Showing papers on "Centroid published in 1990"

Estimation of geometrical parameters and collimator evaluation for cone beam tomography.

Abstract: A method is presented for estimating the geometrical parameters for a cone beam detector geometry from the coordinates of the centroid of a projected point source sampled over 360°. Nonlinear expressions are derived for the coordinates of the centroids in terms of the geometrical parameters which include: the two‐dimensional coordinates of the projection of the center of rotation onto the detectorimage plane; the focal length; the distance from the focal point to the center of rotation; and the spatial coordinates of the point source itself. Experimental data were obtained using a rotating gamma camera with a symmetrically converging collimator. The Marquardt algorithm was used to estimate the parameters for this particular cone beam geometry. The method was able to estimate the geometrical parameters and evaluate the accuracy of the collimator construction.

Structure-factor probabilities for related structures

Abstract: Probability relationships between structure factors from related structures have allowed previously only for either differences in atomic scattering factors (isomorphous replacement case) or differences in atomic positions (coordinate error case). In the coordinate error case, only errors drawn from a single probability distribution have been considered, in spite of the fact that errors vary widely through models of macromolecular structures. It is shown that the probability relationships can be extended to cover more general cases. Either the atomic parameters or the reciprocal-space vectors may be chosen as the random variables to derive probability relationships. However, the relationships turn out to be very similar for either choice. The most intuitive is the expected electron-density formalism, which arises from considering the atomic parameters as random variables. In this case, the centroid of the structure-factor distribution is the Fourier transform of the expected electron-density function, which is obtained by smearing each atom over its possible positions. The centroid estimate has a phase different from, and more accurate than, that obtained from the unweighted atoms. The assumption that there is a sufficient number of independent errors allows the application of the central limit theorem. This gives a one- (centric case) or two-dimensional (non-centric) Gaussian distribution about the centroid estimate. The general probability expression reduces to those derived previously when the appropriate simplifying assumptions are made. The revised theory has implications for calculating more accurate phases and maps, optimizing molecular replacement models, refining structures, estimating coordinate errors and interpreting refined B factors.

On improving the accuracy of the Hough transform

Abstract: The subject of this paper is very high precision parameter estimation using the Hough transform. We identify various problems that adversely affect the accuracy of the Hough transform and propose a new, high accuracy method that consists of smoothing the Hough arrayH(ρ, θ) prior to finding its peak location and interpolating about this peak to find a final sub-bucket peak. We also investigate the effect of the quantizations Δρ and Δθ ofH(ρ, θ) on the final accuracy. We consider in detail the case of finding the parameters of a straight line. Using extensive simulation and a number of experiments on calibrated targets, we compare the accuracy of the method with results from the standard Hough transform method of taking the quantized peak coordinates, with results from taking the centroid about the peak, and with results from least squares fitting. The largest set of simulations cover a range of line lengths and Gaussian zero-mean noise distributions. This noise model is ideally suited to the least squares method, and yet the results from the method compare favorably. Compared to the centroid or to standard Hough estimates, the results are significantly better—for the standard Hough estimates by a factor of 3 to 10. In addition, the simulations show that as Δρ and Δθ are increased (i.e., made coarser), the sub-bucket interpolation maintains a high level of accuracy. Experiments using real images are also described, and in these the new method has errors smaller by a factor of 3 or more compared to the standard Hough estimates.

System for measuring shapes and dimensions of gaps and flushnesses on three dimensional surfaces of objects

Abstract: A system for measuring three-dimensional shapes and dimensions includes: an imaging unit which faces an object with shape changes on the surface and images by means of a TV camera the slit line formed on the object surface by projecting slit light from a slit light source at a fixed angle; and a surface feature extraction member for extracting features representing the shape changes of the object surface, based on the changes of the XYZ coordinate values of the object surface detected by triangulation based on the centroid position of the intensity distribution of the slit line image corresponding to the scanning lines from the TV camera. Based on features thus extracted, the surface shape of the object can be measured rapidly with high precision.

