Lossless compression of binary image descriptors for visual sensor networks

Nowadays, visual sensor networks have emerged as an important research area for distributed signal processing, with unique challenges in terms of performance, complexity, and resource allocation. In visual sensor networks, the energy consumption must be kept low to extend the lifetime of each battery-operated camera node. Thus, considering the large amount of data that visual sensors can generate, all the sensing, processing, and transmission operations must be optimized considering strict energy constraints. In this paper, camera nodes sense the visual scene but instead of transmitting the pixel coded representation, which demands high computation and bandwidth, a compact but yet rich visual representation is created and transmitted. This representation consists of discriminative visual features offering tremendous potential for several image analysis tasks. From all low-level image features available, the novel class of binary features, very fast to compute and match, are well suited for visual sensor networks. In this paper, lossless compression of binary image features is proposed to further lower the energy and bandwidth requirements. The coding solution exploits the redundancy between descriptors of an image by sorting the descriptors and applying DPCM and arithmetic coding. Experimental results show improvements up to 32% in terms of bitrate savings without any impact in the final image retrieval task accuracy.