The increased popularity of multimedia applications places a great demand on efficient data storage and transmission techniques. Network communication, especially over a wireless network, can easily be intercepted and must be protected from eavesdroppers. Unfortunately, encryption and decryption are slow and it is often difficult, if not impossible, to carry out real-time secure image and video communication and processing. Methods have been proposed to combine compression and encryption together to reduce the overall processing time [3, 4, 12, 18, 20], but they are either insecure or too computationally intensive. We propose a novel solution, called partial encryption, in which a secure encryption algorithm is used to encrypt only part of the compressed data. Partial encryption is applied to several image and video compression algorithms in this paper. Only 13%--27% of the output from quadtree compression algorithms [13, 17, 29, 30, 31, 32] is encrypted for typical images, and less than 2% is encrypted for 512 \Theta 512 images compressed by the SPIHT algorithm [26]. The results are similar for video compression, resulting in a significant reduction in encryption and decryption time. The proposed partial encryption schemes are fast, secure, and do not reduce the compression performance of the underlying compression algorithm. EDICS Number: SP 7.8 This research is supported in part by the Motorola Wireless Data Group and the Canadian Natural Sciences and Engineering Research Council under Grant OGP9198 and Postgraduate Scholarship. y Presently at Department of Computer Science, University of Waterloo. z To whom correspondence should be addressed. 1 1

Abstract—We consider a situation where a video sequence is to be compressed and transmitted over a wireless channel. Our goal is to limit the amount of distortion in the received video sequence, while minimizing transmission energy. To accomplish this goal, we consider error resilience and concealment techniques at the source coding level, and transmission power management at the physical layer. We jointly consider these approaches in a novel framework. In this setting, we formulate and solve an optimization problem that corresponds to minimizing the energy required to transmit video under distortion and delay constraints. Experimental results show that simultaneously adjusting the source coding and transmission power is more energy efficient than considering these factors separately. Index Terms—Error concealment, error resilience, expected distortion, optimal mode selection, power and rate control. I.

The loss and delay experienced by packets travelling along an Internet network path are mainly governed by the characteristics of a bottleneck link, such as available data rate and queue size. In this work, we propose a framework for rate-distortion optimized packet scheduling with adaptive rate control for media streaming over bandwidth-constrained bottleneck links. The framework computes optimal packet schedules while continuously adapting its instantaneous rate to the following three factors: the available data rate and the current queue size on the bottleneck link, and the congestion that packets transmitted under the schedules will create on the bottleneck link. Experimental results demonstrate that our framework does not lose in rate-distortion performance over rate-distortion optimized packet scheduling without strict rate control, while producing at the same time a much smoother instantaneous rate feeding the bottleneck queue. This in turn contributes to fairness to other flows sharing the bottleneck link and causes less variations in queue size, thereby avoiding queue overflow and unnecessarily long packet delays on the bottleneck link. 1.

In this paper, we address the problem of the efficient encoding of object boundaries. This problem is becoming increasingly important in applications such as content-based storage and retrieval, studio and television postproduction, and mobile multimedia applications. The MPEG-4 visual standard will allow the transmission of arbitrarily shaped video objects. The techniques developed for shape coding within the MPEG-4 standardization effort are described and compared first. A framework for the representation of shapes using their contours is presented next. Such representations are achieved using curves of various orders, and they are optimal in the rate-distortion sense. Last, conclusions are drawn

In object-based video, the encoding of the video data is decoupled into the encoding of shape, motion and texture information, which enables certain functionalities like content-based interactivity and scalability. However, the problem of how to jointly encode these separate signals to reach the best coding efficiency has never been solved thoroughly. In this paper, we present an operational ratedistortion optimal bit allocation scheme that provides a solution to this problem. Our approach is based on the Lagrangian relaxation and dynamic programming. Experimental results indicate that the proposed optimal encoding approach has considerable gains over an ad-hoc method without optimization. Furthermore the proposed algorithm is much more efficient than exhaustive search. 1.