Focusing SAR data with time-varying Doppler centroid

Abstract: SAR data spatially sampled at the Nyquist limit can be correctly processed if the Doppler centroid is precisely known. Whenever the Doppler centroid shows rapid variations either with range or azimuth, more care is required in order to take advantage of the computational efficiency of frequency domain techniques. It is shown that such focusing techniques can still be exploited, provided that SAR raw data are previously modified and a space-varying nondimensional filter is applied to the focused image. The computational cost increases, but it is still smaller than time-space domain processing. Results obtained with simulated SIR-C/X-SAR data and SPOTlight geometries are presented. >

Centroid target tracking system utilizing parallel processing of digital data patterns

Abstract: Outputs from data bit areas of a target sensing array, above a signal throld level, are obtained by sequential scanning of groups of the data bit area aligned in columns and rows to form a digital pattern of binary data interrogated to determined the offset of the digital pattern from the centroid of the sensor array. Interrogation is performed by simultaneous counting of data bit pulses derived from separate segments of each group on opposite sides of the centroid and comparing such summations to determine the differences therebetween reflecting directional offsets from the centroid.

Shape from texture: the homogeneity hypothesis

Abstract: The authors show how a maximum likelihood estimator can be constructed for homogeneous texture. The estimator turns out to be strikingly simple. It simply enforces, iteratively, coincidence of the centroid of the observed elements with the centroid of the imaging window. Whereas the estimator is based on first moments, error bounds are obtained, the authors show, from second moments. Experiments indicate that, for real textures, orientation estimates fall well inside predicted bounds. >

Class of transforms invariant under shift, rotation, and scaling

Abstract: We introduce a new class of nonreversible transform that is simultaneously invariant under shift, rotation, and scaling. The algorithm is based on a general transformation where the kernel itself contains the function to be transformed. Thus the invariances are achieved by a kind of self-mapping. Preprocessing of the input signal, such as determination of the centroid or coordinate transformation, is not necessary. The transform can be used for multidimensional data. Fast implementation is possible by look-up tables.

Video image position determination

Abstract: An optical beam position controller in which a video camera captures an image of the beam in its video frames, and conveys those images to a processing board which calculates the centroid coordinates for the image. The image coordinates are used by motor controllers and stepper motors to position the beam in a predetermined alignment. In one embodiment, system noise, used in conjunction with Bernoulli trials, yields higher resolution centroid coordinates.

The metabolic centroid method for PET brain image analysis.

Abstract: To the Editor: The recent article' 'The Metabolic Centroid Method for PET Brain Image Analysis\" by Levy et aI. (1989) proposes another new method for positron emission tomography data analysis. What does this method actually do? Simply stated, it reduces the image to three statistics (plus the global metabolic rate) that measure the linear trends in the image, such as right-left (xm), anterior­ posterior (ym), and vertical (zm) differences. Thus, it will be useful only for detecting these sorts of changes in experimental groups such as schizo­ phrenics. For example, if schizophrenics show de­ creased frontal activity, then Ym should decrease. If they show decreased right temporal lobe activity, then Xm should decrease. On the other hand, other sorts of changes may not be detected at all. If a patient shows decreased activity in both temporal lobes, then the metabolic centroid should stay roughly the same, since the temporal lobes are on approximately opposite sides of the brain centroid. Similarly, if both frontal lobe and cerebellum were depressed, then the metabolic centroid might not change. If regions close to the centroid, such as the thalamus, were depressed, this might not be de­ tected either. How can this be overcome? Other choices of \"location\" for the voxels could be used, such as replacing the lateral coordinate x by its absolute value or distance from the plane of symmetry of the brain. This would then detect a decrease in both temporal lobes , but it would not detect a change in right-left temporal lobe differences. The message is that you find only what you are looking for. If you suspect that schizophrenics will have decreased frontal activity, together with de­ creased activity in one hemisphere relative to the other, then the metabolic centroid will be the sta­ tistic for you. If you suspect decreased activity in both temporal lobes, try the metabolic centroid with the absolute value of x. However, there are methods that will select the linear combinations or \"locations\" for you: princi­ pal components (Clark et aI., 1985), factor analysis (Volkow et aI., 1986), and the recently proposed scaled subprofile model (SSM) (Moeller et aI., 1987). They all give two or three sets of scores for each subject [called subprofile scaling factors (SSF) in SSM], which are linear combinations of the re­ gional values, and so they are roughly analogous to the three coordinates Xm' Ym, and Zm of the …

