Passive Stereo Virtual Reality System
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Concept of Stereoscopy
Before introducing our passive stereo virtual reality (Passive stereo VR) system, we will look at the theory and concept of stereoscopy[5]. Scientists have realized that human eyes perceive stereoscopy through depth cues. And there are two kinds of depth cues: monocular and stereoscopic. When we look at a computer screen, TV and photo, we can realize those objects inside the display media are 3 dimensional because of the monocular cues. But in the real world, scientists find out there some extra information other than the monocular cues that cause human eyes to perceive 3 dimensional stereo scenes. This extra element is called stereoscopic cues. In the following, we will elaborate the elements of these two different kinds of cues.
Monocular Depth Cues
This kind of cues is the basic elements that artists use in their painting:
- Light and Shade. This is the most basic element. Shading makes object look solid, and casting shadows make object look resting on top of a surface.
- Relative Size. Memory helps us to make a judgment about the distance of familiar objects.
- Interposition. The objects closer to the viewers will block the sight of other objects behind or farther away from them.
- Textural Gradient. A textured material, like grassy lawn, provides a depth cue because the texture is more apparent as the object is closer to the observer.
- Aerial Perspective. Distant vistas will look bluish haze because of the scattering of red light intervening atmosphere. This effect deteriorates in thick fog.
- Motion Parallax. A familiar example of this cue is seen from a moving train. Observers can notice that the electricity poles move past more quickly than the distant houses.
- Perspective. Observing the railroad track, one can notice that the parallel track seems to recede into a vanishing point.
- Depth Cuing. This is the effect of reducing the intensity of the object in proportion to the distance from the viewer.
Monocular depth cues are part of the real visual world and can be easily generate at the electronic displays. But normal electronic display lack of stereoscopic depth cues. Some of these monocular depth cues, such as the perspective and motion parallax, are important when creating the sense of stereo viewing.
Stereoscopic Depth Cues
Scientists realize that in the real world there is a stereoscopic depth cue caused by the retinal disparity. Normal human eyes are 5 to 7 centimeters apart, and the images formed in each retina are not quite the same. However, human brains are able to fuse those two images together and create a perception of stereoscopic image. Try this experiment: hold your finger in front of your face. When you look at your finger, your eyes are converging on your finger. That is, the optical axes of both eyes cross on the finger. If you continue focus your eyes on your finger, paying attention to the background, you will notice the background appears to be double[5]. (As shown in figure 1.)
figure – 1
On the other hand, when looking at the background your eyes are converging on it, and your finger, with introspection, will appear to be double. (Figure 2)
figure – 2
Therefore, to create stereo viewing experience on a 2D screen, we need to generate left-eyed image and right-eyed image, and then let the observer’s right see only the right-eyed image, and left eye for the left-eyed image. This report will describe how the visualization team at the National Center for High-performance Computing (NCHC) setup the virtual reality theater in their laboratory in stead of introducing all the possible ways for creating stereo viewing.
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