Digital Stereoviewing

As with analog stereoviewing, on-screen stereoviewing requires that each of the two images is perceived by one eye only. There are various hardware and software solutions for this that separate the two images either by color, by time, by polarization, or by viewing angle. The simplest solution is the anaglyph method mentioned above; it requires no special graphic cards or hardware besides cheaply available anaglyph glasses. This is also a useful and financially feasible method for displaying 3D views to a larger audience via video projector, and the authors use it frequently in student courses.

Excellent 3D views can be created with dedicated stereographic cards, suitable display devices, and stereo-eyeware. With the active shutter-glasses method, the two images are displayed alternately with a frequency depending on the monitor refresh rate (Fig. 11-21). LCD shutter

FIGURE 11-20 Two A4 printouts of the images shown in previous figure under a Zeiss mirror stereoscope fitted with 8x magnification binoculars.

FIGURE 11-21 The stereopair shown in Figure 11-19 displayed in quad-buffered stereo mode on a computer monitor, to be viewed with wireless active shutter glasses activated by an infrared emitter (top left and center). Alternatively, the system shown here can be used for anaglyph viewing (red-cyan glasses top right). Photo by IM.

FIGURE 11-21 The stereopair shown in Figure 11-19 displayed in quad-buffered stereo mode on a computer monitor, to be viewed with wireless active shutter glasses activated by an infrared emitter (top left and center). Alternatively, the system shown here can be used for anaglyph viewing (red-cyan glasses top right). Photo by IM.

glasses alternately block one of the images in synchronization with the display. This method relies on high monitor refresh rates (120 Hz minimum are required to avoid flickering) that at the time of writing are still rare with LCD flat-panel monitors and usually only available with the near-extinct large CRT monitors. Another method is the superimposition of the images through polarizing filters and the use of passive polarized glasses. Recent stereo-display systems based on the same technique use two LCD monitors mounted at an angle with a semi-reflecting beamsplitter mirror between them, a dual-head graphics card, and polarized glasses for viewing.

Finally, autostereoscopic display devices have for the last decade or so offered the possibility of aid-free 3D viewing. A stereogram is composed of vertically interleaved lines from the two stereo-images, and an equally slitted barrier or lenticular plate is placed in front of this view. When viewing the screen from the right distance, the barrier or the lenses on the lenticular plate block the view of the left image's lines to the observer's right eye and vice versa. Thus, a true stereoscopic image using the human 3D perception ability is created. Shan et al. (2006) have found that autostereoscopic technology can be employed for photogrammetric stereo-measurements but yield 16-25% lower precision than the more commonly used shutter-glasses methods.

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