Step 15 Displays

Examples of common displays used in virtual reality include head-mounted displays, shutter glasses, autostereoscopic systems, CAVES, and video monitors. Current video monitors in surgery are a common interface for relaying images captured through a variety of scope designs. This interface is the easiest to recreate in simulation, since the standard PC uses a monitor interface anyway. However, as simulation and surgery progress, an understanding of other more immersive-type displays should be understood.

When people think of virtual reality, they often think of stereoscopic display or three-dimensional views that pop right out of the screen. The eye captures two-dimensional images on the retina and the human mind perceives distance or depth by using the many available depth cues. The physiological cues include accommodation, convergence, and binocular parallax. Accommodation is the eye muscle tension needed to change the focal

Stereoscopic display can be achieved with a simple cathode ray computer monitor and shutter glasses.

Autostereoscopic systems do not require glasses and divide two-dimensional displays into two sets of viewable pixels, which are displayed to each eye separately. The downside is that this display required a fixed viewing position.

Head-mounted displays consist of an image source with collimating optics (each eye sees a different display). The head-mounted display image may be a cathode ray tube (like a small TV), a liquid crystal display (like a laptop screen), or a virtual retinal display by which images are displayed directly on the retina.

Volumetric displays are an emerging technology that allows for true three-dimensional display.

CAVE® is a room-sized advanced visualization solution that combines high-resolution DLP™ based stereoscopic projection technology and three-dimensional computer graphics to create the illusion of complete sense of presence in a virtual environment.

length of the eye lens in order to focus at a particular depth. Convergence refers to the muscle tension required to rotate each eye so that it is facing the focal point. Binocular parallax means that each eye sees a slightly different view. Psychological cues that contribute to seeing in three-dimensional including relative size, occlusion of far objects by close ones, linear per-spective, shading, texture gradients, atmospheric effects such as color and haze, and motion parallax.

Common run time graphics (such as Open GL) automatically calculate shading, occlusion, relative size, texture, atmospheric effects, etc. This allows the perception of distance in a two-dimensional display, but objects do not appear to pop out of the screen. To display a scene in stereo, it is necessary to address accommodation, convergence, and/or binocular parallax. Stereoscopic displays typically rely only on binocular parallax, where each object in a virtual scene is shown to each eye at a slightly different angle and view position (relative to each eye). Methods for displaying different views to each eye include color filters, polarizing filters (which requires a three-dimensional projection system), shuttered vision, and directional filters.

Stereoscopic display can be achieved with a simple cathode ray computer monitor and shutter glasses.

Shutter glasses alternately occluded each eye and this is synchronized with the computer screen. This makes it possible to show a different display to each eye. Because stereoscopic display requires two separate alternating views, the refresh rate must be doubled as compared to that required for standard animation. If the overall refresh rate is less than 60 Hz (30 Hz per eye), then many viewers will perceive a flicker in the display. Shutter glasses can also be coupled with large projection displays, such as found in theme parks or IMAX theaters.

It should be noted that using shutter glasses with a display screen is not considered a "true" three-dimensional display. This is because accommodation and convergence are ignored as the screen is always the same distance away. Many people will get a headache from extended viewing with these displays because there remains a conflict between the perceived distance of virtual objects and the actual focal distance of the display.

Autostereoscopic systems do not require glasses and divide two-dimensional displays into two sets of viewable pixels, which are displayed to each eye separately. The downside is that this display required a fixed viewing position.

At the sacrifice of display resolution, multiple sets of pixels can be utilized, which enables more stereo viewing positions and wider viewing freedom. Some advanced systems will track the viewer's eye positions and adjust the aim of each pixel set so that they follow the eyes.

Head-mounted displays consist of an image source with collimating optics (each eye sees a different display). The head-mounted display image may be a cathode ray tube (like a small TV), a liquid crystal display (like a laptop screen), or a virtual retinal display by which images are displayed directly on the retina (34).

A feature typically coupled with head-mounted displays is head tracking. This makes it possible to update the user's view based on where their head is relative to the virtual scene.

Volumetric displays are an emerging technology that allows for true three-dimensional display.

These systems show pixel information in a predefined volume of three-dimensional space, rather than just a flat two-dimensional surface. One application of a volumetric display uses a varifocal mirror that oscillates at a high rate enabling variable focal distances. By synchronizing the image shown on a reflecting screen with the optical power of the mirror, any point of a given volume can be illuminated. Another experimental display uses the concept of emissive volume. This display has given volume filled with a medium that can emit light from any part of its interior as a result of an external excitation (such as from different wavelength lasers). One key problem with this approach is finding the appropriate substrate to serve as the medium. Another approach is the rotating screen. A flat screen rotates at around 600 rpm. For every angular position of the screen, an optical system projects a scene corresponding to the perspective associated with the screen angle. The final result is the three-dimensional image of the object, viewable in 360°. Most volumetric displays are still in experimental development and it may be some time before they find a commercial market.

CAVE® is a room-sized advanced visualization solution that combines high-resolution DLP™ based stereoscopic projection technology and three-dimensional computer graphics to create the illusion of complete sense of presence in a virtual environment.

The CAVE was the first virtual reality technology in the world to allow multiple users to immerse themselves fully in the same virtual environment at the same time. This

CAVES are quite expensive and have not been widely utilized for medical simulation, but are beginning to be explored as a tool for medical team/disaster and immersive training.

An intuitive interface can be crucial for efficient utilization of a simulator.

is accomplished by using four, five, or six projection surfaces and numerous projectors that relay stereoscopic images when viewed with three-dimensional glasses (glasses based on either color filters, polarizing filters, or shuttered vision). CAVE systems are usually coupled with three-dimensional "pointer" tools that can be used to interact with the environment and each other.

CAVES are quite expensive and have not been widely utilized for medical simulation, but are beginning to be explored as a tool for medical team/disaster and immersive training.

Holograms are images made on high-resolution film that captures patterns of light waves emanating from an object when illuminated by a laser. When light shines through this special film, the light patterns are reproduced, giving a three-dimensional effect. Holograms usually involved fixed scenes with no animation.

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