A Survey of Image Compression Algorithms for Visual Sensor Networks <s> SPIHT-Based Schemes for VSN. <s> Among all algorithms based on wavelet transform and zerotree quantization, Said and Pearlman's (1996) set partitioning in hierarchical trees (SPIHT) algorithm is well-known for its simplicity and efficiency. This paper deals with the real-time implementation of SPIHT algorithm using DSP chip. In order to facilitate the implementation and improve the codec's performance, some relative issues are thoroughly discussed, such as the optimization of program structure to speed up the wavelet decomposition. SPIHT's high memory requirement is a major drawback for hardware implementation. In this paper, we modify the original SPIHT algorithm by presenting two new concepts-number of error bits and absolute zerotree. Consequently, the memory cost is significantly reduced. We also introduce a new method to control the coding process by number of error bits. Our experimental results show that the implementation meets common requirement of real-time video coding and is proven to be a practical and efficient DSP solution. <s> BIB001 </s> A Survey of Image Compression Algorithms for Visual Sensor Networks <s> SPIHT-Based Schemes for VSN. <s> In this paper, we proposed a novel scheme for error robust and energy efficient image transmission over wireless sensor networks. The innovations of proposed scheme are two folds: multiple bitstream image encoding to achieve error robust transmission and small fragment burst transmission to achieve efficient transmission. In the case of image encoding, a multiple bitstream image encoding is developed based on the decomposition of images in the wavelet domain. We first group wavelet coefficients into multiple trees according to parent-child relationship and then code them separately by SPIHT algorithm to form multiple bitstreams. We show that such decomposition is able to reduce error propagation in transmission. To further enhance the error robustness, unequal error protection strategy with RCPC/CRC is implemented. The coding rate is designed based on the bit error sensitivity of different bit planes. In the stage of image transmission, we divide the multiple bit streams into small fragments and transmit them in burst. We demonstrate that such transmission strategy matches well with the media access control layer protocol as well as the link layer protocol currently employed in wireless sensor networks. We achieve energy efficient transmission by saving energy consumed on control overhead and device switching from sleep to active. Experimental results show that the proposed scheme has not only high energy efficiency in transmission but also graceful degradation in PSNR performance over high BERs channels in terms of image reconstruction quality at the base station. <s> BIB002 </s> A Survey of Image Compression Algorithms for Visual Sensor Networks <s> SPIHT-Based Schemes for VSN. <s> This paper presents a very low-memory wavelet compression architecture for implementation in severely constrained hardware environments such as wireless sensor networks (WSNs). The approach employs a strip-based processing technique where an image is partitioned into strips and each strip is encoded separately. To further reduce the memory requirements, the wavelet compression uses a modified set-partitioning in hierarchical trees (SPIHT) algorithm based on a degree-0 zerotree coding scheme to give high compression performance without the need for adaptive arithmetic coding which would require additional storage for multiple coding tables. A new one-dimension (1D) addressing method is proposed to store the wavelet coefficients into the strip buffer for ease of coding. A softcore microprocessor-based hardware implementation on a field programmable gate array (FPGA) is presented for verifying the strip-based wavelet compression architecture and software simulations are presented to verify the performance of the degree-0 zerotree coding scheme. <s> BIB003 </s> A Survey of Image Compression Algorithms for Visual Sensor Networks <s> SPIHT-Based Schemes for VSN. <s> A multiview image compression framework for the Wireless Multimedia Sensor Network (WMSN) based on image stitching and SPIHT coding is proposed. This framework is designed to work with the mesh network topology that is very robust. In this framework, the images taken by neighboring sensors are stitched together with an image stitching technique to remove the overlap redundancy. The image stitching process will be carried out in certain intermediate nodes along the way towards a centralized decoder. This will help to conserve more power since the amount of data to be transmitted is reduced. In addition, the tree structure adapted by SPIHT is modified to improve the flexibility in coding the stitched image. Simulation results show that the use of image stitching with SPIHT coding can greatly remove the overlap and spatial redundancy. <s> BIB004
We start by the compression method proposed in BIB002 , where the basic design idea is drawn from the following observation. It is more efficient to send a very long bitstream in small decomposed fragments or bursts than their transmission as one entire block. That is, the suggested scheme in BIB002 uses waveletbased decomposition strategy to create multiple bitstream image encodings which are sent in small bursts. The wavelet coefficients are grouped into multiple trees and encoded separately using SPIHT algorithm. The unequal error protection method is also adopted in order to combat time-varying channel errors. Experimental results show that the proposed scheme has a good energy efficiency in transmission. Another work incorporating SPIHT as a compression tool is presented in BIB003 . The authors use a strip-based processing technique where an image is divided into strips which are encoded separately. Figure 5 shows the block diagram of this suggested method. First, a few lines of image data are wavelet decomposed by DWT module. The lifting-based 5/3 DWT is used for this purpose. After that, the wavelet coefficients are computed and then buffered in a strip buffer. Finally, the bitstream generated is transmitted. The proposed SPIHT coding eliminates the use of lists in its set-partitioning approach. The idea behind the framework developed in BIB004 is the use of image stitching in conjunction with SPIHT coding to remove the overlap and spatial redundancy. Image stitching can be defined as the process of combining multiple images with overlapping fields of view to create a segmented panorama or high-resolution image. Thus, the images taken by neighboring sensors are stitched together by certain intermediate nodes with an image stitching technique to remove the overlap redundancy. For compression purpose, a modified version of the SPIHT compression tool is used, which leads to the reduction in the amount of the transmitted data BIB004 . Implementing SPHIT on power-constrained devices, such as visual sensors, is an excellent idea. Its advantages over JPEG and EZW in terms of high compression ratio, less computational complexity, and low powerconsumption, as well as less complex implementation make it possible to play an interesting role in image compression for power-limited applications. An open research work should be the adaptation of the SPIHT algorithm to the power-constrained VSN. This can be performed by exploiting some ideas, like the substitution of lists by flags BIB001 to reduce the memory usage. An alternative idea is the use of wavelet lifting scheme instead of the convolutional based wavelet used by the original SPIHT BIB003 .