Tracking Partially Occluded Two Dimensional Shapes

Abstract: A method for tracking partially occluded two dimensional polygonal shapes undergoing unknown two dimensional translational and rotational motion has been developed based on Kalman filtering. Observation of a robotic workspace by a machine vision system presents many situations in which known objects may be occluded partially or completely by other objects, fixtures, or the robot itself. Tracking such objects using non-occluded, visible features is an important problem. The method assumes object corners, or some other feature set, can be identified to known accuracy by another technique, and that feature occlusion (absence) can also be detected or recognized. A linear, constant acceleration model is assumed for shape translational and rotation motion in which the shape centroid and angular orientation, as well as their velocities and accelerations, comprise the state. A nonlinear observation model is assumed where the corner or feature locations are measured. The proposed method is investigated under a variety of conditions, including non-constant acceleration, substantial, and total occlusion. Conditions under which tracking is lost are examined.

Real-time video image centroid tracker

Abstract: A real time image centroid tracker has been developed and is presently being used for laser beam alignment in the Aurora Laser Fusion project at Los Alainos National Laboratory. Aurora is one of the three national inertial confinement fusion energy experiments. The Aurora beam control systems are required to achieve angular beam pointing accuracy of up to 0.25 microradians by precision control of optical mirrors up to 44 inches in diameter. The centroid tracker is part of a PS 170 video image processing system which provides the precision beam position information that is the feedback portion of a stochastic control system for beam pointing. Mechanical vibration and electronic noise inherent in the system is used to enhance, rather than degrade, positional resolution. This system. is capable of yielding centroid position information accurate to within a very small fraction of one pixel. The centroid tracker is presently implemented on one Multibus board using primarily HCT 7400 series logic and large PLDs. Digitized R5170 video is supplied as the board's input. The video image's centroid information is supplied by the board every 33 milliseconds asthe board's output. A unique hardware algorithm used on the board allows very high resolution, real time tracking with a relatively small amount of electronics. Included on the board is circuitry used to distinguish image pixels from noise and background pixels. Other applications of the centroid tracker might include pick and place robot arm control, military target tracking and pointing, intruder localization and, in general, very high resolution, real time image position tracking. The board can be modified relatively easily to allow real time, multiple object centroid tracking and we expect to do this in the future. The first of the Aurora beam control systems requires that 96 laser beams be simultaneously controlled. The multiple centroid tracker board will make it possible to do image process- 82 / SPIE Vol. 1304 Acquisition, Tracking, and Pointing /V(1990)ing at a higher rate, leading to increased position resolution, while still maintaining control system bandwidth.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Performance evaluation of motion compensation methods in ISAR by computer simulation

Abstract: Three methods of motion compensation are considered: the scatterer point referencing method, the track-the-target centroid method, and the maximum likelihood (ML) image formation and motion compensation method. Computer simulations of imaging an aircraft in straight flight by a ground-based ISAR (inverse synthetic aperture radar) were carried out. The SNR (signal-to-noise ratio) performance, the susceptibility to target scintillation, and the computational complexity of the three methods are evaluated and compared. It is shown that the scatterer point referencing method is simple in computation, but is susceptible to target scintillation. The track-the-target centroid method and the ML method are able to greatly reduce the track loss arising from target scintillation at the cost of a higher computational load. The SNR performance of the track-the-target centroid method is poorer than that of the other two methods. The ML method suffers from the largest computational complexity among the three methods. Nevertheless, the ML method can provide the best SNR performance of the three methods. >

