Frederik Verbist

Distributed coding of endoscopic video

Triggered by the challenging prerequisites of wireless capsule endoscopic video technology, this paper presents a novel distributed video coding (DVC) scheme, which employs an original hash-based side-information creation method at the decoder. In contrast to existing DVC schemes, the proposed codec generates high quality side-information at the decoder, even under the strenuous motion conditions encountered in endoscopic video. Performance evaluation using broad endoscopic video material shows that the proposed approach brings notable and consistent compression gains over various state-of-the-art video codecs at the additional benefit of vastly reduced encoding complexity.

Wyner-Ziv video coding for wireless lightweight multimedia applications

Wireless video communications promote promising opportunities involving commercial applications on a grand scale as well as highly specialized niche markets. In this regard, the design of efficient video coding systems, meeting such key requirements as low power, mobility and low complexity, is a challenging problem. The solution can be found in fundamental information theoretic results, which gave rise to the distributed video coding (DVC) paradigm, under which lightweight video encoding schemes can be engineered. This article presents a new hash-based DVC architecture incorporating a novel motion-compensated multi-hypothesis prediction technique. The presented method is able to adapt to the regional variations in temporal correlation in a frame. The proposed codec enables scalable Wyner-Ziv video coding and provides state-of-the-art distributed video compression performance. The key novelty of this article is the expansion of the application domain of DVC from conventional video material to medical imaging. Wireless capsule endoscopy in particular, which is essentially wireless video recording in a pill, is proven to be an important application field. The low complexity encoding characteristics, the ability of the novel motion-compensated multi-hypothesis prediction technique to adapt to regional degrees of temporal correlation (which is of crucial importance in the context of endoscopic video content), and the high compression performance make the proposed distributed video codec a strong candidate for future lightweight (medical) imaging applications.

Progressively refined wyner-ziv video coding for visual sensors

Wyner-Ziv video coding constitutes an alluring paradigm for visual sensor networks, offering efficient video compression with low complexity encoding characteristics. This work presents a novel hash-driven Wyner-Ziv video coding architecture for visual sensors, implementing the principles of successively refined Wyner-Ziv coding. To this end, so-called side-information refinement levels are constructed for a number of grouped frequency bands of the discrete cosine transform. The proposed codec creates side-information by means of an original overlapped block motion estimation and pixel-based multihypothesis prediction technique, specifically built around the pursued refinement strategy. The quality of the side-information generated at every refinement level is successively improved, leading to gradually enhanced Wyner-Ziv coding performance. Additionally, this work explores several temporal prediction structures, including a new hierarchical unidirectional prediction structure, providing both temporal scalability and low delay coding. Experimental results include a thorough evaluation of our novel Wyner-Ziv codec, assessing the impact of the proposed successive refinement scheme and the supported temporal prediction structures for a wide range of hash configurations and group of pictures sizes. The results report significant compression gains with respect to benchmark systems in Wyner-Ziv video coding (e.g., up to 42.03% over DISCOVER) as well as versus alternative state-of-the-art schemes refining the side-information.

Encoder-driven rate control and mode decision for distributed video coding

To provide low-complexity encoding for video in unidirectional or offline compression scenarios, this paper proposes an efficient feedback-channel-free distributed video coding architecture featuring a novel encoder-driven rate control scheme in tandem with a designated mode selection process. To this end, the encoder features a novel low-complexity motion estimation technique to approximate the side-information (SI) available at the decoder. Then, a SI-dependent correlation channel estimation between the approximated SI and the original frames is used to derive the theoretically required rate for successful Slepian-Wolf (SW) decoding. Based on the evaluation of the expected trade-off between the estimated required coding rate and the estimated distortion outcome, a novel encoder-side mode decision module assigns a different coding mode to distinct portions of the coded frames. In this context, skip, intra and SW coding modes are supported. To reduce the effect of underestimation, the final SW rate is adjusted upwards using a novel rate formula. Additionally, a successive SI refinement technique is exploited at the decoder to decrease the number of SW decoding failures. Experimental results illustrate the benefit of the different coding options and show similar or superior compression performance with respect to the feedback-based DISCOVER benchmark system. Finally, the low-complexity encoding characteristics of the proposed system are confirmed, as well as the beneficial impact of the proposed scheme on the decoding complexity.

Transform-domain Wyner-Ziv video coding for 1K-pixel visual sensors

Efficient coding of extremely low-resolution video has received a mere shadow of the attention given to their high-definition counterpart. However, low-resolution video still proves suitable for video processing and may enable the design of advanced video-based applications in a cost-effective manner. Indeed, visual sensors with modest frame resolutions can be designed to be small, low-cost and low-power. This paper introduces a novel efficient codec, specifically designed to handle extreme low-resolution video captured by such visual sensors. By applying Wyner-Ziv coding principles, the encoding complexity is reduced since temporal redundancy is exploited at the decoder. The proposed system outperforms H.264/AVC Intra and is therefore particularly attractive to wireless, resource-constraint low-resolution visual sensor applications by reducing the amount of transmitted data at a reduced computational cost.

