Differential block coding of bilevel images.

In this correspondence, a simple one-dimensional (1-D) differencing operation is applied to bilevel images prior to block coding to produce a sparse binary image that can be encoded efficiently using any of a number of well-known techniques. The difference image can be encoded more efficiently than the original bilevel image whenever the average run length of black pixels in the original image is greater than two. Compression is achieved because the correlation between adjacent pixels is reduced compared with the original image. The encoding/decoding operations are described and compression performance is presented for a set of standard bilevel images.

Synthetic aperture pulse-echo imaging with rectangular boundary arrays [acoustic imaging].

The effectiveness of a square boundary array in finite-range, pulse-echo imaging is investigated. The images produced by such an array are quite poor when no additional signal processing is used. It is demonstrated through simulations that a synthetic-aperture signal processing technique called image addition can be used to reduce the sidelobes associated with the square boundary array, thereby improving the image quality. Image addition was originally proposed for narrowband imaging of far-field scenes, but it is also useful for finite-range, pulse-echo imaging. The conclusions are expected to apply to other, nonsquare boundary array geometries.

Coarray synthesis with circular and elliptical boundary arrays.

An elliptical boundary aperture is a collection of points lying on an ellipse from which energy is transmitted and/or received. An important special case is the circular boundary aperture. When these apertures are used with beamforming to produce a narrowband image of a far-field source, the corresponding point spread function (PSF) is characterized by high sidelobes. The concept of the coarray of an imaging system is used here to develop techniques which synthesize the effect of a more desirable PSF with an elliptical boundary aperture. Techniques are given for use in active imaging of spatially coherent sources, as well as passive imaging of spatially incoherent sources. Discrete arrays and continuous apertures are considered separately. The approach shows that the PSF synthesis problem can be solved in many more ways than previously recognized, and this fact is exploited to develop procedures which have a least-squares optimality property.