Paul W. Melnychuck

Conditioning contexts for the arithmetic coding of bit planes

The arithmetic coding of the bit planes of an image represented either in the conventional weighted binary form or by the Gray code, is investigated. Two-dimensional contexts based on the pixels from the current bit plane and three-dimensional contexts on pixels from both the current and previous bit planes are studied. Results indicate that for binary bit planes, a three-dimensional template results in a significant bit rate reduction compared to a two-dimensional template of the same size. The savings are much less for Gray encoded bit planes; since the final bit rates are similar to the binary case, the need for Gray encoding is eliminated. The effect of the order of encoding the bit planes are also studied. Comparisons to conventional lossless schemes, such as predictive coding followed by Huffman or arithmetic coding, are presented. >

Survey Of Lossless Image Coding Techniques

Many image transmission/storage applications requiring some form of data compression additionally require that the decoded image be an exact replica of the original. Lossless image coding algorithms meet this requirement by generating a decoded image that is numerically identical to the original. Several lossless coding techniques are modifications of well-known lossy schemes, whereas others are new. Traditional Markov-based models and newer arithmetic coding techniques are applied to predictive coding, bit plane processing, and lossy plus residual coding. Generally speaking, the compression ratio offered by these techniques are in the area of 1.6:1 to 3:1 for 8-bit pictorial images. Compression ratios for 12-bit radiological images approach 3:1, as these images have less detailed structure, and hence, their higher pel correlation leads to a greater removal of image redundancy.

Effect of noise and MTF on the compressibility of high-resolution color images

There are an increasing number of digital image processing systems that employ photographic image capture; that is, a color photographic negative or transparency is digitally scanned, compressed, and stored or transmitted for further use. To capture the information content that a photographic color negative is capable of delivering, it must be scanned at a pixel resolution of at least 50 pixels/mm. This type of high quality imagery presents certain problems and opportunities in image coding that are not present in lower resolution systems. Firstly, photographic granularity increases the entropy of a scanned negative, limiting the extent to which entropy encoding can compress the scanned record. Secondly, any MTFrelated chemical enhancement that is incorporated into a film tends to reduce the pixel-to-pixel correlation that most compression schemes attempt to exploit. This study examines the effect of noise and MTF on the compressibility of scanned photographic images by establishing experimental information theoretic bounds. Images used for this study were corrupted with noise via a computer model of photographic grain and an MTF model of blur and chemical edge enhancement. The measured bounds are expressed in terms of the entropy of a variety of decomposed image records (e.g., DPCM predictor error) for a zeroeth-order Markov-based entropy encoder, and for a context model used by the Q-coder. The resultsshow that the entropy of the DPCM predictor error is 3-5 bits/pixel, illustrating a 2 bits/pixel difference between an ideal grain-free case, and a grainy film case. This suggests that an ideal noise filtering algorithm could lower the bitrate by as much as 50%.

Extensions to the Kodak Photo CD file format for commercial applications

The Photo CD system provides an inexpensive means of participating in the information age with personal digital images. At the heart of the system is a 35 mm, 2000 pixels/inch scanner to acquire image data from a variety of photographic media. The raw data is then processed into a calibrated color metric, decomposed into a hierarchy of 5 resolution, compressed, encoded, and written to a compact disc using a high-speed CD writer. These discs can then be quickly displayed to a consumer television with a low-cost Photo CD player, that is also a high-quality CD-audio player. Additionally, Photo CD discs can be read into a computer using a CD-ROM XA drive, and manipulated on the desktop for importing into documents, or, for hard-copy printing to a variety of devices. The 35 mm-based system described above is only a part of a more generalized architecture. An add-on component capable of scanning larger format negatives and slides will soon be available, and will utilize extensions to the Photo CD format appropriate for these new image modalities. In particular, the formation of a 6th resolution and its relationship to the existing hierarchy will be discussed. Additionally, the structure of future extensions currently under development will be outlined in the context of the specific applications which they are designed to support.

Fourier Spectra Of Digital Halftone Images Containing Random Position Errors And Reflectance Non-Uniformities

Random laser-writer position errors in raster-scanned monochrome halftone images and the effects of non-uniformities in the recording medium are examined using Fourier analysis techniques. For a high contrast, narrow-exposure latitude recording material (typically used in halftone reproduction) with medium-sized halftone dots (25-85%), a one-dimensional halftone model is developed to derive the signal power spectrum of a halftone image containing position errors in the slow-scan (page-scan) direction. Non-uniformity in the recording medium (via sensitivity or reflectance) is modelled by representing the digital halftone image as a random amplitude-modulated signal. The spectrum of an image containing these combined effects is shown to consist of a periodic component and a random component, which is a function of position error but independent of dot size. The term signal power spectrum, in the context of this work, is the average modulus squared of the Fourier transform of an image containing these errors. The results can be generalized to include any digitally generated halftone image based on a center-growing dot configuration and containing dot size/shape distortions.