Omnidirectional, also referred to as 360º, 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° 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.