Character recognizing device

Abstract: PURPOSE:To enhance a recognition rate by obtaining an inclination from an input character using the centroid of the character and executing correction. CONSTITUTION:Points at equal intervals are obtained from the coordinates of the inputted character by a CPU 2. Respective averages of the x-coordinates and the y-coordinates are obtained from the obtained points at equal intervals, and a centroid G0 11 of the whole of the input character is obtained and stored in a whole centroid memory 4d. Next, another centroid is obtained only from some points at equal intervals having y-coordinates larger than a y-coordinate y0 of the centroid G0 11, and an upper centroid G1 12 is obtained. An inclination theta13 is obtained from the two centroids G0 11 and G1 12, and the input character is corrected from the inclination theta 13. Feature points composed of a start point, an end point, a sharp point, etc., are obtained from the corrected input coordinates and stored in an input feature memory 4c. According to the matching between these feature points and feature points registered in a recognition dictionary 5, feature points in the dictionary 5 closest to the input character are displayed on a display part 6 as the recognized result.

Centroid deciding element output device using neural network

Abstract: PURPOSE: To put a fuzzy control system to practical use by forming a neural network in order to output a centroid deciding element for a fuzzy neuro-fusion model. CONSTITUTION: Plural units, e.g., the neurons 1a, 1b... contained in an input layer 2 correspond in 1:1 to plural coordinate values set on several straight lines. The input value is given to each unit. The units 3a and 3b forming an output layer 4 are connected to the units 1a, 1b... of the layer 2 with the weight that is calculated with use of those coordinate values and then decided from the input values given to the units included in the layer 2 and the coordinate values set on the straight lines corresponding to the units. Then two numerical values necessary for calculation of the centroid are outputted. Thus a fuzzy control system is put to practical use. COPYRIGHT: (C)1991,JPO&Japio

Variance of focal-plane centroids

Abstract: The variance of the unbiased focal-plane estimates of image centroids from a point source is derived for arbitrary strength of scattering by using path-integral methods. The Markov approximation and narrow angular scattering are assumed. Results are presented for an ideal focusing telescope with an aperture transmission that produces no phase errors. Calculations of the leading term are performed for a circular telescope aperture and the case of homogeneous atmospheric turbulence described by the Hill spectrum [ J. Fluid Mech.88, 541– 562 ( 1978)]. When there are irradiance fluctuations, the variance of the unbiased estimate of image centroids depends on the choice of the origin of the weighting vector. The variance of the centroid of a single image depends on three physical parameters related to the Fresnel scale, the inner scale, and the strength of scattering. The variance of the separation between two images from two separated telescope apertures depends on the same parameters in addition to the normalized separation of the apertures. The important contributions from propagation distance and spectral wave number are identified for the different parameter regimes. The magnitude of centroid motion compared to the average image size is also discussed.

ACE: Automatic Centroid Extractor for real time target tracking

Abstract: A high performance video image processor has been implemented which is capable of grouping contiguous pixels from a raster scan image into groups and then calculating centroid information for each object in a frame. The algorithm employed to group pixels is very efficient and is guaranteed to work properly for all convex shapes as well as most concave shapes. Processing speeds are adequate for real time processing of video images having a pixel rate of up to 20 million pixels per second. Pixels may be up to 8 bits wide. The processor is designed to interface directly to a transputer serial link communications channel with no additional hardware. The full custom VLSI processor was implemented in a 1.6 mu m CMOS process and measures 7200 mu m on a side.

