Cylindrical panoramas are intended to be viewed when curved around the inside of a cylinder with the viewer in the centre. When panoramas of this type are viewed flat and without any correction of perspective, curvature of the horizontal axis may be visible.
The concept of a cylindrical panorama can be compared to wrapping an elongated paper print around a sphere such as an translucent globe of the Earth. The print should be wrapped around the globe such that it makes contact with the equator. A powerful light at the centre of the globe would then project a cylindrical panoramic image of the surface of the globe on to the paper. Verticals such as the lines of longitude remain vertical in the panorama, and non-vertical lines become curved. Higher-latitude areas of the globe closer to the poles would be stretched out, whereas equatorial regions would remain unchanged.
This basic technique has been used for centuries to project geographical maps of the surface of the spherical Earth onto the flat surfaces of the pages of a book.
The 360-degree 3D panorama below shows the surface of Mars as seen by the NASA roving vehicle Opportunity.