Use Of Look-up Tables Research Articles

Evaluation of errors from neglecting polarization in the forward modeling of O 2A band measurements from space, with relevance to CO 2 column retrieval from polarization-sensitive instruments

Sensitivity studies have been performed to evaluate the errors resulting from ignoring polarization in analyzing spectroscopic measurements of the O 2 A band from space, using the Orbiting Carbon Observatory (OCO) as a test case. An 11-layer atmosphere, with both gas and aerosol loading, and bounded from below by a lambertian reflecting surface, was used for the study. The numerical computations were performed with a plane-parallel vectorized discrete ordinate radiative transfer code. Beam and viewing geometry, surface reflectance and aerosol loading were varied one at a time to evaluate and understand the individual errors. Different behavior was observed in the line cores and the continuum because of the different paths taken by the photons in the two cases. The errors were largest when the solar zenith angle was high, and the aerosol loading and surface reflectance low. To understand the effect of neglecting polarization on CO 2 column retrievals, a linear error analysis study was performed on simulated measurements from the OCO spectral regions, viz. the 1.61 and 2.06 μm CO 2 bands and the O 2 A band. It was seen that neglecting polarization could introduce errors as high as 10 ppm, which is substantially larger than the required retrieval precision of ∼2 ppm. A variety of approaches, including orders of scattering, spectral binning and the use of lookup tables are being explored to reduce the errors.

Improved memoryless RNS forward converter based on the periodicity of residues

The residue number system (RNS) is suitable for DSP architectures because of its ability to perform fast carry-free arithmetic. However, this advantage is over-shadowed by the complexity involved in the conversion of numbers between binary and RNS representations. Although the reverse conversion (RNS to binary) is more complex, the forward transformation is not simple either. Most forward converters make use of look-up tables (memory). Recently, a memoryless forward converter architecture for arbitrary moduli sets was proposed by Premkumar in 2002. In this paper, we present an extension to that architecture which results in 44% less hardware for parallel conversion and achieves 43% improvement in speed for serial conversions. It makes use of the periodicity properties of residues obtained using modular exponentiation.

Lookup Tables for Predicting CHF and Film-Boiling Heat Transfer: Past, Present, and Future

Lookup tables (LUTs) have been used widely for the prediction of critical heat flux (CHF) and film-boiling heat transfer for water-cooled tubes. LUTs are basically normalized data banks. They eliminate the need to choose between the many different CHF and film-boiling heat transfer prediction methods available.The LUTs have many advantages; e.g., (a) they are simple to use, (b) there is no iteration required, (c) they have a wide range of applications, (d) they may be applied to nonaqueous fluids using fluid-to-fluid modeling relationships, and (e) they are based on a very large database. Concerns associated with the use of LUTs include (a) there are fluctuations in the value of the CHF or film-boiling heat transfer coefficient (HTC) with pressure, mass flux, and quality, (b) there are large variations in the CHF or the film-boiling HTC between the adjacent table entries, and (c) there is a lack or scarcity of data at certain flow conditions.Work on the LUTs is continuing. This will resolve the aforementioned concerns and improve the LUT prediction capability. This work concentrates on better smoothing of the LUT entries, increasing the database, and improving models at conditions where data are sparse or absent.

Templatized refinement of triangle meshes using surface interpolation

AbstractMesh refinement is an important process with regards to achieving good accuracy for computational simulation and analysis. Currently, there is a lack of a high‐fidelity refinement algorithm for the accurate modelling of geometry in the absence of a physical geometric model. This paper focuses on using a surface interpolation procedure based on a quartic triangular Bezier patch to approximate the underlying geometry of a mesh and to determine the locations of new subdivision vertices. A robust methodology is used for feature retention and accurate curve fitting at sharp edges and hard vertices. This extends the applicability of the surface fitting procedure to any arbitrary geometric configuration. The refinement is based on a new 1:9 subdivision scheme and its implementation is discussed in detail. Despite its high order subdivision footprint, computational efficiency is made possible by the effective use of lookup tables. Copyright © 2005 John Wiley & Sons, Ltd.

Moderated histogram equalization, an automatic means of enhancing the contrast in digital light micrographs reversibly.

