Guido M. Schuster

Simultaneous optimal boundary encoding and variable-length code selection

This paper describes efficient and optimal encoding and representation of object contours. Contours are approximated by connected second-order spline segments, each defined by three consecutive control points. The placement of the control points is done optimally in the rate-distortion (RD) sense and jointly with their entropy encoding. We utilize a differential scheme for the rate and an additive area-based metric for the distortion to formulate the problem as a Lagrangian minimization. We investigate the sensitivity of the resulting operational RD curve on the variable length codes used and propose an iterative procedure arriving at the entropy representation of the original boundary for any given rate-distortion tradeoff.

An efficient boundary encoding scheme which is optimal in the rate distortion sense

A major problem in object oriented video coding is the efficient encoding of the shape information of arbitrarily shaped objects. Efficient shape coding schemes are also needed in encoding the shape information of video object planes (VOP) in the MPEG-4 standard. In this paper, we present an efficient method for the lossy encoding of object shapes which are given as 8-connect chain codes (Meier et al., 1997). We approximate the object shape by a second order B-spline curve and consider the problem of finding the curve with the lowest bit rate for a given distortion. The presented scheme is optimal, efficient and offers complete control over the trade-off between bit-rate and distortion. We present results with the proposed scheme using objects shapes of different sizes.

Shape coding using temporal correlation and joint VLC optimization

This paper investigates ways to explore the between frame correlation of shape information within the framework of an operationally rate-distortion (ORD) optimized coder. Contours are approximated both by connected second-order spline segments, each defined by three consecutive control points, and by segments of the motion-compensated reference contours. Consecutive control points are then encoded predictively using angle and run temporal contexts or by tracking the reference contour. We utilize a novel criterion for selecting global object motion vectors, which improves the efficiency. The problem is formulated as a Lagrangian minimization and solved using dynamic programming. Furthermore, we employ an iterative technique to remove dependency on a particular variable length code and jointly arrive at the ORD globally optimal solution and an optimized conditional parameter distribution.

A video compression scheme with optimal bit allocation between displacement vector field and displaced frame difference

We address the fundamental problem of optimally splitting a video sequence into two sources of information, the displaced frame difference (DFD) and the displacement vector field (DVF). We first consider the case of a lossless motion compensated video coder (MCVC) and derive a general dynamic programming (DP) formulation which results in an optimal tradeoff between the DVF and the DFD. We then consider the more important case of a lossy MCVC and present an algorithm which solves optimally the bit allocation between the rate and the distortion. This algorithm is based on Lagrangian relaxation and the DP approach introduced for the lossless MCVC. We then present an H.263-based MCVC which uses the proposed optimal bit allocation scheme and compare its results to H.263. As expected, the proposed coder is superior in the rate-distortion sense.

Inter mode vertex-based optimal shape coding

This paper investigates the problem of optimal lossy encoding of object contours in the inter mode. Contours are approximated by connected second-order spline segments, each defined by three consecutive control points. Taking into account correlations in the temporal direction, control points are chosen optimally in the rate-distortion sense. Applying motion to contours in the reference frame followed by the temporal context extraction, we predict the next control point location, given the previously encoded one. Based on the chosen differential encoding scheme and an additive MPEG-4 based distortion metric, the problem is formulated as Lagrangian minimization. We utilize an iterative procedure to jointly find the optimal solution and the associated DPCM parameter probability mass functions.

Rate-distortion optimal boundary encoding using an area distortion measure

In this paper an optimal boundary encoding algorithm in the rate-distortion sense is proposed. Second-order B-spline curves are used to model object boundaries. An additive area distortion measure between the original boundary and its approximation is employed in the optimization process. The problem is formulated in a Directed Acyclic Graph (DAG) paradigm, and the shortest path solution is used to optimally select control point locations of the B-spline curve approximation based on the desired rate-distortion tradeoff.

Operational rate-distortion optimal bit allocation between shape and texture for MPEG-4 video coding

MPEG-4 is the first multimedia standard that supports the decoupling of a video object into object shape and object texture information, which consequently brings up the optimal encoding problem for object-based video. In this paper, we present an operational rate-distortion optimal bit allocation scheme between shape and texture for MPEG-4 encoding. Our approach is based on the Lagrange multiplier method, while the adoption of dynamic programming techniques enables its higher efficiency over the exhaustive search algorithm. Our work will not only benefit the further study of joint shape and texture encoding, but also make possible the deeper study of optimal joint source-channel coding of object-based video.

An optimal shape encoding scheme using skeleton decomposition

This paper presents an operational rate-distortion (ORD) optimal approach for skeleton-based boundary encoding. The boundary information is first decomposed into skeleton and distance signals, by which a more efficient representation of the original boundary results. Curves of arbitrary order are utilized for approximating the skeleton and distance signals. For a given bit budget for a video frame, we solve the problem of choosing the number and location of the control points for all skeleton and distance signals and for all boundaries within a frame, so that the overall distortion is minimized. The problem is solved with the use of Lagrangian relaxation and a shortest path algorithm in a 4D directed acyclic graph (DAG) we propose. By defining a path selection pattern, we reduce the computational complexity of the 4D DAG shortest path algorithm from O(N/sup -5/) to O(N/sup -4/), where N is the number of admissible control points for a skeleton. A suboptimal solution is also presented for further reducing the computational complexity of the algorithm to O(N/sup -2/). The proposed algorithm outperforms experimentally other competing algorithms.

Exploiting temporal correlation in shape coding

This paper investigates ways to explore the between frame correlation of shape information within the framework of an operationally rate-distortion (ORD) optimal coder. Contours are approximated both by connected second-order spline segments, each defined by three consecutive control points, and by segments of the motion-compensated reference contours. Consecutive control points are then encoded predictively using angle and run temporal contexts. We utilize a novel criterion for selecting global object motion vectors, which further improves efficiency. Formulating this problem as Lagrangian minimization, we employ an iterative technique to remove dependency on a particular VLC and jointly arrive at the ORD optimal solution and its underlying conditional parameter distribution.

Optimal quad-tree-based motion estimator

In this paper we propose an optimal quad-tree (QT)-based motion estimator for video compression. It is optimal in the sense that for a given bit budget for encoding the displacement vector field (DVF) and the QT segmentation, the scheme finds a DVF and a QT segmentation which minimizes the energy of the resulting displaced frame difference (DFD). We find the optimal QT decomposition and the optimal DVF jointly using the Lagrangian multiplier method and a multilevel dynamic program. The resulting DVF is spatially inhomogeneous since large blocks are used in areas with simple motion and small blocks in areas with complex motion. We present results with the proposed QT-based motion estimator which show that for the same DFD energy the proposed estimator uses about 30% fewer bits than the commonly used block matching algorithm.