Aspect of image quality and the Internet
The colour reproduction of images viewed across the Internet is an area of significant importance due to the recent increase in the levels of on-line shopping and the number of digital imaging libraries accessible over the Internet. The quality of images viewed on-line can be affected by variables such as the display and viewing conditions. This thesis describes a programme of work that investigates existing methods for controlling the colour and tone reproduction of images accessed over the Internet. The reproduced colour of images viewed across the Internet may be affected by transformations applied to provide consistent colour reproduction among different web browsers and operating systems for 8-bit depth graphics card adapter display systems. In this work objective measures (ΔE*ab) are used to determine the range of colours that can be reproduced with perceptible colour differences. It is shown that a 24-bit depth image displayed under the 8-bit depth setting of a graphics card adapter results in lower colour differences than the 216 colour image displayed under the same bit depth setting. The standard RGB colour space (sRGB) has been proposed as a means for obtaining accurate reproduction of colour and tone for images displayed across the Internet provided that they are viewed under the reference display and viewing conditions defined in the standard. A survey has indicated that the typical display and viewing conditions of a sample of Internet users frequently deviate from the reference sRGB standard. Psychophysical experiments to determine the perceptibility and acceptability of differences in CRT monitor gamma under controlled and uncontrolled display and viewing conditions are reported. The point of subjective equality (PSE) and the just noticeable difference values of gamma for the imperceptibility and acceptability of gamma differences in images are estimated. A survey of the gamma settings of a sample of computer monitors is reported. It is shown that, due to deviations in the gamma settings, an sRGB image displayed on 50% of the monitors surveyed will be judged as perceptibly different from a monitor complying with the sRGB standard, while 40% of monitors will be judged as acceptably similar, although observers may detect a difference.