Obtaining Centroids Of Digitized Regions Using Square And Hexagonal Tilings For Photosensitive Elements

Abstract: In computer vision and graphics, the square or rectangular tessellation is most commonly used The hexagonal lattice has not been studied as frequently In this paper we project squares and circles on each of these grids, digitize these figures and obtain the error in locating the centroid We adopt a Monte Carlo approach, with the centroid of the actual figures being chosen randomly In the case of the circle, we project various sizes onto the respective grids and study the error in obtaining the centroid In the case of the square we combine different sizes with angles of rotation that vary from 0 to 90 degrees Theoretical formulae are developed for the circle on square tile case These symbolic representations are compared to the results from the Monte Carlo simulation and they are found to be quite close Finally, comparisons are drawn between the two grids In the case of the circle, there is a definite advantage in using the hexagonal grid For the square there is no inherent advantage to either These results are of use if it is decided to build a camera with hexagonal picture elements

A Comparison of Methods for Accurate Image Centroid Position Determination with Matrix Sensors

Abstract: Optical methods for the measurement of position, part dimensions, or alignment are finding increased application in manufacturing operations. Several of these methods utilize analog or digital position sensitive detectors to find the centroid of an illuminated spot on the detector. The objective of this study was to determine the accuracy with which the centroids of a direct laser beam and a circular incoherent image could be determined using a matrix array sensor. Centroids were found using five different analytical methods. The results showed that a simple intensity centroid computation was as accurate as more complex analytical schemes. Maximum errors of less than 1/20 pixel were found for both the laser beam and incoherent image centroids when their diameters were approximately 40 pixels. The effect of laser beam wander on accuracy was also investigated.

Motion analyzing method

Abstract: PURPOSE: To provide a motion analysis method based on dynamics. CONSTITUTION: A human body or an animal body is decomposed to parts as minimum motion units, and data of a human body or animal body model based on properties and restrictions peculiar to individual parts are preliminarily inputted to a data base, and a motion as the analysis object is inputted, and the inputted motion is calculated with dynamics, and the motion and the centroid of each part, the force and the torque applied to each joint, the motion and the centroid of the whole, and the force and the torque applied to the centroid are displayed on a screen independently or over the human body or animal body model of the data base. COPYRIGHT: (C)1992,JPO&Japio

Isolation of integrals for centroid orbits in stellar system models

Abstract: In this paper we present two solutions for isolating integrals of the centroid motion in a non-stationary stellar system under the ellipsoidal hypothesis with point-axial symmetry.

Pattern recognizing device

Abstract: PURPOSE:To attain recognition processing even when an image pattern is inclined by setting up plural circular or ring-like measuring areas arranged around the centroid position of the image pattern and discriminating the image pattern based upon the areas of the image patterns included in respective measuring areas. CONSTITUTION:When an image pattern to be recognized is formed, the centroid position of the image pattern is measured by a centroid measuring part 2 and plural circular or ring-like measuring areas are set up around the centroid position by a measuring area setting part 3. An area measuring part 4 measures the area occupied by the image pattern included in respective measuring areas and a pattern decision part 5 discriminates the image pattern based upon the measuring values of respective areas. Since respective measuring areas are circulars or ring-like shapes, the area occupied by the image patterns in respective measuring areas is fixed independently of the inclination of the image patterns. Since it is unnecessary to store many reference patterns including patterns having inclinations or to match the image patterns with all the reference patterns, rapid recognition processing can be attained.

Device for transferring semiconductor wafer

Abstract: PURPOSE:To obtain a transfer device which can transfer semiconductor wafers by bringing the centroids of semiconductor wafers into coincidence by providing centroid detection means for movement and control in 3-dimensional directions on the basis of gravity information of a pressure sensor, and transfer means for transferring the centroid of the wafer to the rotation center of a rotary stage. CONSTITUTION:Pressure sensors 43 are so disposed at an equal interval on a raising body 42 as to be located at the vertexes of a regular triangle, received by the gravity of a semiconductor wafer 30 upon rising of the body 42, this gravity information is converted to an electric signal or the like and output. A moving shaft 41 is so displaced that its raising center coincides with the centroid of the wafer 30 on the basis of the gravity information from the sensors 43. A fork 51 is so operated as to transfer the wafer 30 detected at its centroid from the body 42 to a rotary stage 2. Accordingly, after the detected centroid of the wafer 30 is brought into coincidence with the rotation center of the stage 2, the wafer 30 is transferred onto the stage 2.