For me the linear gradient hints that the surface is curved on one axis, i.e. is cylindrical, not spherical. Of course the context plays a role too on the perc

For me the linear gradient hints that the surface is curved on one axis, i.e. is cylindrical, not spherical. Of course the context plays a role too on the perceived illusion; the appearance of changing light source may be stronger for lonely button (vs. group of elements sharing the same light source).