The perceptual image quality of natural scenes as a function of the physical system parameter gamma has a definite optimum. This optimum is subject-independent and greater than 1, but was found to vary from one scene to another. If gamma is varied, brightness contrast is the most obviously changing perceptual attribute. Subjects appear to be able to make consistent, global judgments of brightness contrast in natural scenes, despite the fact that local brightness contrast may vary considerably. If scaled perceptual quality is plotted against scaled (perceived) brightness contrast, all curves coincide, suggesting that under the given conditions brightness contrast is the dominant psychological dimension of the perceptual image quality. Taking into account the grey-level distribution of the scene in combination with the luminance- reproduction function of the imaging chain, an effective gamma value can be defined. If scaled perceptual quality and global brightness contrast are plotted against this effective gamma, the differences between scenes disappear, although there are clear differences in the relative sizes of the light and dark parts of the various test scenes. An analysis by scaling global brightness of Gaussian blobs of randomly distributed sizes, modulation depths, and polarities shows that skewness of this distribution does indeed have only a weak effect over a considerable range. The data suggest that the ratio of maximum and minimum luminance determines global brightness contrast for complex scenes under these conditions.

Global brightness contrast and the effect on perceptual image quality

Display units designed for the transfer of alphanumeric information may be found to be quite different in quality. Badly designed units are not read easily and cause visual fatigue when used in long and intensive sessions. This has already received some attention (e.g., Grandjean, 1980; Padmos, this volume; Cakir, Renter, Schmude, & Ambruster, 1978; Cakir, Hart, & Stewart, 1979; Matula, 1981). It became apparent that it was important to match the physical parameters of the displays to the demands of the visual system. However, visual fatigue is known to be very difficult to measure and, consequently, is not an obvious design criterion. Looking for a more adequate measure, therefore, we surveyed other criteria based on two different angles of approach. The most direct candidate is the comfort with which a subject feels he can read the information on the screen. In principle this can be measured at least ordinally by psychometric methods. Another line of approach is the speed at which information is taken in, as reflected by performance measures like reading speed or by eye movement characteristics such as saccadic length and fixation duration.

http://link.springer.com/content/pdf/10.1007/978-3-642-73402-1_5.pdf

Chapter 4. Visual Comfort as a Criterion for Designing Display Units

Visual comfort as a criterion for designing display units

Experimental methods have been developed particularly for the subjeelive evaluation of picture telephone systems. The viewing conditions deviate to some extent from earlier recommendations, and the experimental procedures comprise some new features. The methods have been validaled in a test with participation from laboratories in five countries. The results obtained from different laboratorles are generally in good agree­ ment. Results obtained by different methods within a Iabaratory are very similar. The tests prove that alternative experimental methods can be applied for the subjeelive assessment of picture quality with the same result. 1. Introduetion Within the framework of the COST 211 project, a special Subjective Tests Subgroup was created, with the task to develop, describe and use methods by which to evaluate solutions for picture coding proposed within the project. The activities of the subgroup have mainly comprised: a) preparatien of a document descrihing recommended methods for the subjective assessment of visual tele­ phone picture quality; b) performance of an international validatien test in order to prove that the recommended experimental methods lead to consistent and valid results if ap­ plied in different laboratories. This paper presents the results achieved by the sub­ group.

/pdf/definition-and-validation-of-methods-for-the-subjective-4fpbd0mep3.pdf

Definition and validation of methods for the subjective assessment of visual telephone picture quality

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Definition and validation of methods for the subjective assessment of visual telephone picture quality