i don't know much about elliptic geometries, but a fisheye lens seems to be an exemple of it.

in halfsphereVision (180deg around), an edge contained in vertical frontal plane that contains the point-of-view, wherefrom the observer looks forward, is a horizon of vision, its boundary, perimeter.

now is this vision-edge in 180degFOV an ideal line at infinity ?

what about ideal lines ? do they twist ? is that wherefrom the 'strangeness'(nonorientability) of projective plane comes from ?

anyway, my question is this: can we measure in projective geometry at all, and if so, what is the diameter of the space represented through halfspherical fisheye lens ?

half infinity ? or one infinity ? what kind of ?

or what is the diameter(extent) of RP3 ?

is diameter of RP4 'bigger', like 2 infinities ?