The 3-D effect is produced using stereoscopic lenticular printing. Ripples in lenticular bedding are both laterally and horizontally discontinuous. Corrugated images that change when viewed from different angles predate the development of lenticular printing. "Chenopodium" seeds vary in shape between lenticular and cylindrical. Gypsum flowers are most often acicular, scaly, stellate, and lenticular. Bright colours (called iridescence) are sometimes seen along the edge of lenticular clouds. Cases can be classified further, such as regular or irregular and lenticular or corneal. The physical version of the album functions with lenticular printing to mimic the animation effect. One of the main difficulties in lenticular printing is colour synchronisation. Around 1966 several companies started producing lenticular postcards. Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies. Wavy bedding marks the boundary between flaser and lenticular bedding. Each book featured 3D lenticular front and back covers. This fairly common phenomenon is most often observed in altocumulus, cirrocumulus, lenticular, and cirrus clouds. In many respects the composition of lenticular galaxies is like that of ellipticals.