Masahiro Iwahashi

A development of symmetric extension method for subband image coding

Describes the development of a technique for subband image coding called the symmetric extension method which utilizes the nature of a symmetrically extended image to achieve high quality coding. There are, however, some restrictions on the usable subband coding system. The development is done based on the property of a symmetrically extended signal in the analysis process to remove the restriction. The behavior of the sequence after two processes, filtering and decimation, is examined and formulated. The development extends the application area of the technique.

Two layer lossless coding of HDR images

This report proposes a two layer lossless coding system for high dynamic range (HDR) images expressed in a floating point data format. Its encoder outputs compressed image data in two layers. From the base layer, a standard low dynamic range (LDR) image is decoded. Merging with the enhancement layer, the original HDR image is decoded without any loss. We introduce a reversible logarithmic mapping to reduce bit depth of pixel values. We also introduce bit plane classification to divide data into two layers. It was confirmed that our method has better coding performance than existing methods especially at high bit rate coding of LDR images.

Lossy compression of sparse histogram image

In this paper, a lossy data compression for a sparse histogram image signal is proposed. It is extended from an existing lossless coding which is based on a lossless histogram packing and a lossless coding. We introduce a lossy mapping, which has less computational load than the rate-distortion optimized Lloyd-Max quantization, and combine it with a lossless coding. It was confirmed that the proposed method attains higher performance in the rate-distortion plane than existing methods. This is because it can utilize histogram sparseness of images, and also its inverse mapping does not magnify quantization noise.

Non separable 2D factorization of separable 2D DWT for lossless image coding

In this report, we propose a reversible two dimensional (2D) discrete wavelet transform (DWT) compatible to the irreversible 9–7 DWT for lossy coding in the JPEG 2000. All the filters and scalings are factorized into lifting steps, and signals are rounded into integers, so that the proposed DWT becomes reversible and applicable to lossless coding. Furthermore, we factorize the separable 2D transfer function of the 2D DWT into non separable 2D functions to reduce the lifting steps and rounding operations. As a result, errors due to the rounding operations are reduced and the compatibility to the irreversible 9–7 DWT is improved.

A new lifting structure of non separable 2D DWT with compatibility to JPEG 2000

In this report, we reduce the number of lifting steps of the two dimensional (2D) discrete wavelet transform (DWT) in JPEG 2000 by introducing a factorization of the separable 2D transfer function into some non separable 2D (NS-2D) functions. The order of the 1D lifting steps in vertical and in horizontal are properly replaced so that all of them are synthesized into NS-2D functions. As a result, the number of the lifting steps of the 2D 9/7 DWT is reduced to 75 [%] maintaining compatibility to the JPEG 2000. Since the number of the lifting steps is essentially proportional to a delay of output signals, it contributes to construct a low latency DWT under the assumption that direct accessing to a 2D intermediate memory is permitted.

Lossless coding of still images with four channel prediction

In this paper a new lossless coding of digital image data with a four channel filter bank (4ch FB) having optimized filter coefficients is proposed. The method has an advantage of less restrictions on optimizing the filter coefficients compared to the cascade form of two channel filter banks (2ch FB). Simulation results are shown to confirm its effectiveness.

Three dimensional discrete wavelet transform with deduced number of lifting steps

This report reduces the total number of lifting steps in a three-dimensional (3D) double lifting discrete wavelet transform (DWT), which has been widely applied for analyzing volumetric medical images. The lifting steps are necessary components in a DWT. Since calculation in a lifting step must wait for a result of former step, cascading many lifting steps brings about increase of delay from input to output. We decrease the total number of lifting steps introducing 3D memory accessing for the implementation of low delay 3D DWT. We also maintain compatibility with the conventional 5/3 DWT defined by JPEG 2000 international standard for utilization of its software and hardware resources. Finally, the total number of lifting steps and rounding operations were reduced to 67 % and 33 %, respectively. It was observed that total amount of errors due to rounding operations in the lifting steps was also reduced.

An integer tone mapping operation for hdr images expressed in floating point data

This report proposes a new tone mapping operation which is implemented in integer input and integer output. A tone mapping operation (TMO) generates a low dynamic range (LDR) image from a high dynamic range (HDR) image. Since pixel values of an HDR image are generally expressed in a floating point data format, e.g. RGBE, OpenEXR, a TMO is also implemented in floating point calculations in conventional approaches. However, it requires huge memory resources, even though a resulting LDR image is expressed in simple integer. We perform a TMO with integer input and integer output to reduce memory resources. It is experimentally confirmed with PSNR and contrast evaluations that the proposed method offers LDR images of visually high quality comparable to the conventional method.

A New Unified Lossless/Lossy Image Compression Based on a New Integer DCT

In this paper, we propose a new one-dimensional (1D) integer discrete cosine transform (Int-DCT) for unified lossless/lossy image compression. The proposed 1D Int-DCT is newly designed to reduce rounding effects by minimizing number of rounding operations. The proposed Int-DCT can be operated not only lossless coding for a high quality decoded image but also lossy coding for a compatibility with the conventional DCT-based coding system. Both theoretical analysis and simulation results confirm an effectiveness of the proposed Int-DCT.

Non separable 3D lifting structure compatible with separable quadruple lifting DWT

This report reduces the total number of lifting steps in a 3D quadruple lifting DWT (discrete wavelet transform). In the JPEG 2000 international standard, the 9/7 quadruple lifting DWT has been widely utilized for image data compression. It has been also applied to volumetric medical image data analysis. However, it has long delay from input to output due to cascading four (quadruple) lifting steps per dimension. We reduce the total number of lifting steps introducing 3D direct memory accessing under the constraint that it has backward compatibility with the conventional DWT in JPEG 2000. As a result, the total number of lifting steps is reduced from 12 to 8 (67 %) without significant degradation of data compression performance.