Active contours

Majid Mirmehdi, Xianghua Xie, Ronghua Yang

Active contours finding boundaries in the brainActive contour models, commonly known as snakes, have been widely used for object localisation, shape recovery, and visual tracking due to their natural handling of shape variations. The introduction of the Level Set method into snakes has greatly enhanced their potential in real world applications.

Since 2002, we have developed some novel active contour models. The first one aims to bridge (image gradient) boundary based approach and region-based approach. In this work, a level set based geometric snake, enhanced for more tolerance towards weak edges and noise, is introduced. It is based on the principle of the conjunction of the traditional gradient flow forces with new region constraints. We refer to this as the Region-aided Geometric Snake or RAGS. The image gradient provides local information of object boundaries, while the region information offers global definition of boundaries. In this framework, the region constrains can be conveniently customerised and plugged into the snake model.

The second model, called Charged Contour Model (CCM),is a migration of Charged Particle Model (CPM) into the active contour framework. The basic idea is to introduce particle dynamics into contour based deformable models. CCM performs better than CPM in the sense that it guarantees closed contours, i.e. it eliminates the ambiguities in contour reconstruction. Also, CCM is much more efficient. In comparison to geodesic snake, CCM is more robust to weak edges and less sensitive to noise interference.

The third model, CACE (Charged Active Contour model based on Electrostatics), is a further development of the CCM. The snake, implicitly embedded in level sets, propagates under the joint influence of a boundary attraction force and a boundary competition force. Its vector field dynamically adapts by updating itself when a contour reaches a boundary (which differs from CCM). The model is then more invariant to initialisation and possesses better convergence abilities. Analytical and comparative results are presented on synthetic and real images.

MAC model is a result of our most recent effort in developing new active contour models. The proposed external force field that is based on magnetostatics and hypothesized magnetic interactions between the active contour and object boundaries. The major contribution of the method is that the interaction of its forces can greatly improve the active contour in capturing complex geometries and dealing with difficult initializations, weak edges and broken boundaries. The proposed method is shown to achieve significant improvements when compared against six well-known and state-of-the-art shape recovery methods, including the geodesic snake, the generalized version of GVF snake, the combined geodesic and GVF snake, and the charged particle model.

For more information, please see our active contours site.

Error resilience and transport

The Group’s relationship with the Communication Systems and Networks Group has produced a body of successful research into the reliable transport of video. The group has proposed a number of error resilient methods which reduce the propagation of errors and conceal, rather than correct them.

Early work (EU FP4 Project “WINHOME”) found that error resilience methods, based on EREC could be combined with adaptive packetisation strategies and data partitioning to provide a robust MPEG-2 transport for in-home TV distribution. WINHOME provided the first European demonstration of robust video transport over WLANs, highlighting the weakness of media-unaware systems and the potential of selective reuse of corrupted packets.

In the EPSRC funded project SCALVID (GR/L43596/01) robust and scalable video coding schemes for heterogeneous communication systems were investigated. In particular the Group investigated a new coding approach based on matching pursuits. SCALVID significantly reduced the complexity of matching pursuits through a hierarchical (primitive operator) correlator structure (patented by NDS) and through the optimisation of basis function dictionaries. This work was widely cited internationally. SCALVID was first to show that matching pursuits can form the basis of an inherently robust coding system.

With BT and JISC funding, JAVIC (Joint Audio Visual Internet Coding) investigated packet-loss-robust internet video coding for multicast applications. Using H.263, reliable streaming with up to 10% packet loss was demonstrated, by combining cross packet FEC, prioritisation and judicious reference frame selection. Following this, the 3CResearch-ROAM4G project, delivered a novel 3-loop Multiple Description Coding scheme which, with minimal redundancy, provides highly robust video transmission over congestion-prone networks (Figure 2). Extended recently to exploit path-diversity in MIMO video systems this has, for the first time, shown that MDC with spatial-multiplexing can deliver up to 8dB PSNR improvement over corresponding SDC systems. ROAM4G also produced a 3D wavelet embedded coder which competes well with MPEG4-SVC and which additionally provides excellent congestion management. Trials are underway with Thales Research.

In the EU FP5 WCAM project (Wireless Cameras and Seamless Audiovisual Networking), the Group in collaboration with Thales Communications (France) and ProVision Communications (UK) produced a wireless H.264 platform incorporating a new spatio-temporal error concealment algorithm, which provides substantial gains (up-to 9dB PSNR improvement over H.264-JM) with up to 20% packet loss (Figure 3). This was singled out by the reviewers and has been patented and successfully licensed. WCAM has also provided an understanding of link adaptation (switching between a range of modulation and FEC schemes according to channel conditions). Realising that throughput based switching metrics are inherently flawed for video, new quality-derived metrics were developed which substantially outperform existing methods.

Work on High Definition coding and transport has progressed further under the Group’s participation in the EU FP5 MEDIANET project where pre- and post-processing algorithms have been developed. Finally, the 3CResearch-project VISUALISE integrated much of the above work into a live-viewing infrastructure where video compression and streaming technology are efficiently deployed over wireless broadband networks in difficult environments. This collaboration between BT Broadcast, Inmarsat, Node, ProVision, U4EA and ISC has developed a way for spectators at large-scale live events to have near real-time access to events as they unfold via portable terminals for an enhanced experience.