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An image processing software package called Hoku is available to the public, free-of-charge, for use in recovering erased text from multispectral images of palimpsests. This software was developed by the author for his own use in recovering text, but has been built with an intuitive user interface that makes it suitable for non-technical users. It is written in Java and runs on both Windows and macOS computers. In this article, examples will be given of Hoku image processing results on data sets from three imaging systems—MegaVision, Phase One, and Lumiere Technology.

In August 2002, the near-Earth asteroid 2002 NY40, made its closest approach to the Earth. This provided an opportunity to study a near-Earth asteroid with a variety of instruments. Several of the telescopes at the Maui Space Surveillance System were trained at the asteroid and collected adaptive optics images, photometry and spectroscopy. Analysis of the imagery reveals the asteroid is triangular shaped with significant self-shadowing. The photometry reveals a 20-hour period and the spectroscopy shows that the asteroid is a Q-type.

Telescope images of astronomical objects and man-made satellites are frequently characterized by high dynamic range and low SNR. We consider the problem of how to enhance these images, with the aim of making them visually useful rather than radiometrically accurate. Standard contrast and histogram adjustment tends to strongly amplify noise in dark regions of the image. Sophisticated techniques have been developed to address this problem in the context of natural scenes. However, these techniques often misbehave when confronted with low-SNR scenes that are also mostly empty space. We compare two classes of algorithms: contrast-limited adaptive histogram equalization, which achieves spatial localization via a tiling of the image, and gradient-domain techniques, which perform localized contrast adjustment by non-linearly remapping the gradient of the image in a content-dependent manner. We extend these to include a priori knowledge of SNR and the processing (e.g. deconvolution) that was applied in the preparation of the image. The methods will be illustrated with images of satellites from a ground-based telescope.

This paper presents the initial results of the second phase of the Livingstone Spectral Imaging Project (2013–16), a digital humanities initiative funded by the National Endowment for the Humanities. The results suggest that spectral imaging technology can be applied successfully to illuminate the composition history of David Livingstone’s 1870 Field Diary and to enhance and/or reveal material features of the text otherwise invisible to the naked eye. The results represent an important advance in the use of spectral imaging in the study of manuscripts. Preliminary review of the results also indicates that more detailed analysis will reveal new and important information about Livingstone, his manuscript, the African cultures among which he traveled, and the many environments through which his diary traveled and was preserved.

Intensity Interferometry is a form of imaging developed in the 1950’s by Hanbury Brown and Twiss, which gave very early results for estimates of the diameters of stellar discs. It relies on the statistical properties of light to form an image by correlating the electronic signals measured independently and simultaneously at two or more separate collection telescopes. Its benefits are that it can provide very high resolution, can be very low in cost, does not require precision path matching, and is insensitive to atmospheric effects. Its disadvantages are that it has relatively poor SNR properties for larger telescope separations. An experiment is performed with three telescopes in Kihei, HI to investigate the potential for large-separation, high-resolution, multi-telescope operation. Simulations were performed to address key issues related to the experiment. Correlations were measured during lab checkouts, and also for early field testing. A compression scheme was developed to archive the raw data. The compression process had the added advantage of eliminating spurious electronic interference signals.

During his explorations of Africa, David Livingstone kept a diary and wrote letters about his experiences. Near the end of his travels, he ran out of paper and ink and began recording his thoughts on leftover newspaper with ink made from local seeds. These writings suffer from fading, from interference with the printed text and from bleed through of the handwriting on the other side of the paper, making them hard to read. New image processing techniques have been developed to deal with these papers to make Livingstone's handwriting available to the scholars to read. A scan of the David Livingstone's papers was made using a twelve-wavelength, multispectral imaging system. The wavelengths ranged from the ultraviolet to the near infrared. In these wavelengths, the three different types of writing behave differently, making them distinguishable from each other. So far, three methods have been used to recover Livingstone's handwriting. These include pseudocolor (to make the different writings distinguishable), spectral band ratios (to remove text that does not change), and principal components analysis (to separate the different writings). In initial trials, these techniques have been able to lift handwriting off printed text and have suppressed handwriting that has bled through from the other side of the paper.

The Archimedes Palimpsest imaging team has developed a spectral imaging system and associated processing techniques for general use with palimpsests and other artifacts. It includes an illumination system of light-emitting diodes (LEDs) in 13 narrow bands from the near ultraviolet through the near infrared (uλ≤ 40nm), blue and infrared LEDs at raking angles, high-resolution monochrome and color sensors, a variety of image collection techniques (including spectral imaging of emitted fluorescence), standard metadata records, and image processing algorithms, including pseudocolor color renderings and principal component analysis (PCA). This paper addresses the development and optimization of these techniques for the study of parchment palimpsests and the adaptation of these techniques to allow flexibility for new technologies and processing capabilities. The system has proven useful for extracting text from several palimpsests, including all original manuscripts in the Archimedes Palimpsest, the undertext in a privately owned 9th-century Syriac palimpsest, and in a survey of selected palimpsested leaves at St. Catherine's Monastery in Egypt. In addition, the system is being used at the U.S. Library of Congress for spectral imaging of historical manuscripts and other documents.

