Optimal edge detector design I: parameter selection and noise effects

This paper comments on the optimality of the Laplacian of a Gaussian edge detection filter which localizes edges through zero crossings in the filtered image. The arguments of both Marr and Hildreth, and Dickey and Shanmugam are reviewed to establish that the filter is optimal in the sense of maximizing output image energy near edge features. This filter's principal advantage over other edge detectors is that its response is user-adjustable through selection of a single parameter, the Gaussian standard deviation. However, no clear method for the selection of this parameter has been provided. The problem is addressed here by applying the filter to two ideal periodic edge models blurred by a Gaussian distribution point-spread function. The observed response to the edge spacing and blur standard deviation is then translated into a filter parameter design procedure. The problems of optimum filter performance in the presence of additive Gaussian noise are then addressed. The problem of selecting the sampled filter's coefficient word size is dealt with in a companion paper.     

The Importance of Free Fatty Chain Length on the Lipid Organization in the Long Periodicity Phase

The skin’s barrier ability is an essential function for terrestrial survival, which is controlled by intercellular lipids within the stratum corneum (SC) layer. In this barrier, free fatty acids (FFAs) are an important lipid class. As seen in inflammatory skin diseases, when the lipid chain length is reduced, a reduction in the barrier’s performance is observed. In this study, we have investigated the contributing effects of various FFA chain lengths on the lamellar phase, lateral packing. The repeat distance of the lamellar phase increased with FFA chain length (C20–C28), while shorter FFAs (C16 to C18) had the opposite behaviour. While the lateral packing was affected, the orthorhombic to hexagonal to fluid phase transitions were not affected by the FFA chain length. Porcine SC lipid composition mimicking model was then used to investigate the proportional effect of shorter FFA C16, up to 50% content of the total FFA mixture. At this level, no difference in the overall lamellar phases and lateral packing was observed, while a significant increase in the water permeability was detected. Our results demonstrate a FFA C16 threshold that must be exceeded before the structure and barrier function of the long periodicity phase (LPP) is affected. These results are important to understand the lipid behaviour in this unique LPP structure as well as for the understanding, treatment, and development of inflammatory skin conditions.     

The influence of surface condition on the localized corrosion of 316L stainless steel orthopaedic implants

The localized corrosion of austenitic stainless steel 316L intended for use as orthopaedic implants is determined as a function of the surface condition and metallurgical state. From the examination of samples exposed to a ferric chloride solution, at both 22 and 37 °C, the independent contribution of crevice and pitting corrosion to localized corrosion is determined. Both forms of localized corrosion occur to a greater extent at the higher temperature. The results indicate that weight loss measurements may not be sufficient to determine the extent of crevice corrosion separately from the influence of pitting corrosion. More importantly, the surface conditions required for the best resistance to crevice or pitting corrosion differ. Electropolished surfaces provide the best resistance to crevice corrosion, while bead blasted surfaces provide the best resistance to pitting corrosion. The implication of this result in terms of the serviceability as orthopaedic implants is discussed. The current results indicate the cold-worked state exhibits improved resistance to pitting corrosion. However, the influence of the metallurgical state could not be separated from a possible compositional effect. © 1999 Kluwer Academic Publishers     

Evaluations of P/M Gamma Titanium Aluminides

For the last five years, the Structures, Materials and Propulsion Laboratory of the Institute for Aerospace Research, National Research Council of Canada, has devoted a significant amount of effort investigating powder metallurgy gamma titanium aluminides. The main results of the studies on microstructural evolution, hot workability, creep behaviour and microstructure-mechanical property relationships for P/M gamma alloys are reviewed in this article. Work is in progress to apply the knowledge gained to cast gamma alloys for gas turbine applications.     

