Shawmin Lei

An embedded still image coder with rate-distortion optimization

It is well known that the fixed rate coder achieves optimality when all coefficients are coded with the same rate-distortion (R-D) slope. In this paper, we show that the performance of the embedded coder can be optimized in a rate-distortion sense by coding the coefficients with decreasing R-D slope. We denote such a coding strategy as rate-distortion optimized embedding (RDE). RDE allocates the available coding bits first to the coefficient with the steepest R-D slope, i.e., the largest distortion decrease per coding bit. The resultant coding bitstream can be truncated at any point and still maintain an optimal R-D performance. To avoid the overhead of coding order transmission, we use the expected R-D slope, which can be calculated from the coded bits and is available in both the encoder and the decoder. With the probability estimation table of the QM-coder, the calculation of the R-D slope can be just a lookup table operation. Experimental results show that the rate-distortion optimization significantly improves the coding efficiency in a wide bit rate range.

An overview of the visual optimization tools in JPEG 2000

The human visual system plays a key role in the final perceived quality of the compressed images. It is therefore desirable to allow system designers and users to take advantage of the current knowledge of visual perception and models in a compression system. In this paper, we review the various tools in JPEG 2000 that allow its users to exploit many properties of the human visual system such as spatial frequency sensitivity, color sensitivity, and the visual masking effects. We show that the visual tool sets in JPEG 2000 are much richer than what is achievable in JPEG, where only spatially invariant frequency weighting can be exploited. As a result, the visually optimized JPEG 2000 images can usually have much better visual quality than the visually optimized JPEG images at the same bit rates. Some visual comparisons between different visual optimization tools, as well as some visual comparisons between JPEG 2000 and JPEG, will be shown.

Point-wise extended visual masking for JPEG-2000 image compression

One common visual optimization strategy for image compression is to exploit the visual masking effect where artifacts are locally masked by the image acting as a background signal. In this paper, we present a point-wise extended visual masking approach that nonlinearly maps the wavelet coefficients to a perceptually uniform domain prior to quantization by taking advantages of both self-contrast masking and neighborhood masking effects, thus achieving very good visual quality. It is essentially a coefficient-wise adaptive quantization without any overhead. It allows bitstream scalability, as opposed to many previous works. The proposed scheme has been adopted into the working draft of JPEG-2000 Part II.

An efficient color re-indexing scheme for palette-based compression

This paper presents a fast and efficient way for color re-indexing that tends to maximize the compression performance of a palette-based compression system. The proposed scheme relates the index difference of neighboring pixels to the potential cost of bits. It optimizes the assignment of index values to colors in a one-step look-ahead greedy fashion. Experimental results suggest that the proposed re-indexing scheme can reduce the bit rate by up to 43%, when compared to a previously proposed intensity-based color-indexing scheme. Furthermore, we show that with the proposed color re-indexing scheme, the palette-based JPEG-LS and palette-based JPEG-2000 can often outperform the graphical interchange format (GIF) significantly.

Arbitrary shape wavelet transform with phase alignment

We propose a wavelet transform for an arbitrary shape object. Compared with previous schemes, the proposed transform generates exactly the same number of coefficients as that of the original object. What is more, the phase of the horizontal wavelet filter is aligned so that the subsequent vertical transform is applied on coefficients with coherent phases and thus the transform efficiency is improved. We denote the scheme as the arbitrary shape wavelet transform with phase alignment (ASWP). The scheme was proposed at the MPEG4 meeting at Stockholm in July 1997, and was adopted by MPEG4 CD version 1 for wavelet transform of an arbitrary shape object.

Visual masking in wavelet compression for JPEG-2000

We describe a nonuniform quantization scheme for JPEG2000 that leverages the masking properties of the visual system, in which visibility to distortions declines as image energy increases. Derivatives of contrast transducer functions convey visual threshold changes due to local image content (i.e. the mask). For any frequency region, these functions have approximately the same shape, once the threshold and mask contrast axes are normalized to the frequencys threshold. We have developed two methods that can work together to take advantage of masking. One uses a nonlinearity interposed between the visual weighting and uniform quantization stage at the encoder. In the decoder, the inverse nonlinearity is applied before the inverse transform. The resulting image- adaptive behavior is achieved with only a small overhead (the masking table), and without adding image assessment computations. This approach, however, underestimates masking near zero crossings within a frequency band, so an additional technique pools coefficient energy in a small local neighborhood around each coefficient within a frequency band. It does this in a causal manner to avoid overhead. The first effect of these techniques is to improve the image quality as the image becomes more complex, and these techniques allow image quality increases in applications where using the visual systems frequency response provides little advantage. A key area of improvement is in low amplitude textures, in areas such as facial skin. The second effect relates to operational attributes, since for a given bitrate, the image quality is more robust against variations in image complexity.

Visual optimization tools in JPEG 2000

We review the various tools in JPEG 2000 that allow the users to take advantages of the various properties of the human visual system such as spatial frequency sensitivity and the visual masking effect. We show that the visual tool sets in JPEG 2000 are much richer than what was available in JPEG, where only locally invariant frequency weighting can be exploited.

Adaptive wavelet transforms with spatially varying filters for scalable image coding

An adaptive wavelet transform algorithm for scalable image coding is proposed. A quadtree segmentation scheme based on an entropy criterion is proposed to segment the image into several regions to allow for adaptive filtering using different types of prototype wavelet filters. A joint bit allocation and coding method is presented to efficiently code the segmented image in an embedding fashion. Experimental results show that the proposed scheme generally provides higher peak signal-to-noise ratios (PSNRs) at higher bit rates, and significantly better visual quality at lower bit rates, compared to fixed-filter wavelet transform based coding schemes.