A means for improving the contrast in the images produced from digital light micrographs is described that requires no intervention by the experimenter: zero-order, scaling, tonally independent, moderated histogram equalization. It is based upon histogram equalization, which often results in digital light micrographs that contain regions that appear to be saturated, negatively biased or very grainy. Here a non-decreasing monotonic function is introduced into the process, which moderates the changes in contrast that are generated. This method is highly effective for all three of the main types of contrast found in digital light micrography: bright objects viewed against a dark background, e.g. fluorescence and dark-ground or dark-field image data sets; bright and dark objects sets against a grey background, e.g. image data sets collected with phase or Nomarski differential interference contrast optics; and darker objects set against a light background, e.g. views of absorbing specimens. Moreover, it is demonstrated that there is a single fixed moderating function, whose actions are independent of the number of elements of image data, which works well with all types of digital light micrographs, including multimodal or multidimensional image data sets. The use of this fixed function is very robust as the appearance of the final image is not altered discernibly when it is applied repeatedly to an image data set. Consequently, moderated histogram equalization can be applied to digital light micrographs as a push-button solution, thereby eliminating biases that those undertaking the processing might have introduced during manual processing. Finally, moderated histogram equalization yields a mapping function and so, through the use of look-up tables, indexes or palettes, the information present in the original data file can be preserved while an image with the improved contrast is displayed on the monitor screen.

Binary mixtures of hard spheres: how far can one go with the virial equation of state?

We describe a technique to calculate partial virial coefficients up to seventh order in a binary mixture of hard spheres, using hit-and-miss Monte-Carlo (MC) numerical integration. The algorithm makes use of look-up tables of all the blocks contributing to each partial virial coefficient. All topologically equivalent graphs are listed in this table so as to improve the statistical efficiency of the calculation. For the case of additive hard spheres, we report the partial contributions to the sixth and seventh virial coefficients, for size ratios ranging from 0.1 to 0.9. For the non-additive mixture we truncated the expansion at sixth order and only considered one set of potential parameters: size ratio 0.1 and non-additivity factor +0.1. In line with previous work, our results indicate that for additive spheres with a size ratio in the region of 0.1, there would appear to be a liquid–liquid de-mixing transition but at an overall packing fraction that would imply that this is meta-stable with respect to a fluid–solid transition. A positive non-additivity serves to increase the tendency for liquid–liquid de-mixing, but appears to have an adverse effect on the rate of convergence of the virial series.

Atmospheric corrections of single broadband channel and multidirectional airborne thermal infrared data: Application to the ReSeDA experiment

This study focused on atmospheric corrections of airborne thermal infrared remote sensing data acquired with a multidirectional and single broadband channel sensor during the ReSeDA experiment. For single channel sensors, atmospheric corrections are generally performed using atmospheric radiative transfer models such as MODTRAN 3.5 along with radiosoundings. A sensitivity study was performed using MODTRAN 3.5 simulations to assess the accuracy of the processing regarding the experimental context. It was shown that the local topography and the atmosphere spatial variability could affect significantly the radiosounding representativeness, whereas the fluctuations of the flight altitude around the nominal value induced non-negligible inaccuracies. Moreover, using a broadband sensor induced non-linear effects that required a second order correction and also the use of look-up tables to reduce computation time. A theoretical error was next proposed to account for both the sensor accuracy and the experimental uncertainties considered when performing the sensitivity study. This theoretical error was about 1°C, and agreed well with the results obtained when validating against field measurements.

Efficient Systolic Array Mapping of FIR Filters Used in PAM-QAM Modulators

This paper presents efficient techniques for mapping FIR filter computation circuits for PAM and QAM modulators onto systolic arrays. The exploitation of the inherent symmetry of these problem instances and the use of Look-Up Tables (L.U.T.) in conjunction with the use of systolic architectures, increases the performance while keeping the VLSI area minimal. Exploiting parallelism and pipelining enhances the throughput and results in linear expandability of the FIR filter with respect to the bit accuracy and to the stage count.

Monte-Carlo integration for virial coefficients re-visited: hard convex bodies, spheres with a square-well potential and mixtures of hard spheres

Techniques to adapt the hit-and-miss Monte-Carlo numerical integration are proposed with the aim to determine virial coefficients up to eighth order in fluids of hard convex bodies, hard spheres with an attractive square-well potential and a two-component mixture of hard spheres. These algorithms make use of look-up tables of all the blocks contributing to the coefficients. Each type of block is represented in the tables by several entries. These correspond to all possible topologically equivalent graphs that can be generated by the Monte-Carlo process. This rendered the Monte-Carlo method statistically more efficient. In the case of a two-component system the look-up tables had to have representations of blocks having two sorts of vertices. The reported data are: improved values of the seventh and eighth virial coefficients for hard spheres, the sixth, seventh and eighth coefficients of spheroids, spherocylinders and cutspheres, fifth virial coefficient of spheres with a square-well potential of relative range 1.25; 1.5; 1.75 and 2.0 and the partial contributions of the sixth virial coefficient for a mixture of hard spheres with the size ratio 0.1.