In this paper, we review a general framework for the optimal bit allocation among dependent quantizers based on the minimum maximum (MINMAX) distortion criterion. Pros and cons of this optimization criterion are discussed and compared to the well-known Lagrange multiplier method for the minimum average (MINAVE) distortion criterion. We argue that, in many applications, the MINMAX criterion is more appropriate than the more popular MINAVE criterion. We discuss the algorithms for solving the optimal bit allocation problem among dependent quantizers for both criteria and highlight the similarities and differences. We point out that any problem which can be solved with the MINAVE criterion can also be solved with the MINMAX criterion, since both approaches are based on the same assumptions. We discuss uniqueness of the MINMAX solution and the way both criteria can be applied simultaneously within the same optimization framework. Furthermore, we show how the discussed MINMAX approach can be directly extended to result in the lexicographically optimal solution. Finally, we apply the discussed MINMAX solution methods to still image compression, intermode frame compression of H.263, and shape coding applications.

We discuss new theoretical and experimental results on the Dynamic Rate Shaping (DRS) approach for transcoding compressed video bitstreams (MPEG-1, MPEG-2, MPEG-4, H.261 as well as JPEG). A set of low complexity algorithms for both constrained and unconstrained DRS are presented. We present the first extensive experimental study on the various DRS algorithms (causally optimal, memoryless, and rate-based) both in their constrained and generalized forms. The study proves the computational viability of the DRS approach to transcoding and identifies a range of rate shaping ratios for which it is better than requantization, both complexity-wise as well as in performance. We then substantiate the almost-optimal experimental performance of the memoryless algorithm by analyzing the behavior of the DRS problem assuming a first order Autoregressive source. By deriving the statistical and rate-distortion characteristics of different components of the inter-frame rate shaping problem, we offer an explanation as to why the set of optimal breakpoint values for any frame is somewhat invariant to the accumulated motion compensated shaping error from past frames. This result is significant as it opens up the way to construct much simpler memoryless algorithms that give minimal penalty in achieved quality, not just for this, but possibly other types of algorithms. Of equal, if not more, importance is the very first use of matrix perturbation theory for tracking the spectral behavior of the auto-correlation matrix of the source signal and the motion residual it yields. 1 1

In this paper, we present fast and efficient methods for the lossy encoding of object boundaries which are given as 8-connect chain codes. We approximate the boundary by a polygon and consider the problem of finding the polygon which leads to the smallest distortion for a given number of bits. We also address the dual problem of finding the polygon which leads to the smallest bit rate for a given distortion. We consider two different classes of distortion measures. The first class is based on the maximum operator and the second class is based on the summation operator. For the first class, we derive a fast and optimal scheme which is based on a shortest path algorithm for a weighted directed acyclic graph. For the second class we propose a solution approach which is based on the Lagrange multiplier method, which uses the above mentioned shortest path algorithm. Since the Lagrange multiplier method can only find solutions on the convex hull of the operational rate distortion function, w...

A major problem in object oriented video coding and MPEG-4 is the encoding of object boundaries. In our previous work, we presented efficient methods for the lossy encoding of object boundaries which were optimal in the rate distortion sense. In this paper, we extend our work to utilize both the original image and an initial segmentation to obtain the optimal shape representation based on the criteria we impose. The boundary detection and encoding problems are considered simultaneously. If there is low confidence on the location of the boundary, a large approximation error is allowed when encoding the boundary and vide versa. Experimental results demonstrate the effectiveness of the proposed algorithm.

Multimedia applications involving the transmission of video over communication networks are rapidly increasing in popularity. Such applications can greatly benefit from adapting video coding parameters to network conditions as well as adapting network parameters to better support the application requirements. These two dimensions can both be viewed as allocating source and network resources to improve video quality. In this paper, we highlight recent advances in optimal resource allocation for real-time video communications over unreliable and resource constrained communication channels. More specifically, we focus on point-to-point coding and delivery schemes in which the sequences are encoded on the fly. We present a high-level framework for resource-distortion optimization. The framework can be used for jointly considering factors across network layers, including source coding, channel resource allocation, and error concealment. For example, resources can take the form of transmission energy in a wireless channel, and transmission cost in a DiffServ-based Internet channel. This framework can be used to optimally trade off resource consumption with end-to-end video quality in packet-based video transmission. After giving an overview of this framework, we review recent work in two areas—energy efficient wireless video transmission and resource allocation for Internet-based applications. Keywords—Cross-layer design, energy efficient, error resilience, distortion estimation, internet video, wireless video. I.