C. Cafforio

Motion compensation and multiresolution coding

Abstract Multiresolution techniques have become more and more appealing in current image coding. Image multispectral representation produces many important features such as spectral shaping of coding noise according to human eye perception, good image energy compaction, coder tuning with respect to any band characteristics, and allows for multilevel layered transmission that is one of the main targets pursued by the broadcaster. Despite these appealing capabilities, multiresolution techniques have failed to give the expected results. One of the reasons for this failure is the difficulty of exploiting the temporal redudancy present in image sequences. This paper addresses the problem of motion compensation in a multiresolution environment, considering both QMF-SBC and wavelet transform approaches. Different motion compensation schemes are derived and their efficiency is considered with regard to scalability and to the lengths of subband analysis and synthesis filters. Simulation results are used to support relevant conclusions where needed.

Flexible Dynamic Block Adaptive Quantization for Sentinel-1 SAR Missions

The letter introduces a novel quantizer suited for medium to high-resolution synthetic aperture radar (SAR) systems, like the forthcoming SENTINEL-1 SAR. The Flexible Dynamic Block Adaptive Quantization (FDBAQ) extends the concept of the Block Adaptive Quantization (BAQ), used in spaceborne SAR since the Magellan mission, by adaptively tuning the quantizer rate according to the local signal-to-noise-ratio (SNR). A design is presented aiming to optimize the average bit-rate, while constraining the minimum SNR. FDBAQ optimized performance is then evaluated using backscatter maps derived from ENVIronment SATellite (ENVISAT) data.

Doppler centroid estimation for ScanSAR data

We introduce a novel accurate technique to estimate the Doppler centroid (DC) in ScanSAR missions. The technique starts from the ambiguous DC measures in the subswaths and uses a method alternative to standard unwrapping to undo the jumps in estimates induced by modulo pulse repetition frequency (PRF) measures. The proposed alternative is less error prone than the usual unwrapping techniques. Doppler Ambiguity is then solved by implementing a maximum-likelihood estimate that exploits the different PRFs used in different subswaths. An azimuth pointing of the antenna that does not change with subswaths, or that changes in a known way, is assumed. However, if the PRF diversity is strong enough, unknown small changes in azimuth pointing are tolerated and accurately estimated. This estimator is much simpler and more efficient, than those in the literature. Results achieved with both RADARSAT 1 and ENVISAT ScanSAR data are reported.

Remarks on the differential method for the estimation of movement in television images

Abstract In this paper a physical interpretation is given of a method for the estimation of movements in television images. The method, already presented by the author in other papers, is based on a linear regression of the image derivatives, both spatial and temporal. It is shown that a centered finite differences approximation of the image differentials is mandatory to obtain good performances. Experimental results are issued to support theoretical conclusions.

Combining DCT and subband coding into an intraframe coder

Abstract The advent of broadband digital networks will make it possible to transmit high quality digital television. Subband coding is a very interesting coding scheme as it accommodates a transmission quality that can flexibly adapt to the available channel capacity. Subband coding (SBC) and block cosine-transform techniques have already been used together, but only to code the low-pass image. Residual spatial correlation present in higher frequencies bands has only partially been taken care of. This paper presents the results obtained with a simulated coder in which DCT is used in all bands where expected spatial correlation is high enough. The use of a pyramid vector quantizer is proposed. Results are good and compare favourably with other results published in the literature.

Optimal block quantization for SAR data

A variable bit rate coding scheme designed for new generation SAR satellites is presented. This coding is based on previous schemes of Block Adaptive Quantizer (BAQ) [1] and is intended as a possible improvement over Sentinel-1 FDBAQ [3,4]. In this project, the number of quantizers is considered a design choice and the optimization is carried out looking for a set of reflectivity classes in which partition all the imaged areas on the Earth. Superior performances to those envisaged for Sentinel-1 FDBAQ are obtained in terms of SNR vs. average rate. Results come from simulations based on a 16-bit mosaic of C band reflectivity obtained by ESRIN from ENVISAT GM data [6] and Wien University database of ENVISAT WSM radar reflectivity [10].

Motion Compensation Processing of Airborne SAR Data

This paper presents a motion compensation algorithm to focus high resolution SAR data taken from an airborne sensor. The basic idea is to fragment the sensor trajectory into smaller segments that can be modelled as linear or quadratic. The synthetic antenna is then considered as an array of smaller subarrays. The algorithm can easily adapt and trade efficiency for sensor path irregularity. In fact the algorithm is more efficient if the segments are longer. Experimental results with real airborne data confirm the correctness of the approach.

A Simple and Effective Edge Detector

Nonlinear filtering based on a two concentric circular windows operator is introduced as a simple and effective way to find edges in image processing. The dual windows edge detector can operate with no fixed threshold, is isotropic, i.e. its response does not depend on edge orientation and has good noise immunity in the vicinity of edges. Computational requirements are limited and experimental results are good enough.

Motion estimation and region segmentation via functional optimization

High performance coding of image sequences requires efficient motion estimation and compensation. Block matching techniques are the usual choice. However they produce irregular motion fields and require subsequent regularization. This paper reports on a motion estimation technique that merges block matching with polynomial approximation and region growing to achieve such a regularization of the measured motion field. As a byproduct, a segmentation of the scene into regions with smoothly varying motion is obtained. Experimental results show that the obtained motion fields, though much more regular than the ones produced by simple block matching, still are very effective in giving a small prediction error.

Edge enhancement for subband-coded images

Subband coding is a well-established technique. Coding algorithms, however, rarely try to exploit residual similarities between subbands in order to get both coding gain and improvement in visual appearance. In this work we propose a simple postprocessing technique for edge enhancement and ringing-artifact reduction in subband-coded images. The proposed technique employs morphological filtering for preliminary image simplification and uses a novel dual-window nonlinear operator for subsequent edge enhancement. Visual appearance and numerical results confirm the validity of the approach. A simple coding scheme, designed to test the efficiency of the approach, gives good results.