Most of the perceptual cues that humans use to visualize the world's 3D structure are available in 2D projections. This is why we can make sense of photographs and images on a television screen, at the cinema, or on a computer monitor. Such cues include occlusion, perspective, familiar size, and atmospheric haze. Four cues are missing from 2D media: stereo parallax - seeing a different image with each eye, movement parallax - seeing different images when we move our heads, accommodation - the eyes' lenses focus on the object of interest, and convergence - both eyes converge on the object of interest. All 3D display technologies (stereoscopic displays) provide at least stereo parallax. Autostereoscopic displays provide the 3D image without the viewer needing to wear any special viewing gear.