Volumetric display
A volumetric display device is a display device that forms a visual representation of an object in three physical dimensions, as opposed to the planar image of traditional screens that simulate depth through a number of different visual effects. One definition offered by pioneers in the field is that volumetric displays create 3D imagery via the emission, scattering, or relaying of illumination from well-defined regions in (x,y,z) space.
A true volumetric display produces in the observer a visual experience of a material object in three-dimensional space, even though no such object is present. The perceived object displays characteristics similar to an actual material object by allowing the observer to view it from any direction, to focus a camera on a specific detail, and to see perspective – meaning that the parts of the image closer to the viewer appear larger than those further away.
Volumetric 3D displays are technically not autostereoscopic, even though they create three-dimensional imagery visible to the unaided eye. This is because the displays do not generate stereoscopic images; they naturally provide focally-accurate holographic wavefronts to the eyes. Due to this, they have accurate characteristics of material objects such as focal depth, motion parallax, and vergence.
Volumetric displays are one of several kinds of 3D displays. Other types are stereoscopes, view-sequential displays,[1] electro-holographic displays,[2] "two view" displays,[3][4] and panoramagrams.
Although first postulated in 1912, and a staple of science fiction, volumetric displays are not widely used in everyday life. There are numerous potential markets for volumetric displays with use cases including medical imaging, mining, education, advertising, simulation, video games, communication and geophysical visualisation. When compared to other 3D visualisation tools such as virtual reality, volumetric displays offer an inherently different mode of interaction, providing the opportunity for a group of people to gather around the display and interact in a natural manner without having to don 3D glasses or other head gear.
Human–computer interfaces[edit]
The unique properties of volumetric displays, which may include 360-degree viewing, agreement of vergence and accommodation cues, and their inherent "three-dimensionality", enable new user interface techniques. There is recent work investigating the speed and accuracy benefits of volumetric displays,[15] new graphical user interfaces,[16] and medical applications enhanced by volumetric displays.[17][18]
Also, software platforms exist that deliver native and legacy 2D and 3D content to volumetric displays.[19]
Technical challenges[edit]
Known volumetric display technologies also have several drawbacks that are exhibited depending on trade-offs chosen by the system designer.
It is often claimed that volumetric displays are incapable of reconstructing scenes with viewer-position-dependent effects, such as occlusion and opacity. This is a misconception; a display whose voxels have non-isotropic radiation profiles are indeed able to depict position-dependent effects. To-date, occlusion-capable volumetric displays require two conditions: (1) the imagery is rendered and projected as a series of "views", rather than "slices", and (2) the time-varying image surface is not a uniform diffuser. For example, researchers have demonstrated spinning-screen volumetric displays with reflective and/or vertically diffuse screens whose imagery exhibits occlusion and opacity. One system[20][21] created HPO 3D imagery with a 360-degree field of view by oblique projection onto a vertical diffuser; another[22] projects 24 views onto a rotating controlled-diffusion surface; and another[23] provides 12-view images utilizing a vertically oriented louver.
So far, the ability to reconstruct scenes with occlusion and other position-dependent effects have been at the expense of vertical parallax, in that the 3D scene appears distorted if viewed from locations other than those the scene was generated for.
One other consideration is the very large amount of bandwidth required to feed imagery to a volumetric display. For example, a standard 24 bits per pixel, 1024×768 resolution, flat/2D display requires about 135 MB/s to be sent to the display hardware to sustain 60 frames per second, whereas a 24 bits per voxel, 1024×768×1024 (1024 "pixel layers" in the Z axis) volumetric display would need to send about three orders of magnitude more (135 GB/s) to the display hardware to sustain 60 volumes per second. As with regular 2D video, one could reduce the bandwidth needed by simply sending fewer volumes per second and letting the display hardware repeat frames in the interim, or by sending only enough data to affect those areas of the display that need to be updated, as is the case in modern lossy-compression video formats such as MPEG. Furthermore, a 3D volumetric display would require two to three orders of magnitude more CPU and/or GPU power beyond that necessary for 2D imagery of equivalent quality, due at least in part to the sheer amount of data that must be created and sent to the display hardware. However, if only the outer surface of the volume is visible, the number of voxels required would be of the same order as the number of pixels on a conventional display. This would only be the case if the voxels do not have "alpha" or transparency values.