  The conversion between different spherical image and video projection formats
requires highly accurate resampling techniques in order to minimize the
inevitable loss of information. Suitable resampling algorithms such as nearest
neighbor, linear or cubic resampling are readily available. However, no
generally applicable resampling technique exploits the special properties of
spherical images so far. Thus, we propose a novel viewport-adaptive resampling
(VAR) technique that takes the spherical characteristics of the underlying
resampling problem into account. VAR can be applied to any mesh-to-mesh capable
resampling algorithm and shows significant gains across all tested techniques.
In combination with frequency-selective resampling, VAR outperforms
conventional cubic resampling by more than 2 dB in terms of WS-PSNR. A visual
inspection and the evaluation of further metrics such as PSNR and SSIM support
the positive results.
