A case study in digitizing a photographic collection

This paper reviews the processes involved in the digitisation, display and storage of medium size collections of photographs using mid-range commercially available equipment. Guidelines for evaluating the performance of these digitisation processes based on aspects of image quality are provided. A collection of photographic slides, representing first-generation analogue reproductions of a photographic collection from the nineteenth century, is treated as a case study. Constraints on the final image quality and the implications of digital archiving are discussed. Full descriptions of device characterisation and calibration procedures are given and results from objective measurements carried out to assess the digitisation system are presented. The important issues of file format, physical storage and data migration are also addressed

Influence of luminance and resolution on the perceived quality of black-and-white images on soft displays

The perceived quality of an image displayed on a computer monitor depends on a number of different factors that can affect viewers' preferences. Two of these are the luminance of the monitor display and the resolution of the image. The effect of luminance is of interest for applications such as on-line access of images where the computer displays used for viewing these images could have different luminance settings. A relationship between cathode-ray tube (CRT) display luminance and resolution has been shown in previous studies. It was therefore interesting to investigate whether there is a relationship between image resolution and CRT display luminance on perceived image quality. Image resolution is related to the image file size, which is an important factor for applications such as on-line access of images. This work used a CRT display to study the effect of the above-mentioned factors on the perceived quality of the displayed image. Three sets of black-and-white images, each set with a different resolution, were presented to observers at three brightness settings of the computer monitor. Results are discussed regarding the effect of monitor display luminance and image resolution on perceived image quality and the interaction between them. Evaluation of results is further extended to the influence of the different backgrounds of the images. The scene content of the images was also shown to affect the viewers' judgement

Perceptibility and acceptability of gamma differences of displayed sRGB images

The standard RGB (sRGB) colour space was developed to ensure accurate colour reproduction of images when viewed on Cathode Ray Tube (CRT) displays under specified conditions. Typical display and viewing conditions may vary, however, especially when accessing on-line images. Previous work has been carried out on the effects of different display white points and phosphor chromaticities and on modeling gamma tolerances and display conditions for the sRGB colour space. The work described in this paper investigates the effects of different gamma values on viewing sRGB images. Ten images with variable scene content were selected and converted to the sRGB colour space. A set of test images was generated for each scene by adjusting the display gamma of the sRGB image to a value in the range 1.8 to 2.6. Comparative judgments were conducted in which the reference sRGB image (calibrated for displays with gamma equal to 2.2) in each set was compared to each of the images adjusted to different display gammas. Each pair of images was displayed on the same monitor using software specially developed for the purpose. In the first series, the observers' responses concerned the perceptible difference between the reference sRGB image and the images calibrated to the different gamma value. In the second test their response concerned the acceptable gamma difference. The experimental results were evaluated and discussed. Conclusions were drawn regarding the effects of gamma differences on perceived image quality while viewing on-line sRGB images

The effect of reduced color depth on the color reproduction of web images

Accurate color reproduction of images viewed across the Internet can be a difficult task due to problems,which include variable viewing conditions.The standard RGB color space (sRGB)has been proposed to overcome this problem without the need for special software applications. Different graphics adapter bit-depth settings also affect the number of displayable colors on a web browser.For this reason,the 216 colors "web safe" palette has been introduced. To understand the effect of device characteristics on the process of image reproduction via the web,the characterization of some common devices was carried out. This created a controlled environment whereby experiments could be conducted with reproducible results.Device characterization also enabled the accurate conversion of scanned images to sRGB by using a step-by-step process that followed the specification of the color space. The investigation of color reproduction on a Cathode Ray Tube (CRT)display under different bit-depth settings has been conducted using a suitable test target,which was converted to the sRGB color space and also to the "web safe"(216 colors)palette.These two images were displayed under the 8-bit and 24-bit depth settings of the graphics card adaptor. Colorimetric measurements concerning color differences were performed and evaluated.Experimental investigation on the true color (24-bit color depth)image while displayed under 8-bit setting via different browsers followed.The outcome of the investigations was evaluated and combined to determine the relationship between the different parameters described above and their effect on image quality