Development of the I3A CPIQ spatial metrics

The I3A Camera Phone Image Quality (CPIQ) initiative aims to provide a consumer-oriented overall image quality metric for mobile phone cameras. In order to achieve this goal, a set of subjectively correlated image quality metrics has been developed. This paper describes the development of a specific group within this set of metrics, the spatial metrics. Contained in this group are the edge acutance, visual noise and texture acutance metrics. A common feature is that they are all dependent on the spatial content of the specific scene being analyzed. Therefore, the measurement results of the metrics are weighted by a contrast sensitivity function (CSF) and, thus, the conditions under which a particular image is viewed must be specified. This leads to the establishment of a common framework consisting of three components shared by all spatial metrics. First, the RGB image is transformed to a color opponent space, separating the luminance channel from two chrominance channels. Second, associated with this color space are three contrast sensitivity functions for each individual opponent channel. Finally, the specific viewing conditions, comprising both digital displays as well as printouts, are supported through two distinct MTFs.

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