Mihai Mitrea

Printed romanian modelling: a corpus linguistics based study with orthography and punctuation marks included

This paper is part of a larger study dedicated by the authors to the description of printed Romanian language as an information source. Here, the statistical investigation attempts to get an answer concerning the mathematical model of the language with orthography and punctuation marks included into the alphabet. To come out to an accurate result, the authors processed the information obtained out of multiple data sets sampled from a corpus linguistics, by using the following statistical inferences: probability estimation with multiple confidence intervals, test of the hypothesis that the probability belongs to an interval, and test of the equality between two probabilities. The second type statistical error probability involved in the tests was considered. The experimental results, which are new for printed Romanian, refer to the letter, digram and trigram statistical structure in a corpus linguistics of 93 books (about 50 millions characters).

Video watermarking based on spread spectrum and wavelet decomposition

An original statistical approach making it possible to accurately model video sequences in the wavelet domain as Gaussian laws is presented. By partitioning the wavelet coefficients into classes with independent elements, we rigorously handle the dependency existing among the successive frames in the video sequence. Further on, four statistical tests are applied to each class in the partition, with the following purposes: (1) to check up the Gaussian law; (2) to validate the data partition and (3) to reveal a homogeneity behaviour among the classes in the partition. Finally, the obtained results are fusioned so as to provide a global information characterising the whole sequence. At the same time, an a posteriori proof concerning an ergodicity behaviour for video sequences is obtained. We integrated these results within a robust video watermarking scheme. The mark is generated according to a CDMA (Code Division Multiple Access) procedure, starting from a 64 bit message (a serial number, a logo, etc). The embedding procedure is a weighted addition of the watermark into the wavelet coefficients featuring the Gaussian behaviour. The detection procedure is based on matched filters, the optimality of which is ensured under the considered framework. The experiments feature firm results concerning all the requirements stated nowadays: obliviousness, transparency, robustness, and probability of false alarm.

Wavelet versus DCT-based spread spectrum watermarking of image databases

This paper addresses the issue of oblivious robust watermarking, within the framework of colour still image database protection. We present an original method which complies with all the requirements nowadays imposed to watermarking applications: robustness (e.g. low-pass filtering, print & scan, StirMark), transparency (both quality and fidelity), low probability of false alarm, obliviousness and multiple bit recovering. The mark is generated from a 64 bit message (be it a logo, a serial number, etc.) by means of a Spread Spectrum technique and is embedded into DWT (Discrete Wavelet Transform) domain, into certain low frequency coefficients, selected according to the hierarchy of their absolute values. The best results were provided by the (9,7) bi-orthogonal transform. The experiments were carried out on 1200 image sequences, each of them of 32 images. Note that these sequences represented several types of images: natural, synthetic, medical, etc. and each time we obtained the same good results. These results are compared with those we already obtained for the DCT domain, the differences being pointed out and discussed.

2D approaches to 3D watermarking: state of the art and perspectives

With the advent of the Information Society, video, audio, speech, and 3D media represent the source of huge economic benefits. Consequently, there is a continuously increasing demand for protecting their related intellectual property rights. The solution can be provided by robust watermarking, a research field which exploded in the last 7 years. However, the largest part of the scientific effort was devoted to video and audio protection, the 3D objects being quite neglected. In the absence of any standardisation attempt, the paper starts by summarising the approaches developed in this respect and by further identifying the main challenges to be addressed in the next years. Then, it describes an original oblivious watermarking method devoted to the protection of the 3D objects represented by NURBS (Non uniform Rational B Spline) surfaces. Applied to both free form objects and CAD models, the method exhibited very good transparency (no visible differences between the marked and the unmarked model) and robustness (with respect to both traditional attacks and to NURBS processing).

Digital image protection by means of cryptographic mixing transformations

A lot of research studies have been devoted to image protection, either by cryptographic methods (e.g. Opt. Eng. 35, Sept. 1996) or by watermarking techniques (e.g. Proc. SPIE Vol. 3971). In a previous study, the authors reconsidered and improved the cryptographic mixing transformations proposed by CE Shannon for natural language, obtaining a strong cipher for images, as well. The present paper goes deeply inside of the image protection problem: (1) by presenting some variants for the cryptographic mixing transformations which are good even when burst errors appear in the cryptogram; (2) by enabling the use of an m-gram substitution in the mixing functions; (3) by advancing a bridge between cryptographic methods and watermarking techniques. The illustrations are obtained out of processing: (a) computer-simulated random images obeying different probability laws and autocorrelation functions, (b) natural images, and (c) test images.

Cryptographic mixing transformations for image applications

Several image encryption methods have been recently considered. This paper presents a new image enciphering method starting from the mixing transformations suggested by C.E. Shannon for natural language. There are two ways in which images are involved here: first, as a means of visual perception of the mixing transformation features (the very sophisticated diffusion and confusion); secondly, as the application field itself. Both the theoretical and experimental aspects pointed out here show the high quality of this method of image enciphering. The method illustrates were made either for simulated images with different probability laws and autocorrelation functions, or for natural images.

Wavelet based mobile video watermarking: spread spectrum vs. informed embedding

The cell phone expansion provides an additional direction for digital video content distribution: music clips, news, sport events are more and more transmitted toward mobile users. Consequently, from the watermarking point of view, a new challenge should be taken: very low bitrate contents (e.g. as low as 64 kbit/s) are now to be protected. Within this framework, the paper approaches for the first time the mathematical models for two random processes, namely the original video to be protected and a very harmful attack any watermarking method should face the StirMark attack. By applying an advanced statistical investigation (combining the Chi square, Ro, Fisher and Student tests) in the discrete wavelet domain, it is established that the popular Gaussian assumption can be very restrictively used when describing the former process and has nothing to do with the latter. As these results can a priori determine the performances of several watermarking methods, both of spread spectrum and informed embedding types, they should be considered in the design stage.