A spectral imaging system comprising a 39-Mpixel monochrome camera, LED-based narrowband illumination, and acquisition/control software has been designed for investigations of cultural heritage objects. Notable attributes of this system, referred to as EurekaVision, include: streamlined workflow, flexibility, provision of well-structured data and metadata for downstream processing, and illumination that is safer for the artifacts. The system design builds upon experience gained while imaging the Archimedes Palimpsest and has been used in studies of a number of important objects in the LOC collection. This paper describes practical issues that were considered by EurekaVision to address key research questions for the study of fragile and unique cultural objects over a range of spectral bands. The system is intended to capture important digital records for access by researchers, professionals, and the public. The system was first used for spectral imaging of the 1507 world map by Martin Waldseemueller, the first printed map to reference "America." It was also used to image sections of the Carta Marina 1516 map by the same cartographer for comparative purposes. An updated version of the system is now being utilized by the Preservation Research and Testing Division of the Library of Congress.

The Archimedes Palimpsest is a thousand-year old overwritten parchment manuscript, containing several treatises by Archimedes. Eight hundred years ago, it was erased, overwritten and bound into a prayer book. In the middle of the twentieth century, a few pages were painted over with forged Byzantine icons. Today, a team of imagers, scholars and conservators is recovering and interpreting the erased Archimedes writings. Two different methods have been used to reveal the erased undertext. Spectral information is obtained by illuminating the manuscript with narrow-band light from the ultraviolet, through the visible wavebands and into the near-infrared wavelengths. Characters are extracted by combining pairs of spectral bands or by spectral unmixing techniques adapted from remote sensing. Lastly, since all of the text was written with iron gall ink, X-Ray fluorescence has been used to expose the ink underneath the painted icons. This paper describes the use of color to enhance the erased text in the processed images and to make it visible to the scholars. Special pseudocolor techniques have been developed that significantly increase the contrast of the erased text and make it possible to be read by the scholars despite the presence of the obscuring, overlaid text.

Invisible watermark images can be incorporated in printed halftone images using specially designed halftone screens. The watermark information is embedded into the image by varying the spatial correlation of the halftone texture. The halftone screen with embedded watermarks can be used exactly as a normal halftone screen, so there is no additional image processing required for processing individual images to embed watermarks. Once the binary output image is printed on the paper, the correlation of the binary image is converted into physical spatial correlation between neighboring areas of the printed image. This correlation relation is not visible to the eye but it can be detected by scanning the printed image on a desktop scanner and processing the scanned image. Printer and scanner distortions can interfere with the self-alignment of the scanned image, so localized adjustments are made to detect the embedded spatial correlation information in the watermarked image. The retrieval of this watermark is robust to copying and distortion and it can be detected in reproductions of the halftone image.

A method has been developed to induce symmetric edge enhancement in error diffusion with threshold modulation. The existing threshold modulation algorithm induces edge enhancement by subtracting a term proportional to the input image from the threshold. The edge enhancement that results in asymmetric due to the asymmetric shape of the error diffusion filter. The new method induces a symmetric edge enhancement by subtracting a term from the threshold that is proportional to a filtered version of the input image. The filtering is accomplished with a recursive filter, applied during the error diffusion algorithm, which cancels out the error diffusion filter and imposes a symmetric sharpening filter. The result is a symmetric edge enhancement that is more pleasing to the eye.

As we approach the new millennium, error diffusion is approaching the 25th anniversary of its invention. Because of its exceptionally high image quality, it continues to be a popular choice among digital halftoning algorithms. Over the last 24 years, many attempts have been made to modify and improve the algorithm - to eliminate unwanted textures and to extend it to printing media and color. Some of these modifications have been very successful and are in use today. This paper will review the history of the algorithm and its modifications. Three watershed events in the development of error diffusion will be described, together with the lesions learned along the way.

