Perceptual Analysis of Perspective Projection for Viewport Rendering in 360° Images

Omnidirectional, also referred to as 360
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, visual content provides an immersive experience since it allows users to view a visual scene from different directions. The overall content typically covers a full sphere, and omnidirectional videos or images are processed to obtain a projection on a 2D plane of a fraction of the sphere (aka viewport), which is shown to the user. Therefore, users can look around freely and only a restricted field of view (FoV) of the entire content is shown, according to the selected viewing direction. To maximize the immersion and engagement that omnidirectional content can provide, viewports must be generated and displayed with a large FoV. However, when large FoVs are used, geometric distortions (e.g., shape and scale deformations) can be present. In this paper, the perceptual impact of the perspective projection for viewport rendering of 360
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images is evaluated. This study is based on subjective experiments conducted at Instituto Superior Técnico (IST), aiming to assess the perceptual impact of the rendering projection parameters, such as FoV and centre of projection, as well as the influence of the image content.