and the scene as a ray reordering measure. We reorder rays by using a space-filling curve based on their hit points. Since a hit point of a ray is available only after performing the ray intersection test with the scene, we compute an approximate hit point for the ray by performing an intersection test between the ray and simplified representations of the original models. Our method is a highly modular approach, since our reordering method is decoupled from other components of common ray tracing systems. We apply our method to photon mapping and path tracing and achieve more than an order of magnitude performance improvement for massive models that cannot fit into main memory, compared to rendering without reordering rays. Also, our method shows a performance improvement even for ray tracing small models that can fit into main memory. This performance improvement for small and massive models is caused by reducing cache misses occurring between different memory levels including the L1/L2 caches, main memory, and disk. This result demonstrates the cache-oblivious nature of our method, which works for various kinds of cache parameters. Because of the cache-obliviousness and the high modularity, our method can be widely applied to many existing ray tracing systems and show performance improvements with various models and machines that have different cache parameters.

R-LODs: fast LOD-based ray tracing of massive models

We present a novel LOD (level-of-detail) algorithm to accelerate ray tracing of massive models. Our approach computes drastic simplifications of the model and the LODs are well integrated with the kd-tree data structure. We introduce a simple and efficient LOD metric to bound the error for primary and secondary rays. The LOD representation has small runtime overhead and our algorithm can be combined with ray coherence techniques and cache-coherent layouts to improve the performance. In practice, the use of LODs can alleviate aliasing artifacts and improve memory coherence. We implement our algorithm on both 32-bit and 64-bit machines and are able to achieve up to 2–20 times improvement in frame rate of rendering models consisting of tens or hundreds of millions of triangles with little loss in image quality.

Algorithm optimizations and mapping scheme for interactive ray tracing on a reconfigurable architecture

This paper presents a mapping scheme of an optimized octree-based ray tracing algorithm and its implementation on a SIMD reconfigurable architecture, MorphoSys, with appropriate hardware incorporated. A two-level SIMD mapping scheme for ray tracing is chosen to get better trade-off between coherence exploitation efficiency and bandwidth requirements. We apply a SIMD octree traversal algorithm that supports ray traversals of any origins and directions. Moreover, we have applied the bottom-up traversal order for shadow and reflection rays to avoid unnecessary testing. The memory overhead of the parallel execution of ray tracing in SIMD systems is analyzed to direct memory optimization. Pre-fetching is utilized to hide data fetch latency behind the computation. A Spatial Partitioning Tree buffer reduces the latency due to the interleaved accesses to the shared memory. It also dynamically exploits ray coherence to save memory bandwidth. A Pointer Update Unit and a Pointer Buffer are combined to remove the overhead resulted from pointer-calculations and stack pushes during the parallel depth-first-traversal process. The associated hardware cost is less than 2% of the whole system. In order to include diffuse effects into the output, we apply spherical harmonic. Post-synthesis simulation shows that the target chip is estimated to be 33 mm 2 and consumes less than 1 W in the worst case. Cycle-accurate simulation demonstrates that interactive ray tracing for medium-sized scenes is achieved on MorphoSys.

Image parallel ray tracing using static load balancing and data prefetching

We propose an image parallel ray tracing with a static load balancing and data prefetching scheme on distributed memory multicomputers. We use a scattered decomposition of which the allocation unit is one pixel and each processor takes part in pixels scattered around the whole image space, which can make a load balance efficiently. In a scattered decomposition scheme, remote data accesses may be frequent since it can not utilize ray coherence sufficiently. To reduce the communication overhead due to this, we propose data prefetching by multicasting (DPM) where requested object data are also transferred to the processors dealing with the adjacent pixels if a processor requests a remote data. We implemented our approach on the Intel's Paragon and showed the linear speedup. However, the advantages of data prefetching did not materialize due to the machine's inefficient multicast operation.