Joint DC coefficient band decoding and motion estimation in Wyner-Ziv video coding

In contrast to traditional predictive coding, Wyner-Ziv video coding enables low-cost encoding architectures, in which the computationally expensive tasks for performing motion estimation are shifted to the decoder-side. In Wyner-Ziv video coding, side-information generation is a key aspect profoundly affecting the compression capacity of the system. This paper presents a novel technique which enables side-information refinement after DC coefficient band decoding in a transform-domain Wyner-Ziv video codec. The proposed side-information refinement approach performs overlapped block motion estimation and compensation, utilizing multi-hypothesis pixel-based prediction. The experimental results show that the presented Wyner-Ziv video codec incorporating the proposed technique yields significant and systematic compression gains of up to 23.22% with respect to the state-of-the-art DISCOVER codec.

Iterative Wyner-Ziv decoding and successive side-information refinement in feedback channel-free hash-based distributed video coding

This work presents a novel rate control scheme that suppresses the need for a feedback channel in hash-based distributed video coding (DVC) architectures. Our state-of-the-art DVC schemes generate side-information (SI) at the decoder by means of hash-based overlapped block motion estimation followed by probabilistic motion compensation, where key frames or previously decoded non-key frames, called Wyner-Ziv frames, are used as references. These DVC systems employ powerful low-density parity-check accumulate codes to code the Wyner-Ziv frames in the transform domain. Our previous DVC architectures use a classical decoder-driven rate control scheme with a feedback channel. Specifically, chunks of accumulated syndrome bits are sent from the encoder to the decoder upon request from the latter until successful decoding is achieved. In order to suppress the feedback channel, the encoder of the DVC system, proposed in this work, approximates the SI available to the decoder using a novel low complexity SI generation technique. Subsequently, the conditional probabilities of the original Wyner-Ziv frames, given the approximation of the SI at the encoder, are used to generate an estimate of the required rate for channel decoding. Hence the presence of a feedback channel is evaded. Additionally, the proposed feedback channel-free DVC system is equipped with advanced reconstruction techniques to reduce the impact of failed channel decoding. In this context, our DVC architecture features iterative refinement of the SI at the decoder. The latter allows for reattempting to decode Wyner-Ziv data for which the channel decoding failed in previous decoding steps when only a lower quality version of the SI was available. Experimental results show competitive compression performance of our novel feedback channel-free hash-based DVC system with respect to the feedback channel-based benchmark in DVC.

Efficient hash-driven Wyner-Ziv video coding for visual sensors

Recent advances in wireless visual sensor technology, have been calling for innovative architectures realizing efficient video coding under stringent processing and energy restrictions. Driven by profound findings in network information theory, Wyner-Ziv video coding constitutes a suitable paradigm for video sensor networks. This work presents a novel hash-driven Wyner-Ziv video coding architecture for visual sensors, which coarsely encodes a low resolution version of each Wyner-Ziv frame to facilitate accurate motion-compensated prediction at the decoder. The proposed method for side-information generation comprises hash-based multi-hypothesis pixel-based prediction. Once critical Wyner-Ziv information is decoded, the derived dense motion field is further enhanced. Experimental validation illustrates that the proposed hash-driven codec achieves significant compression gains with respect to state-of-the-art Wyner-Ziv video coding, even under demanding conditions.

Demo: depth estimation for 1K-pixel stereo visual sensors

This demonstrator explores the applicability of depth estimation based on stereo video with extremely low resolution, i.e., 30×30 pixels. To handle this resolution, a disparity estimation technique, composed of local correlation-based matching of two low-resolution stereo images followed by segmentation-driven post-processing, is proposed. The demonstrator includes a setup where a stereo visual sensor is connected to a laptop computer, running the proposed depth estimation method in realtime and displaying the resulting disparity maps. In addition, an interface is available to give the user control over the proposed algorithm parameters. To illustrate the superior performance, the results of proposed method can be readily compared to disparity maps generated using a typical global correlation-based depth estimation algorithm.

Efficient intra-frame video coding for low resolution wireless visual sensors

This paper proposes a low-complexity intra-frame video coding system for very low resolution sequences. The application focuses on power constrained wireless visual sensors, recording extremely low-resolution video, namely, 30×30 pixels. In the proposed framework, high-performance video coding techniques such as block-based transforms, intra prediction and entropy coding are employed. The proposed system features several different configurations, all of which are evaluated both in terms of coding efficiency and computational complexity. Based on the experimental results, the best configuration is identified. Experimental results show that the proposed configuration achieves better coding performance than the H.264/AVC Intra frame codec, at only 23.7% of its encoding complexity.