Omnidirectional images generally have nonlinear distortion in radial direction. Unfortunately. traditional algorithms such as scale-invariant feature transform (SIFT) and Descriptor-Nets (D-Nets) do not work well in matching omnidirectional images just because they are incapable of dealing with the distortion. In order to solve this problem. https://www.roneverhart.com/Christmas-Ornament-Ice-Ball-Molds-Design-Set-Options/