A simplified arithmetic detector for EEG sharptransients-preliminary results

A simplified method is described for the detection of electroencephalographic sharp transients (STs), i.e. spikes and sharp waves. The detector consists of two stages. The first is a different filter (i.e. the difference between the current sample point and one given a number of samples earlier), which emphasizes the principal frequency components of STs and can be viewed as a combined slope-duration evaluator. The second stage consists of a product operator, the inputs for which are the output of the difference filter and a slightly delayed version of the latter, thereby imposing some smoothing (low-pass filtering). For signaling the occurrence of STs, the output of the product operator (which, within its passband, can be considered a squarer) is compared to a threshold that is estimated probabilistically (e.g., p=0.001) from the entire current EEG epoch (nominally 4 s). The method, for which some preliminary results are included, uses simple arithmetic procedures and has some intrinsic artifact-rejection features     

Microstructural study of a high-strength stress-corrosion resistant 7075 aluminium alloy

A heat-treatment procedure providing for enhanced stress-corrosion cracking resistance without any sacrifice of yield strength in 7075 aluminium alloy is investigated using transmission electron microscopy. It is suggested that the heat treatment (known as retrogression and re-ageing) provides for large grain-boundary precipitates and coherent matrix precipitates. The latter provides for the high strength levels while the grainboundary precipitates provide for enhanced stress-corrosion cracking resistance. A hydrogen embrittlement mechanism of stress-corrosion cracking is assigned to this alloy system.     

Simulation of a Visual Receptor Network

An analogue method is presented for simulating a visual receptor network of the type described by Hartline, Ratliff, and Miller as a model for inhibitory interaction in the retina. The number of elements required in the model is reduced drastically by using a process of repeated approximation which converges. The activity of a large receptor field is simulated by breaking it up into much smaller, overlapping subfields, the activity of which can be determined using the model equations.     

An Inexpensive Reading Instrument with a Sound Output for the Blind

An alternative auditory code is suggested to alleviate the reading problem presented by the Optophone in its older form. A ``musical' code is discussed which has a high degree of coalescence. Physically it takes the form of an FM squarewave. As for the Optophone, a slit scanner is employed to scan along the line of print. Using Ny photocells, the same number of binary signals b1. . . bn . . . bny are generated by the print patterns. Photocell 1 is located at the bottom of the scanning slit; the cell with n = Ny is at the top. When the cell is over black, bn = 1; over white, bn = 0. Two modulating functions have been used. The first is the familiar binary digit to analog transformation. It is easy to instrument. The disadvantage is the large number of values produced. The second coding function produces fewer f values by discarding information mainly connected with letter thickness. Using either version of the musical code, certain general features in print letters can be recognized. Single straight lines are easy; repeated or symmetrical print shapes, e.g., the letter ``w,' are again easily identified. The code is weakest at representing curves. For example, the letters ``o' and ``c' sound nearly the same. Features used to recognize the lower-case alphabet letters are given. All letters of the alphabet (both lower and upper case) and numbers can be identified by their corresponding code sounds, which are reported to be quite pleasant.     

Primary creep behavior of Ti-48Al-2W as a function of stress and lamellar morphology

The fully lamellar microstructure of powder metallurgy Ti-48Al-2W after cooling from the α region to 1280 °C, followed by air cooling and aging at 950 °C for up to 96 hours, is presented. Aging times as short as 5 hours result in acicular-shaped precipitates of W-rich β ₀ along lamellar interfaces, with the β ₀ size increasing with aging time. The β ₀ precipitates nucleate and grow in the α ₂ lamellae. Concurrently, with the formation of β ₀, the α ₂ decomposes into discontinuous lamellae. Aging to precipitate β ₀ along lamellar interfaces increases the 760 °C tensile strength (with a slight reduction of ductility) and reduces the instantaneous creep strain, since β ₀ precipitates at lamellar interfaces hinder interface dislocation mobility. The deformed microstructures from interrupted creep tests at 140 to 276 MPa at 760 °C indicate that the precipitation of β ₀ along interfaces substantially reduces the primary creep strain, primarily due to the influence of β ₀ on interface dislocation emission and motion. These results are discussed in terms of the influence of lamellar morphology on the instantaneous creep strain and primary creep transient, and the possible creep mechanisms are highlighted. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee of Mechanical Behavior of Materials.