Marc Gazalet

Reduction of blocking effect in DCT-coded images based on a visual perception criterion

Abstract Blocking effect constitutes one of the main drawbacks of the actual DCT-based compression methods. We propose in this paper a new block reduction technique; it is based on a space-variant non-linear filtering operation of the blocking artifacts present in the image to be reconstructed. To account for the perceptual importance of the distortion, the amount of smoothing is adapted to the visibility of the blocking effect. A visibility parameter is computed for each artifact using the psychovisual properties of the human visual system (HVS). The postprocessing algorithm is in conformity with actual existing compression standards; it provides a way to greatly reduce the artifacts without degrading high-frequency information of the original image. First the proposed method is described and then experimental results are presented, showing the effectiveness of the correction.

Acoustic shear wave propagation in Paratellurite with reduced spreading

An original configuration of propagation of acoustic shear wave in Paratellurite slightly off the [110] axis in the (001) plane is described. It allows greatly reducing the spreading of the acoustic energy. The main characteristics of an acousto‐optic interaction with this acoustic shear wave are then discussed. An experimental validation based on the schlieren method is finally proposed. It shows the effective reduction of the acoustic beam diffraction of the shear wave propagating with a 5° angular tilt. It is compared with the acoustic diffraction obtained for a shear wave propagating in TeO2 along the [110] axis without angular tilt.

Blocking artifact reduction of DCT coded image sequences using a visually adaptive postprocessing

The blocking effect is known to be the major degradation in block based video compression techniques. In this paper, we describe an adaptive postprocessing algorithm for the reduction of blocking effect in video coded sequences. It is based on a visual model for the prediction of blockiness visibility. The postfiltering operation is based on a 3D adaptive filter. A space-variant spatial filter is first used to smooth the pixels at block boundaries where the blocking effect is highly visible. It is followed by motion compensated nonlinear filtering in the temporal domain. Experimental results are presented showing that the proposed algorithm can remove the blocking effect while keeping image sharpness.

Univariant assessment of the quality of images

To evaluate the quality of images, most methods com- pare a degraded image to a perfect reference. Nevertheless in many cases, a reference does not exist. We propose an original univariant (i.e., without a reference) method based on the use of artificial neu- ral networks. The principle behind it is to first teach a neural network to assess image quality using images taken from a pool of known examples, then use it to assess the quality of unknown images. The defects considered are compression artifacts, ringing, local singu- larities, etc. To simplify, only images with defects that are not mixed with one another were first used. Two illustrative examples are pre- sented: assessment of the quality of JPEG compressed images and detection of local defects. The quality of the images is assessed without a reference and with error less than 6%-7% compared to the bivariant method that was learned. Our method can even be used to model some very simple visual comportment.

QoS considerations for DMT-based ADSL and VDSL systems

Thanks to their high bandwidth capability, asymmetric digital subscriber lines (ADSL) and very high speed DSL (VDSL) are access technologies that permit the transmission of several applications simultaneously on telephone subscriber lines. Considering that these applications may require a different quality of service (QoS), and particularly different bit error rates (BER), for transmission, this paper addresses the problem of providing simultaneously two BERs for transmission over a DMT-based ADSL or VDSL link. Both coded and uncoded systems are considered.

A postprocessing technique for block effect elimination using a perceptual distortion measure

One of the drawbacks of the discrete cosine transform (DCT) is visible block boundaries due to coarse quantization of the coefficients. In this paper, an algorithm for the reduction of blocking artifacts is presented. The proposed method allows to produce higher quality reconstructed images by adaptively filtering the video signal according to the noise visibility. A visual model is therefore defined for predicting the block edge visibility across each picture of the coded sequence. This model that accounts for the perceptual characteristics of the block distortion is described. Experimental results are presented for low bit-rate coded sequences. They show that the postfiltering operation yields significant results with enhanced visual quality.

A method for analyzing the performance of comb-type pilot-aided channel estimation in power line communications

In this paper, we propose an original method for analyzing the performance of comb-type pilot-aided channel estimation for power line communications. Our method is based on a unified mathematical decomposition of the channel estimation error, which takes both the signal-to-noise ratio of the pilot sub-carrier locations and the interpolation kernel used into account. We demonstrate that the channel estimation error is composed of two terms that evolve differently according to the pilot spacing and the interpolation technique used. The error due to additive noise over channel transmission is formulated by a simple easily used analytic expression. Our results for some practical situations show that good performances can be achieved with a simple linear interpolation.

An adaptive postprocessing technique for the reduction of color bleeding in DCT-coded images

We propose a new postprocessing technique developed specifically for the reduction of color bleeding in DCT-coded color images. The proposed algorithm, based on a complete analysis of this particular kind of distortion, is used to identify and reduce color bleeding artifacts while attempting to preserve the sharpness of the reconstructed images. Simulation results show that the method successfully reduces color bleeding in reconstructed images and improves the visual quality, both objectively and subjectively.

Measurement of the thickness of thin layers by ultrasonic interferometry

An ultrasonic interferometer working in a pulsed mode is described in this paper. It allows for the measurement of coating thicknesses as thin as 5 μm with a 5% precision over different substrates at a very high rate (up to 1000 times per second). The optimal conditions for this interferometric measurement are defined theoretically and the probe characteristics have been optimized technologically. This, together with the design of a very large frequency bandwidth (from 90 to 510 MHz) electronic setup, leads to interesting performances. The advantages of the system for achieving thickness measurements are discussed and comparisons are made with other systems.

High dynamic range, bifrequency acousto-optic modulator

Multifrequency acousto-optic interaction is limited mainly by the two-tone, third-order intermodulation products. The tangential phase-matching (TPM) anisotropic interaction can greatly reduce the phase-grating intermodulations. We consider the two operating modes, high or low frequency, for the slow shear wave propagating in paratellurite with an angular tilt from the [110] axis. As two different angles lead to the same central operating frequency, we show that the low-frequency configuration allows one to greatly increase the dynamic range, compared with that obtained in the high-frequency mode. For experimental validation, two bifrequency acousto-optic modulators have been realized and tested. For a central frequency in the low-frequency configuration and a diffraction efficiency , the dynamic range is shown to be enhanced by 5 dB.