Showing papers on "Digital image correlation published in 1995"

Image correlation velocimetry

Abstract: This paper focuses on the correlation of two successive scalar images for the purpose of measuring imaged fluid motions. A method is presented for deforming, or transforming, one image to another. Taylor series expansions of the Lagrangian displacement field are used, in conjunction with an integral form of the equations of motion, to approximate this transformation. The proposed method locally correlates images for displacements, rotations, deformations, and higher-order displacement gradient fields, and applies a global minimization procedure to insure a global consistency in the results. An integral form of the equations of motion is employed. No explicit spatial or temporal differentiation of the image data is required in estimating the displacement field. As a consequence, this method is appropriate for both continuous-scalar as well as discrete-particle-image data. Successive two-dimensional digital CCD images of fluid motion marked with dye, are used to verify the capabilities of the method. The utility of the method is also illustrated using a pair of Voyager 2 images of Jupiter.

Strain measurement in wood using a digital image correlation technique

Abstract: The suitability of the Digital Image Correlation Technique (DICT) for full-size test specimens of wood and wood-based composites was evaluated in this study. The technique utilizes pairs of digitized video images of undeformed and deformed test specimens and an image correlation computer routine to measure the displacements of any or all points on the surface of the test specimen. New methods for image acquisition and image correlation were developed and evaluated in this study. Evaluation and calibration were performed using an aluminum alloy block for comparison and axial compression of small, clear specimens of wood and in accordance with ASTM D143-83 (1986a). Comparison of strain measurements obtained using an independent measurement technique and strain obtained with the DICT showed close agreement. Utilizing DICT for full-field strain distributions through an increasing load series revealed progressive failure development in the wood specimens, the eventual failure mode, and a shift in strain concentrations during load application.

Aspheric optical elements for extended depth-of-field imaging

Abstract: We provide experimental verification of the performance of an optical-digital imaging system that delivers near diffraction limited imaging over a wide depth of field. A custom aspheric optical element is used to modify an incoherent optical system so that the generated optical image is nearly independent of misfocus-induced blur. The resulting image, called an intermediate image, is not spatially diffraction limited. Digital processing of the intermediate image produces a final image that forms a close approximation to the diffraction limited image. The combined effect of the optical-digital system is to image objects independently of focus or range, that is, the system has an extended depth of field.

Optical transform system for three-dimensional object recognition

Abstract: An image processing method and apparatus in which a range image of a three-dimensional object is obtained, and the inclination of a plane contained in the range image and an approximate area of the plane are instantaneously detected, thereby realizing high-speed recognition of a three-dimensional object. The apparatus includes a range image obtaining device (401) for obtaining a range image as three-dimensional information about an object, an image display device (402) for displaying the obtained range image as a phase distribution, a Fourier transform optical system (403) which employs approximately parallel coherent light for reading out the image displayed on the image display device, and a Fourier transform lens for carrying out Fourier transformation on the image read out by the coherent light, a Fourier transform spectrum detecting device (404) for detecting a Fourier spectrum image obtained by the Fourier transform optical system, and a recognition device (405) for identifying a three-dimensional object which is to be recognized by comparing the data detected by the detecting device with previously inputted information concerning a reference object, thereby detecting a normal to a plane contained in the obtained range image and an approximate area of the plane at high speed.

In Situ Measurement of Sintering Shrinkage in Powder Compacts by Digital Image Correlation Method

Abstract: An In Situ system for monitoring sintering shrinkage of powder compacts using digital image correlation has been developed. This system will provide completely automated non-contact measurement of two dimensional deformation, and thus enables In Situ quantification of non-uniform and anisotropic shrinkage, if any, of a powder compact during sintering. A brief description is given of the procedure for identifying micro-areas on a specimen surface before and after deformation by the digital image correlation method. A new digital image processing algorithm has been developed to determine the two-dimensional distribution of shrinkage rate in powder compacts during sintering. A measurement of the shrinkage non-uniformity in a die pressed carbonyl nickel powder compact during sintering from room temperature to 1573 K is presented. The results showed successful identification of non-uniform and anisotropic shrinkage arising from powder filling inhomogeneities and the neutral zone effect of die wall fric...

Image Correlation From Local Frequency Information

Abstract: A new approach to the comparison and matching of image signals for disparity measurements is introduced. It is proposed that localized frequency data, rather than spatial data, should be used as the basis for matching signals against each other. Working in the frequency domain one can use local phase and amplitude information to construct a dimensionless measure of similarity that has high localization. Of greater interest, it is shown that phase information can be used to eeciently guide the search for a point in one signal that best matches a selected point in another signal. It is also shown that the representation of a signal in terms of phase and amplitude information on a logarithmic frequency scale provides a powerful domain for the comparison of signals.