A Terrain Rendering Method Using Vertical Ray Coherence

Ray casting may be applied to the terrain rendering from DTMs (digital terrain models). In this paper we propose a ray casting method which exploits exact vertical ray coherence. This method not only achieves a good performance, but also allows any set of viewing parameters. To analyse the vertical ray coherence property, we define the following terms: a ray plane which is perpendicular to the sea-level plane and passes through the COP (centre of projection), and the corresponding ray line which is the line of intersection of the ray plane and the projection plane. Vertical ray coherence can be stated as: if two rays pass through the same ray line, they pass over the identical set of points on the DTM base plane. After a ray is cast on the DTM, we can get a good starting point to cast another ray using this property. Our method tries to cover the projection plane with ray lines. Since ray lines are not parallel to each other, it is a difficult task to cover the entire plane. In fact all ray lines intersect at a point (COP′). We first generate a set of ray lines sufficient to cover the projection plane near COP′. As pixels get further from COP′, more ray lines are generated to fill most of the gaps between pixels covered by the previous set of ray lines. Rays passing through the same ray line are effectively cast using vertical ray coherence. To fill remaining pixels not covered by ray lines, pixel values of vertical/horizontal neighbours are interpolated. To cast rays efficiently, successive points generated by a line algorithm are followed along the projection of the rays on the DTM base plane. © 1997 by John Wiley & Sons, Ltd.

A real-time photo-realistic visual flythrough

In this paper we present a comprehensive flythrough system which generates photo-realistic images in true real-time. The high performance is due to an innovative rendering algorithm based on a discrete ray casting approach, accelerated by ray coherence and multiresolution traversal. The terrain as well as the 3D objects are represented by a textured mapped voxel-based model. The system is based on a pure software algorithm and is thus portable. It was first implemented on a workstation and then ported to a general-purpose parallel architecture to achieve real-time performance.

Accelerated ray tracing using an nCUBE2 multicomputer

AbstractAcceleration techniques for rendering a dynamic sequence of frames (animations) and static scenes using ray tracing are presented. The first technique discusses temporal acceleration for dynamic scenes which takes advantage of ray coherence, while the second technique discusses acceleration for complex static scenes based on parallelism. Several practical aspects of the parallel implementation on an nCUBE2 hypercube computer are included.

Ray Coherence Between a Sphere and a Convex Polyhedron

AbstractUsing the two ray coherence theorems of Ohta and Maekawa the computation time of ray tracing algorithms for scenes of spheres and convex polyhedra can be reduced considerably. This paper presents further theorems which, together with the first two, may enable further reduction in computation time.

A Particular Structure of B-doped μc-Si/a-Si:H Layers on Insulator

ABSTRACTMicrocrystalline films of thicknesses ranging from 0.3 to 1.1 μm have been deposited on oxidized silicon wafers by PECVD in a 50KHz capacitive discharge reactor at 450°C. Two series of films have been elaborated over a wide range of boron concentrations at the same H2:SiH4 ratio of 9:1. Cross section TEM micrographs showed the films to consist of two sublayers of distinct crystalline nature, whose relative thickness depends on the preparation conditions. With a strongly <220> textured microcrystalline structure, the overlayer snowed a columnar morphology, while the amorphous underlayer reached thicknesses of 350nm. Two additional striking features were observed by TEM: - The grains took two symmetrical orientations relative to the preferential [220] growth axis;- The interface between the amorphous and crystalline regions had a sawtooth pattern with a period around 300nm. In contrast to these microstructural results, the B-profile as measured by SIMS was found to be fiat accross the whole thickness. These local measurements are compared to the results of grazing X ray diffraction and Raman measurements. We observe and discuss a discrepancy between the X ray coherence length and the dimensions of the columns as observed by TEM. While optimized conditions lead to a vanishing amorphous sublayer, the more original features described above are tentatively interpreted taking into account the high compressive strains in the layers deposited at low plasma frequencies.

Improved techniques for ray tracing parametric surfaces

Several techniques for acceleration of ray tracing parametric surfaces are presented. Some of these are entirely new to ray tracing, while others are improvements of previously known techniques. First a uniform spatial subdivision scheme is adapted to parametric surfaces. A new space- and time-efficient algorithm for finding raysurface intersections is introduced. It combines numerical and subdivision techniques, thus allowing utilization of ray coherence and greatly reducing the average ray-surface intersection time. Non-scanline sampling orders of the image plane are proposed that facilitate utilization of coherence. Finally, a method to handle reflected, refracted, and shadow rays in a more efficient manner is described. Results of timing tests indicating the efficiency of these techniques for various environments are presented.

Realistic computer graphics and free form surfaces

An image generation approach in computer graphics leading to images of considerable optical quality is ray tracing, deduced from geometric optics. Crucial for the efficiency of ray tracing is to find quickly an intersection point closest to a ray's origin. This requires to restrict the candidate patches of a given scene as well as to find the intersections of a ray with a patch. We survey approaches to solve these two problems, and present a new method, based on space sweep and patch subdivision. Its advantages are subdivision adapted to the distribution of rays and consideration of ray coherence. This implies that useless subdivisions are avoided.