For two years, we have been teaching a quarter-long compiler construction course where students implement an interpreter for a variant of the XML query language XQuery. Our goal is to motivate students' interest in the course by exposing them to an interesting and powerful new language which they see as relevant to potential future experiences. In this paper, we first explain the workings of the course itself, and then describe some pedagogically interesting variants of the XQuery language. We close with a discussion of challenges faced and conclusions.

No abstract available.

We consider the scale-space properties of quadratic feature detectors and, in particular, investigate whether, like linear detectors, they permit a scale selection scheme with the "causality property", which guarantees that features are never created as the scale is coarsened. We concentrate on the design of one dimensional detectors with two constituent filters, with the scale selection implemented as convolution and a scaling function. We consider two special cases of interest: the constituent filter pairs related by the Hilbert transform, and by the first spatial derivative. We show that, under reasonable assumptions, Hilbert-pair quadratic detectors cannot have the causality property. In the case of derivative-pair detectors, we describe a family of scaling functions related to fractional derivatives of the Gaussian that are necessary and sufficient for causality. In addition, we report experiments that show the effects of these properties in practice. We thus demonstrate that at least one class of quadratic feature detectors has the same desirable scaling property as the more familiar detectors based on linear filtering.

In this paper, a statistical error analysis is developed to show how the performance of a stereo ranging system relies on system parameters. Formulas for expected absolute depth error, expected relative depth error, and lower and upper bounds of depth error are derived. We show that for a given convergence angle, a simple, but important, parameter called generalized depth resolvability completely determines the performance of the system. These results are useful for the design of fixed ranging systems in constrained environments, or for the active selection of system parameters in "purposive" stereo systems.>

The author introduces a framework for investigating the properties of energy edge detectors and uses it to derive some results of interest. He shows a necessary condition on the form of constituent linear filters in quadratic detectors, subject to some conditions, and demonstrates some limitations of such detectors. It is shown that no quadratic detector can detect an edge at 0 for both a sinewave and a cosine wave, which has implications for detecting narrowband edges with spatially local filters. It is also shown that the scale-space behavior of energy detectors is not well-behaved, in that it contains bifurcations as scale increases, i.e. new edges can be created as the image is smoothed.>

The authors consider the problem of occlusion in computing stereo disparity from a pair of images. Usual approaches to stereo matching, e.g. area-based, feature-based, etc., can perform poorly in the neighborhood of occluding boundaries if no surface models are assumed. Qualitative improvements are possible based on conditions derived from the geometry of occlusion. A pair of correspondence processes, using information near an occlusion boundary to localize the boundary, can determine the sign of associated depth discontinuity unambiguously. The proposed method is able to identify the presence and extent of occlusion regions and assign disparities in a consistent way near the occlusion regions.>

A standard probabilistic formulation of a local shape estimator due to Witkin (1980) has been demonstrated to work to some extent on a range of data, but it also has been shown to overestimate slant. The author identifies and analyzes three sources of this error: the choice of prior probability distribution on shape; a systematic bias in the observation of image events; and a kind of nonergodicity in the stochastic process associated with surface events. As a result of this analysis, the author formulates a corrected probabilistic local shape estimator, proves a correctness theorem for it, and demonstrates its superiority to the Wilkin estimator in a set of experiments.>

Marking boundaries in an image between regions that are differently textured depends on computing descriptions of the regions that show them to be differently textured. Many texture description functions have been introduced in the literature; however, the texture description problem has never been treated in full generality. The author proposed shift-invariant vector-valued piecewise continuous functions of bounded support on images as a general class of texture description functions, and consider approximating functions in this class with polynomials. He proves a theorem that relates the degree of such a polynomial description function to the statistical properties of textured regions it can discriminate, and discusses some implications of the theorem.>

An analysis is presented of the imaging of surfaces modeled by fractal Brownian elevation functions of the sort used in computer graphics. It is shown that, if Lambertian reflectance modest surface slopes and the absence of occlusions and self shadowing are assumed, a fractal surface with Fourier power spectrum proportional to f/sup beta / produces an image with power spectrum proportional to f/sup 2- beta /; here, f is the spatial frequency and beta is related to the fractional dimension value. This allows one to use the spectral falloff of the images to predict the fractal dimension of the surface.>