Channel-matched hierarchical table-lookup vector quantization

In this correspondence, we propose a channel-matched hierarchical table-lookup vector quantizer for a discrete memoryless channel. We show that this scheme simultaneously provides low encoding complexity and robustness against transmission noise. Additionally, we propose the use of lookup tables for transcoding in heterogeneous networks. We show, through an example motivated by an application in multicasting, that the proposed table-lookup transcoding approach is an elegant alternative to the expensive and delay-introducing method of transcoding achieved by decoding and re-encoding.

Look-up table techniques for adaptive digital predistortion: a development and comparison

Simple techniques are presented for performing digital predistortion by use of look-up tables. Performance is measured by the traditional spectral regrowth analysis which quantifies the out-of-band distortion. Also included in the performance evaluation is constellation distortion which quantifies the in-band distortion which is important for bit error rate (BER) performance. As an added performance measure, predistortion is also analyzed with respect to its effect on amplifier efficiency. A nonuniform spacing technique is described which is simple to implement and has a performance very close to the cited optimum approach. In addition, scaling the gain of these tables provides further improvement in terms of correction and efficiency performance. Finally, adaptive indexing is introduced. These algorithms allow the various techniques to adapt to changing signal and power amplifier characteristics.

Comments on "An arithmetic free parallel mixed-radix conversion algorithm"

For original paper see D.F. Miller and W.S. McCormick, ibid., vol.45, pp.158-62 (Jan. 1998). In the above paper, two algorithms based on look-up tables for mixed-radix conversion are presented. Here, we show that one of the algorithms had been previously published in 1978, and we also take this opportunity to speak of the use of look-up tables for the residue number system with present and future technologies.

Correction for pressure dependence of the refractive index in measurements of lubricant film thickness with image analysis

The lubricant film thickness between contacting bodies may be determined from interferograms by image analysis. Look-up tables employed in the analysis are obtained from calibration interferograms of contacts of bodies of known shape. Calibration and measurement interferograms are, however, obtained under different pressure conditions and, since the refractive index of lubricants is pressure dependent, the use of look-up tables can introduce significant errors (in the range of 10 per cent). In this paper it is shown that a straightforward correction for this error is available. In an iterative procedure the uncorrected film thickness is first determined through looking up tables. This allows the pressure distribution over the contact to be calculated and a corrected refractive index may then be obtained. The corrected refractive index in turn allows a corrected film thickness to be calculated. This procedure can be followed iteratively. After two iterations the corrections have been found to be of the same order as the resolution yielded by the method, as has been implemented.

An integrated vegetation mapping approach for northern Alaska (1:4 M scale)

A six-step integrated vegetation mapping approach is described for making a small-scale (1:4 million) map of northern Alaska. The method uses two primary maps: (1) a Phytogeographic subzones and Floristic subprovinces Map (PFM) adjusted to Advanced Very High Resolution Radiometer false colour infrared (AVHRR CIR) imagery, and (2) an Integrated Vegetation-Complex Map (IVCM). The IVCM map-polygon boundaries are guided by information from a variety of remote-sensing data (AVHRR imagery, maximum greenness maps and classified images) and hard-copy source maps (surficial geology, bedrock geology, soils, percentage water cover). The map-polygon boundaries are integrated so that they conform to terrain features that are interpretable on the AVHRR CIR. The PFM and IVCM are overlaid in a geographic information system (GIS), and a series of derived maps is created through the use of look-up tables. Northern Alaska is a prototype area for the Circumpolar Arctic Vegetation Mapping (CAVM) project, which has a goal of producing a new vegetation map of the region north of the arctic tree line by the year 2001. The method could be modified and adapted to any region of the Arctic based on locally available information.

Efficient implementation of error-feedback LSL algorithm

An error feedback LSL algorithm is presented which has a reduced computational complexity and makes use of both a priori and a posteriori prediction errors. A simple numerical convention is proposed to assure numerical stability for poor persistent excitation. Simulation results illustrate that divisions can be implemented with reduced wordlength without leading to a significant decrease in the overall accuracy even in floating-point implementations. This fact allows the use of lookup tables for implementing divisions. Moreover, the proposed algorithm has an inherent parallelism that can be advantageously exploited for fast implementations.

