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Target Distance (target + distance)

Distribution by Scientific Domains

Medical Sciences	50%

Selected Abstracts

Virtual path signature: An approach for flexible searching in object-oriented databases

INTERNATIONAL JOURNAL OF INTELLIGENT SYSTEMS, Issue 1-2 2004
Pichayotai Mahatthanapiwat
Signature technique that is proposed for multikey indexing is used for flexible searching in the area of databases. In this article, we present a new signature technique called Virtual Path Signature for supporting query processing of aggregation hierarchy as a tree in object-oriented databases (OODBs). We derive cost formulas for its storage overhead as well as the retrieval cost. Comparing with the Tree Signature Scheme, the Virtual Path Signature shows significant improvement in the retrieval operation, especially when the target distance between the target class and the predicate class is high. © 2004 Wiley Periodicals, Inc. [source]

Targeted driving using visual tracking on Mars: From research to flight

JOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 3 2009
Won S. Kim
This paper presents the development, validation, and deployment of the visual target tracking capability onto the Mars Exploration Rover (MER) mission. Visual target tracking enables targeted driving, in which the rover approaches a designated target in a closed visual feedback loop, increasing the target position accuracy by an order of magnitude and resulting in fewer ground-in-the-loop cycles. As a result of an extensive validation, we developed a reliable normalized cross-correlation visual tracker. To enable tracking with the limited computational resources of a planetary rover, the tracker uses the vehicle motion estimation to scale and roll the template image, compensating for large image changes between rover steps. The validation showed that a designated target can be reliably tracked within several pixels or a few centimeters of accuracy over a 10-m traverse using a rover step size of 10% of the target distance in any direction. It also showed that the target is not required to have conspicuous features and can be selected anywhere on natural rock surfaces excluding rock boundary and shadowed regions. The tracker was successfully executed on the Opportunity rover near Victoria Crater on four distinct runs, including a single-sol instrument placement. We present the flight experiment data of the tracking performance and execution time. © 2009 Wiley Periodicals, Inc. [source]

The effect of prism adaptation on the response AC/A ratio

OPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 3 2000
Bill B. Rainey
Summary Vergence adaptation, also known as prism adaptation, is a phenomenon in which a patient's heterophoria changes after prolonged viewing through prism. The effect of prism adaptation on the accommodation,convergence relationship, quantified by the AC/A ratio, is not known. Previous studies of AC/A ratio stability and alterability have used only stimulus AC/A ratio calculations, or have measured accommodative responses to only one or two stimuli. The ideal study of AC/A ratio stability and alterability would measure accommodative responses to several accommodative stimuli, and use these along with vergence responses to calculate response AC/A ratios, rather than stimulus AC/A ratios. In addition, the gradient method should be used to avoid any effect of proximal vergence resulting from changes in target distance. This paper describes a project which investigated the effect of vergence (prism) adaptation on the gradient response AC/A ratio, using accommodative responses measured for five different accommodative stimuli. The response AC/A ratio did not significantly change following a period of adaptation to base-in prism for six of the eight subjects in this study. In addition, the response AC/A ratio did not significantly change following a period of adaptation to base-out prism for six of the eight subjects. [source]

Effects of higher-order wavefront aberrations on the eye's depth of focus

ACTA OPHTHALMOLOGICA, Issue 2008
N CHATEAU
Purpose To evaluate the impact of higher order aberrations (HOA), defined by individual Zernike polynomial coefficients, on the eye's depth of focus using an adaptive optics (AO) system. Methods A crx1 AO visual simulator (Imagine Eyes, France) was used to introduce different amounts of individual 3rd and 4th order HOA in 10 healthy eyes. These HOA included coma (Z(3,-1)) and trefoil (Z(3,-3)) at magnitudes of +/-0.3 µm, and spherical aberration (SA) (Z(4,0)) at magnitudes of +/-0.3,+/-0.6 and +/-0.9 µm through a fixed 6-mm pupil diameter. A through-focus response (TFR) curve was assessed by recording the percentage of optotype letters of fixed 20/50 size that the subject could identify while these letters were presented at various target distances. Testing was performed under cycloplegia. For each applied HOA, the subject's depth of focus (DoF) and center of focus (CoF) were computed as, respectively, the half-maximum width and the midpoint of the TFR curve. Results The introduction of SA resulted in linearly shifting the CoF by 1.3 D for each 0.5 µm of wavefront. The shift was hyperopic with positive SA, myopic with negative SA. The simulation of either positive or negative SA also had the effect of enhancing the DoF, up to a maximum increase of 2 D with 0.6 µm of SA. The enhancement became smaller when the SA was further increased. Trefoil and coma appeared to neither shift the CoF nor significantly modify the DoF. Conclusion AO technology allowed us to selectively test the visual impact of several HOA on the DoF. The introduction of SA significantly shifted and expanded the subjects' overall DoF. This technique could help in designing optimal corrections for presbyopia and allowing patients to preview refractive surgery outcomes. Commercial interest [source]