Method of and apparatus for improved image correlation

Abstract: A method of and apparatus for computing correlating images using the one dimensional Fast Fourier Transform (FFT) and the symmetry property of the Fourier Transform. In one aspect, the invention is a method of correlating images in the form of a pair of real number arrays. Apparatus for carrying the method out is also disclosed and claimed.

Three-dimensional Fourier analysis methods for digital processing and 3D visualization of confocal transmission images

Abstract: We have developed digital 3D Fourier transform methods for comparing the 3D spatial frequency content and hence the axial and transverse resolution of confocal versus conventional microscope images. In particular, we have utilized these techniques to evaluate the performance of our recently-developed confocal transmission microscope for bright field and Nomarski DIC imaging. We have also found that Fourier methods, such as the Hibert transform, can be successfully employed to overcome the difficulty of visualizing differentially-shaded phase objects, in 3D, that have been acquired using transmission DIC optics.

An adaptive smoothness constraint for computing optical flow on sequences of cardiac images

Abstract: Different algorithms are available for processing an optical flow field between two successive frames. In most situations, two constraints are necessary to estimate such a displacement vector field. The first constraint is based on the gray level variations. The second one is usually based on the smoothness of the flow field. Unfortunately, the results of these constraints are not really adapted to the movements of the ventricle. Here, the authors propose a new method specifically adapted to sequences of cardiac images, for processing the optical flow. They calculate the amplitude image of a sequence of images by the Fourier transform. Unlike methods which identically smooth the optical flow in each direction, the authors developed a Fourier adaptive smoothness method which restricts the variation of the flow field in the directions with small variations of the Fourier amplitude. Moreover, the authors have generalized this method to sequences of non-cyclic images.

Feature correlation for particle image velocimetry: an application of pattern recognition

Abstract: Particle Image Velocimetry (PIV) has been used successfully for measuring instantaneous two dimensional velocity fields. Analyzing PIV images involves matching particle images captured sequentially. In the usual practice, correlation (auto- or cross-correlation) is used to find the displacement (hence velocity) of the particles within a large number of small 'interrogation areas' in the field of view. An image correlation within an interrogation area is inherently sensitive to the rotation and distortion of the fluid flow within the area, and is also sensitive to the brightness of particles. It can only find the mean spatial offset of particles weighted by their brightness. Rotation, dilation and distortion of fluid flow within the interrogation region and intensity changes of particles introduced correlation errors or bias. As a consequence, the size of the interrogation area and the time interval between images must be kept small to avoid these problems. A feature-recognition method is proposed here for analyzing PIV images. It first extracts structural features of the particle pattern after their locations have been isolated from images. A preliminary process is to replace the particle images by the Cartesian coordinates of particle centers. In this way the brightness of particle images plays no further part, and the point positions are used to establish structural features: topological relations between each point and its neighbors. The interrogation area is defined by a limited number of neighboring points. The size and shape of each interrogation area varies with the distribution of neighbors. A fit to motion, rotation and distortion among the neighbors is then carried out in the space of topological relations of successive images. In this way changes of structural features define fluid spatial translation, rotation, and deformations within each interrogation region. Measurement of feature space in two successive images demands knowledge of the locations of corresponding points derived from individual particles in the two images. Classification of point correspondences, despite confusingly discordant displacements from one image to the next, can be made by taking advantage of physical limitations on the possible movement of particles between the two images. It is found that feature space search and correlation is a much more efficient procedure than correlation operations in the two dimensional image domain.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Comparison between cross-correlation and optical flow methods for patient motion detection in SPECT

Abstract: An automatic procedure for the detection and quantification of motion during single photon emission computerized tomography (SPECT) is proposed. The method computes the optical flow vector field between two successive views. The proposed method is also compared with the cross-correlation method.

3D motion analysis of MR imaging using optical flow method

Abstract: Optical flow is the velocity distribution of each pixel in an image. In this paper, the authors extend the 2D optical flow method to 3D optical flow analysis and also improve its performance around boundaries. Dynamic analysis of heart and knee is under investigation.

Refractive surface flow visualization using image processing

Abstract: The importance of the wake–free-surface interaction in the detection, classification, and tracking of submerged objects has led to the development of a simple but effective free-surface visualization technique for use in controlled water-tunnel experiments. An experiment was performed to verify the effectiveness and the applicability of this method. Digital images of a spatially varying sinusoidal grid were acquired as seen through the disturbance pattern on the water surface. Image-processing techniques were used to perform phase demodulation of the distorted image. The resulting image details the outline, location, and extent of the surface deformation in a gray-scale format. Optimal digital filter specifications and spatial grid frequencies were determined experimentally for various surface-flow conditions.

Tomographic approach to 3D confocal microscopy

Abstract: The nature of 3D imaging with confocal optical system using principles of tomography and geometrical optics is analyzed. Image intensity in any plane of image space is described by the 2D section of a 3D summary image of the sample is obtained.