An Efficient Algorithm for the Detection of Infrequent Rapid Bursts in Time Series Data

Searching through data for infrequent rapid bursts is a common requirement in many areas of scientific research. In this paper, we present a powerful and flexible analysis method that, in a single pass through the data, searches for statistically significant bursts on a set of specified short timescales. The input data are binned, if necessary, and then quantified in terms of probabilities rather than rates or ratios. Using a measure-like probability makes the method relatively count rate independent. The method has been made computationally efficient by the use of lookup tables and cyclic buffers, and it is therefore particularly well suited to real-time applications. The technique has been developed specifically for use in an X-ray astronomy application to search for millisecond bursts from black hole candidates such as Cyg X-1. We briefly review the few observations of these types of features reported in the literature, as well as the variety of ways in which their statistical reliability was challenged. The developed technique, termed the burst expectation search (BES) method, is illustrated using some data simulations and archived data obtained during ground testing of the proportional counter array (PCA) experiment detectors on the Rossi X-Ray Timing Explorer (RXTE). A potential application for a real-time BES method on board RXTE is also examined.

Studies on the effect of filtering, digitization, and computation algorithm on the ABC-DQ current transformation in PWM inverter drive system

In the automotive industry, PWM inverter controlled drive systems find extensive applications in electric vehicles and other motion control systems. A PWM inverter controlled three-phase brushless synchronous motor drive requires a microprocessor based controller to implement the control algorithms. The practical implementation of the control algorithms sometimes calls for the use of lookup tables involving discretized quantities. All these processes and the devices introduce nonidealities and errors in the drive system. Of particular importance in such a system is the ABC-DQ current transformations when vector control is implemented. In automotive applications, the emphasis is to design and realize accurate motion control systems in the most economic manner possible. Hence the effects of filtering, digitization, quantization, and digital computation algorithms and the accompanying inaccuracies introduced by them during the ABC-DQ current transformation in PWM drive system have to be dealt with in a careful manner for an optimum and economic choice of system design to be possible. Some of these aspects are discussed in this paper. The studies lead to a better understanding of the importance of various parameters of power electronics systems and in turn contribute towards the proper selection of parameters and therefore help the design process.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of the influence of O_2A-band absorption on atmospheric correction of ocean-color imagery

Two satellite-borne ocean-color sensors scheduled for launch in the mid 1990's each have a spectral band (nominally 745-785 nm) that completely encompasses the O(2) A band at 762 nm. These spectral bands are to be used in atmospheric correction of the color imagery by assessment of the aerosol contribution to the total radiance at the sensor. The effect of the O(2) band on the radiance measured at the satellite is studied with a line-by-line backward Monte Carlo radiative transfer code. As expected, if the O(2) absorption is ignored, unacceptably large errors in the atmospheric correction result. The effects of the absorption depend on the vertical profile of the aerosol. By assuming an aerosol profile-the base profile-we show that it is possible to remove most of the O(2)-absorption effects from atmospheric correction in a simple manner. We also investigate the sensitivity of the results to the details of the assumed base profile and find that, with the exception of situations in which there are significant quantities of aerosol in the stratosphere, e.g., following volcanic eruptions or in the presence of thin cirrus clouds, the quality of the atmospheric correction depends only weakly on the base profile. Situations with high concentrations of stratospheric aerosol require additional information regarding vertical structure for this spectral band to be used in atmospheric correction; however, it should be possible to infer the presence of such aerosol by a failure of the atmospheric correction to produce acceptable water-leaving radiance in the red. An important feature of our method for removal of the O(2)-absorption effects is that it permits the use of lookup tables that can be prepared in the absence of O(2) absorption by the use of more efficient radiative transfer codes.

A new safe-point thinning algorithm based on the mid-crack code tracing

A new thinning algorithm for binary images, based on the safe-point testing and mid-crack code tracing, is presented. Thinning is treated as the deletion of nonsafe border pixels from the contour to the center of the object layer-by-layer. The deletion is determined by masking a 3/spl times/3 weighted template and by the use of lookup tables. The resulting skeleton does not require cleaning or pruning. The obtained skeleton possesses single-pixel thickness and preserves the object's connectivity. The algorithm is very simple and efficient since only boundary pixels are processed at each iteration and lookup tables are used. >

A Moving Window Shannon Reconstruction Algorithm for Image Interpolation

The ubiquitous problem of interpolating digital image arrays can be reliably met by a moving window Shannon reconstruction. After a discussion of the application of Shannon's theorem within a finite sampling interval, the article discusses the moving window algorithm and describes how truncation effects are avoided. The choice of the window width in which each interpolated value is computed depends upon the type of image to be reconstructed and upon the computational resources. To achieve maximum efficiency, the implementation of the algorithm can make use of look-up tables of transcendental coefficients; jitter errors introduced by the use of such tables are briefly discussed. The paper presents a comparative series of results obtained with various interpolating algorithms, both on test functions and on real images. The behavior of the moving window reconstruction is outstanding in all cases, provided that a good interpolating kernel and a proper window are adopted. A comparison is also made with the results obtained by a Shannon reconstruction based upon the entire sampling interval.