  Despite substantial efforts dedicated to the design of heuristic models for
omnidirectional (i.e., 360$^\circ$) image quality assessment (OIQA), a
conspicuous gap remains due to the lack of consideration for the diversity of
viewing behaviors that leads to the varying perceptual quality of 360$^\circ$
images. Two critical aspects underline this oversight: the neglect of viewing
conditions that significantly sway user gaze patterns and the overreliance on a
single viewport sequence from the 360$^\circ$ image for quality inference. To
address these issues, we introduce a unique generative scanpath representation
(GSR) for effective quality inference of 360$^\circ$ images, which aggregates
varied perceptual experiences of multi-hypothesis users under a predefined
viewing condition. More specifically, given a viewing condition characterized
by the starting point of viewing and exploration time, a set of scanpaths
consisting of dynamic visual fixations can be produced using an apt scanpath
generator. Following this vein, we use the scanpaths to convert the 360$^\circ$
image into the unique GSR, which provides a global overview of gazed-focused
contents derived from scanpaths. As such, the quality inference of the
360$^\circ$ image is swiftly transformed to that of GSR. We then propose an
efficient OIQA computational framework by learning the quality maps of GSR.
Comprehensive experimental results validate that the predictions of the
proposed framework are highly consistent with human perception in the
spatiotemporal domain, especially in the challenging context of locally
distorted 360$^\circ$ images under varied viewing conditions. The code will be
released at https://github.com/xiangjieSui/GSR
