  Human visual sensitivity to spatial details declines towards the periphery.
Novel image synthesis techniques, so-called foveated rendering, exploit this
observation and reduce the spatial resolution of synthesized images for the
periphery, avoiding the synthesis of high-spatial-frequency details that are
costly to generate but not perceived by a viewer. However, contemporary
techniques do not make a clear distinction between the range of spatial
frequencies that must be reproduced and those that can be omitted. For a given
eccentricity, there is a range of frequencies that are detectable but not
resolvable. While the accurate reproduction of these frequencies is not
required, an observer can detect their absence if completely omitted. We use
this observation to improve the performance of existing foveated rendering
techniques. We demonstrate that this specific range of frequencies can be
efficiently replaced with procedural noise whose parameters are carefully tuned
to image content and human perception. Consequently, these frequencies do not
have to be synthesized during rendering, allowing more aggressive foveation,
and they can be replaced by noise generated in a less expensive post-processing
step, leading to improved performance of the rendering system. Our main
contribution is a perceptually-inspired technique for deriving the parameters
of the noise required for the enhancement and its calibration. The method
operates on rendering output and runs at rates exceeding 200FPS at 4K
resolution, making it suitable for integration with real-time foveated
rendering systems for VR and AR devices. We validate our results and compare
them to the existing contrast enhancement technique in user experiments.
