Wolfgang Niehsen

Fast full-search block matching

A fast full-search block-matching algorithm is developed. The matching criterion is the sum of absolute differences or the mean-square error. The algorithm evaluates lower bounds for the matching criteria for subdivided blocks in order to reduce the number of search positions. It also uses the lower bounds for a fast calculation of the matching criterion for the remaining search positions. The computational complexity of the algorithm is evaluated and compared to the three-step search strategy. The search result of the algorithm is identical to the search result of the exhaustive search.

Covariance analysis of motion-compensated frame differences

The second-order statistics of motion-compensated frame differences in a low-bit-rate hybrid video coding scheme with overlapped block motion compensation are investigated. Based on the empirical covariance sequence, an adequate compound covariance model is developed. The prediction gain for motion-compensated frame differences is evaluated, and the performance of the discrete cosine transform for interframe transform coding is discussed.

Detection of close cut-in and overtaking vehicles for driver assistance based on planar parallax

Image processing is widely considered an essential part of future driver assistance systems. This paper presents a motion-based vision approach to initial detection of static and moving objects observed by a monocular camera attached to a moving observer. The underlying principle is based on parallax flow induced by all non-planar static or moving object of a 3D scene that is determined from optical flow measurements. Initial object hypotheses are created in regions containing significant parallax flow. The significance is determined from planar parallax decomposition automatically. Furthermore, we propose a separation of detected image motion into three hypotheses classes, namely coplanar, static and moving regions. To achieve a high degree of robustness and accuracy in real traffic situations some key processing steps are supported by the data of inertial sensors rigidly attached to our vehicle. The proposed method serves as a visual short-range surveillance module providing instantaneous object candidates to a driver assistance system. Our experiments and simulations confirm the feasibility and robustness of the detection method even in complex urban environment.

Boundary filters without DC leakage for paraunitary filter banks

The construction of boundary filters without DC leakage for two-channel paraunitary FIR filter banks is considered. The design procedure is based on orthogonal boundary filters which are optimal in a weighted mean square error sense in the Fourier domain and on Householder transformation of boundary filter matrices. Simulation results are presented for boundary filters based on minimum-phase Daubechies filters.

Energy compaction performance of paraunitary FIR filter banks for finite-length signals

The energy compaction performance of two-channel paraunitary finite impulse response (FIR) filter banks for finite-length signals is investigated. A detailed non-iterative design procedure for boundary filters which are optimal in a weighted mean square error (MSE) sense in the Fourier domain is presented. Simulation results are given for two-channel paraunitary FIR filter banks based on minimum-phase Daubechies filters and least-asymmetric Daubechies filters, respectively.

Fast full search block matching based on combined SAD and MSE measures

A new fast block matching algorithm is presented. The sum of absolute differences (SAD) and the mean square error (MSE) are used to find a suitable motion vector. A lower bound for both error measures is exploited to reduce the number of search positions and therefore the computational requirements. The error measures for the remaining search positions are calculated simultaneously so that the computational load for these calculations only slightly increases. The algorithm is compared to a fast full search block matching algorithm based on the same concept but only using the SAD or the MSE as the matching criterion. It is shown that the algorithm using both error measures combines the advantages of both algorithms using only on the SAD or the MSE.

On entropy coded and entropy constrained lattice vector quantization

Two lattice vector quantization methods are compared. The first, classical method uses Dirichlet domains of lattice points as quantization cells and assigns them reconstruction vectors minimizing the distortion. The second method uses the lattice as codebook but modifies the shapes of the quantization cells by searching for each input vector the mapping onto either the nearest lattice points or one of its neighbors, such that an error criterion subject to an entropy constraint is minimized. Both methods use entropy coding for the codevector indices and an entropy constrained global optimization scheme to find the best lattice scale. Examples demonstrate that the rate-distortion performances of both methods are nearly identical. Hence, especially for non-stationary sources the selection of one of these methods should be based on other criteria: while the first method requires the transmission of the new codebook, the decoder for the second method can permanently adapt itself without side information. The drawback is a higher search complexity for encoding.

Block motion estimation using orthogonal projection

A block motion estimation algorithm which is able to deal with varying gray levels along motion trajectories is introduced. Variations of the illuminating sources and shadows or reflections caused by moving objects are taken into account by the underlying scene model. Orthogonal projection is used to estimate appropriate scaling factors for the luminance components of the image sequence frames. An entropy constrained quantizer for the scaling factor estimates is developed. The proposed algorithm and the standard block matching algorithm are compared with respect to rate-distortion performance and computational load.

Simultaneous optimization of boundary filters and stationary filters for paraunitary FIR filter banks

The optimization of two-band paraunitary finite impulse response (FIR) filter banks for finite-length signals is considered. Boundary filters which are optimal in a weighted mean square error (MSE) sense in the Fourier domain and the associated stationary filters are optimized simultaneously. Simulation results reveal that the maximally flat Daubechies filters are nearly optimal with respect to energy compaction performance.

AR-modeling and low bit rate encoding of motion-compensated frame differences

The hybrid coding scheme is modified. The discrete cosine transform used for encoding displaced frame differences is replaced by a predefined set of transform matrices based on autoregressive models up to the fourth order. The autoregressive models are parameterized using reflection coefficients. It is shown that the coding efficiency can be improved though side information for the chosen transform has to be transmitted to the decoder.