Lindsay W. MacDonald

A robotic system for digital photography

The European collaborative research project IST-2000-28008-VITRA (Veridical Imaging of Transmissive and Reflective Artefacts) developed an innovative system for high-resolution digital image acquisition for conservation and heritage applications. Using a robotic platform to carry both camera and lighting, it can capture colorimetric images up to 15 metres above floor level, thus eliminating the need for scaffold towers. Potential applications include wall-paintings, tapestries, friezes and stained glass windows in historic buildings such as churches, cathedrals, palaces and monuments. Evaluation of the system was conducted at four sites in Germany and the UK. In the course of the project a number of significant technical innovations were made, including a new panoramic image viewer for the Internet.

Assessing the enhancement of image sharpness

This study investigated four different image sharpness enhancement methods. Two methods applied standard sharpening filters (Sharpen and Sharpen More) in PhotoShop and the other two were based on adjustment of the image power spectrum using the human visual contrast sensitivity function. A psychophysical experiment was conducted with 25 observers, the results of which are presented and discussed. Five conclusions are drawn from this experiment: (1) Performance of the sharpening methods; (2) Image dependence; (3) Influence of two different colour spaces on sharpness manipulation; (4) Correlation between perceived image sharpness and image preference; and (5) Effect of image sharpness enhancement on the image power spectrum.

Evaluating contrast sensitivity

The problem for proper rendering of spatial frequencies in digital imaging applications is to establish the relative contrast sensitivity of observers at suprathreshold contrast levels in typical viewing environments. In an experimental study two methods of evaluating spatial contrast sensitivity were investigated, using targets of graded tonal modulation, at which observers were asked to point to the perceived threshold locations. The results produced by these two methods were rather different from those of the classical methods of vision science, showing a much lower sensitivity over a broader range of spatial frequencies. These may be regarded as complementary to CSF data derived from single- frequency Gabor stimuli and prove to be better suited to the needs of practical imaging applications.

Internet-based assessment of image sharpness enhancement

Two internet-based psychophysical experiments were conducted to investigate the performance of an image sharpness enhancement method, based on adjustment of spatial frequencies in the image according to the contrast sensitivity function and compensation of MTF losses of the display. The method was compared with the widely-used unsharp mask (USM) filter from PhotoShop. The experiment was performed in two locations with different groups of observers: one in the UK, and the second in the USA. Three Apple LCD displays (15 studio, 23 HD cinema and 15 PowerBook) were used at both sites. Observers assessed the sharpness and pleasantness of the displayed images. Analysis of the results led to four major conclusions: (1) Performance of the sharpening methods; (2) Influence of MTF compensation; (3) Image dependency; and (4) Comparison between sharpness perception and preference judgement at both sites.

Measuring gloss by digital photography

The measurement of gloss is conventionally made by specialised instruments that determine the ratio of reflected to incident illumination at a single fixed angle. This study investigated whether digital photography with flash illumination could be used as an alternative. Multiple exposures were combined by a high dynamic range (HDR) imaging technique to produce a two-dimensional intensity profile of the reflectance around the specular point. The method was tested for six paper samples of varying gloss, and the results were found to correlate well with instrumental measurements. The image gloss profiles, however, provided more information about the distribution of the reflection over a range of angles and also gave an indication of the surface texture.

Digitization of stained glass

Digital photography was applied to the capture of images of the stained glass windows in the historic parish church in Fairford, Gloucestershire, England. Because of their size, the windows had to be photographed in 45 separate sections in order to capture all the detail present in the painting on the glass. The digital images of each section, approximately 3000 by 2300 pixels, were then mosaiced together in order to construct the very high resolution image needed for the complete window. A special backlight panel was constructed for the purpose, and techniques developed for minimizing the effects of reflected light and for calibrating the color of the images. Improvements in the technology for mounting and positioning the camera were identified as the most significant factors currently preventing the widespread adoption of this technology for virtual heritage applications.

White ink, measurement methods

Printing with white ink plays an important role in many printing processes, but white is difficult to integrate into colour management processes since conventional measurements are uncorrelated with the ink amount. A control method for white ink is proposed in which white is printed and measured over black. The resulting colorimetric densities can be modelled by polynomial regression, allowing accurate prediction of tonal value. The model can readily be inverted to predict the colorant amount required to match a given colorimetric density, and hence is a suitable method of measurement that can support process control and colour management.