We propose an object-based approach to image subsampling in the chroma domain which outperforms the 4:2:0/4:2:2 chroma formats of MPEG-2 and other techniques proposed in the literature. This scheme is motivated by the fact that sharp color edges and detailed areas in the color domain represent significant visual information, and therefore, require better color definition. The object-based color subsampling approach is based on segmenting the C/sub B/ C/sub R/ components of an image into high and low-activity areas. The relatively large and less informative low activity regions are decimated to one pixel while high-activity objects are preserved entirely. A fuzzy filter is incorporated in our segmentation scheme to circumvent the effect of noise which could lead to isolated pixels. The object-based approach demonstrates superior results when measured perceptually. This scheme is particularly attractive as a preprocessing step to object-based coding schemes such as MPEG-4 where segmentation can be already available for content-based manipulation and generation of hybrid video data.

A new coding scheme based on the scalar-vector quantizer (SVQ) is developed for compression of medical images. The SVQ is a fixed rate encoder and its rate-distortion performance is close to that of optimal entropy-constrained scalar quantizers (ECSQs) for memoryless sources. The use of a fixed-rate quantizer is expected to eliminate some of the complexity of using variable-length scalar quantizers. When transmission of images over noisy channels is considered, our coding scheme does not suffer from error propagation that is typical of coding schemes using variable-length codes. For a set of magnetic resonance (MR) images, coding results obtained from SVQ and ECSQ at low bit rates are indistinguishable. Furthermore, our encoded images are perceptually indistinguishable from the original when displayed on a monitor. This makes our SVQ-based coder an attractive compression scheme for picture archiving and communication systems (PACS). PACS are currently under study for use in an all-digital radiology environment in hospitals, where reliable transmission, storage, and high fidelity reconstruction of images are desired.

Presents a new interframe coding method for medical images, in particular magnetic resonance (MR) images. Until now, attempts in using interframe redundancies for coding MR images have been unsuccessful. The authors believe that the main reason for this is twofold: unsuitable interframe estimation models and the thermal noise inherent in magnetic resonance imaging (MRI). The interframe model used here is a continuous affine mapping based on (and optimized by) deforming triangles. The inherent noise of MRI is dealt with by using a median filter within the estimation loop. The residue frames are quantized with a zero-tree wavelet coder, which includes arithmetic entropy coding. This particular method of quantization allows for progressive transmission, which aside from avoiding buffer control problems is very attractive in medical imaging applications.

We present a new inter-frame coding for medical images, in particular magnetic resonance (MR) images. Until now, attempts in using inter-frame redundancies for coding MR images have been unsuccessful. We contend that the main reason for this is twofold: bad inter-frame estimation models and ignoring the thermal noise inherent in MRI. Our inter-frame model is a continuous affine mapping based on (and optimized by) deforming triangles. The inherent noise of MRI is dealt with by using a median filter within the estimation loop. Simulations demonstrate the viability of this algorithm.>

A key issue in subband coding is the efficient compression of the less informative but perceptually important upper frequency bands of the decomposed image. A new approach capable of effectively encoding the upper-bands is described. An intra-band vector quantization (VQ) technique is employed for compression of the base-band while the upper frequency bands are encoded by a hierarchical inter-band VQ method. The proposed inter-band vector quantization scheme exploits the redundancies that exist between pels of significant perceptual importance across the upper frequency bands of the same resolution. Such pels are observed to be at or around the edge-locations, displaying similar discontinuity behavior across the subbands. Therefore, an edge-detector is applied on the reconstructed base-band to extract the edge locations, and as a result no overhead information is transmitted to identify the position of these pels in the upper-bands. Furthermore, a residual subband, being the difference between the original base-band of each layer and its encoded version, is incorporated in the proposed inter-band VQ model. Thus, the proposed interband VQ scheme is simply the vector quantization of pixels across the upper-bands together with their corresponding residual band. Compression results are presented for both digital images and video sequences which demonstrate high subjective visual qualities.>

In this paper, a finite-state vector quantizer called Dynamic Finite-State Vector Quantization (DFSVQ) is investigated with regard to its subcodebook construction. In DFSVQ each input vector encoded by a small codebook, called subcodebook, is created from a much larger codebook called supercodebook. The subcodebook is constructed by selecting (reordering procedure) a set of appropriate codevectors from the supercodebook. The performance of the DFSVQ depends on this reordering procedure, therefore, several reordering procedures are introduced and their performances are evaluated in this paper. The reordering procedures that are investigated are the conditional histogram, address prediction, vector prediction, nearest neighbor design, and the frequency usage of codevectors. The performance of the reordering procedures are evaluated by comparing their hit ratios (the number of blocks encoded by the subcodebook) and their computational complexity. Experimental results are presented for both still images and video. It is found that for still images the conditional histogram performs the best and for video the nearest neighbor design performs the best.

An interframe vector quantization scheme with finite memory called dynamic finite-state vector quantization (DFSVQ) is presented. The encoding of a current input vector consists of a subcodebook, representing the best matchable code vectors, selected from a larger codebook, i.e., a supercodebook. The choice for the entries in the supercodebook is based on the information obtained from the previously encoded blocks where directional conditional block probability matrices are used in the selection of the entries. An adaptive DFSVQ scheme is also proposed in which, when encoding an input vector, first the supercodebook is searched for a matching code vector to satisfy a prespecified waveform distortion. If such a code vector is not found then the whole supercodebook is checked for a better match and a signaling flag along with the corresponding address of the code vector is transmitted to the receiver. The performances of the DFSVQ encoder and its adaptive version are evaluated for several video sequences.>

A new subband video coding technique is introduced which utilizes several memoryless vector quantizing (VQ) schemes for encoding the various layers. A set of quadrature mirror filter banks was applied to the motion compensated frame differences (MCFDs) of the moving sequences to produce seven nonuniform bands. The upper bands displayed a significant amount of information at edge locations, these being the positions where the baseband of each layer was observed to have a similar behavior. Therefore, an edge-detecting operator, e.g., Laplacian or a Gaussian, was incorporated into the video compression model to extract the perceptually important locations of the upper bands from their correspondingly encoded baseband, thus eliminating the need for transmission of their addresses. Promising results were obtained which are suitable for low-bit-rate video applications where videophone and videoconferencing systems may be realized.>