Advances in digital imaging technology have presented new opportunities for scholars to better study ancient texts through image clarification and enhancement, Textual material exists over a period of at least 5000 years, written on a great variety of material including leather, clay, stone, papyrus, copper and, of course, paper. Much of the knowledge we have of the past emerges through this textual material. The written material provides us with the knowledge and traditions of our past. The histories of religion, technology, science, medicine, astronomy, cartography and commerce are ensconced in the ancient writings left by civilizations and traditions of the past. Not only is knowledge transmitted from this ancient past but records and events of the past are there for use and applications in current times. Records of astronomical occurrences, migratory patterns, climatic incidences, warfare, visions, predictions, pestilence, legends, myths, histories, heroes and despots all play a role in understanding current and future schemes and strategies. Our knowledge base builds on the past and the more we learn about the past the better we can understand and deal with the present and plan for the future.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A method will be shown to incorporate digital watermarks in printed halftone images using stochastic screens. The watermark is not visible to the eye and introduces no loss in image quality. Although it cannot be seen, the watermark can be extracted at a later time with post processing. Watermarks of high contrast are incorporated in the image by varying the statistics of the stochastic screen. The watermark information can be made visible by comparing the relative changes in spatial correlation in the halftone texture of the image. Watermarking allows a printed image to be tested for the purposes of identifying the owner or the source of the image. Arbitrary customer information can be incorporated into the image, including variable information, such as the data or time of day. The technique is robust to copying of the printed image and the watermark can be detected in reproductions of the halftoned image.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We present an approach to the annotation of natural scene images that used a simple color segmenter to indicate the presence of skin, sky and grass in an image. The results of the color based segmentation are then used to determine higher level description of images as 'People' or 'Outdoors'. The images are analyzed in a luminance-chrominance color space in order to reduce the influence of the scene illumination on the segmentation. The distribution of the chrominance components of the pixels belonging to each object class is modeled as a Gaussian PDF, allowing an image-adaptive setting of the object-class thresholds.

Threshold modulation has been shown to produce strong effects in error diffusion images. In particular a threshold modulation that is proportional to the input image induces edge enhancement in the output error diffused image. In this paper, examples are shown for image- dependent threshold modulation incorporated into error diffusion implemented on a non- standard raster, such as a serpentine raster or a space filling curve. The amount and type of edge enhancement induced is shown to vary for different raster directions.

Error diffusion has proven to be a very powerful method of producing binary images that are visually similar to the original grayscale images. It has become so popular that many attempts have been made to improve it. In this paper, two modifications to error diffusion, the serpentine raster and threshold modulation, are analyzed from a theoretical viewpoint. The two analyses reveal the origins of the image quality improvements of the modifications and quantify their benefits.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes an under-compensated error diffusion algorithm that is intended for multi-level printing and copying. The new error diffusion algorithm is based on the fact that error calculations and feedback are less important in multi-level systems than in bi-level systems.

Error diffusion is a digital halftoning algorithm that produces high quality binary images, with good detail rendition and little apparent texture noise.

The concept of an error image is defined for the error diffusion algorithm. Ordinarily hidden from view, the error image is a visual representation of the internally generated errors from which the algorithm derives its name. In this paper, it is shown that the error image contains a linear component of the input image, which induces edge enhancement in the output error diffusion image. Examples are shown for three different error weight distributions: a 1-D one- ahead distribution, the standard 4-element distribution, and a 12-element error distribution. The amount of edge enhancement in the corresponding algorithm is shown to vary with the amount of input image information present in the error image.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A recently developed mathematical analysis of error diffusion by Weissbach et al.1 describes the output spectrum of the error diffusion algorithm in terms of a linear filter equation. Their analysis showed that the output spectrum is given by the sum of the input spectrum and a high-pass filtered error spectrum. This analysis has been extended to error diffusion implemented on a serpentine raster. A serpentine raster is one in which the direction of processing is reversed for every scanline. The rationale behind this method is that the repeated reversal of the processing direction eliminates the directional artifacts in the output image that are caused by the asymmetrical distribution of errors in the algorithm. The new analysis of error diffusion on the serpentine raster shows that the nonsymmetric parts of the error spectrum are not eliminated but instead are reduced in magnitude and shifted to half the scanning frequency.

The authors explore the current use of modern image processing techniques to decipher ancient texts, and look toward future applications, including “discovering” original texts from overwritten documents.

Error diffusion is a popular algorithm for the binarization of continuous-tone images. A modification of the error-diffusion algorithm is shown that introduces an input-dependent threshold into the process. This modification establishes a fast and efficient way to increase or decrease edge enhancement in the algorithm.

A technique has recently been proposed for processing astronomical photographs to remove the effects of turbulence. The method involves averaging over many short-exposure photographs of an object to reconstruct a diffraction-limited image of the object. A two-dimensional computer simulation of the reconstruction technique is presented. The results indicate that diffraction-limited imaging with a 100 in. telescope is possible even with 2 arc sec seeing conditions.