the respective lenses and the superimposed images are viewed on 

 an aluminized screen that preserves polarized light. The viewers 

 wear circular polarized glasses and the human brain then 

 reconstructs a stereoscopic image from the left and right 2-D 

 images that impinge on the left and right retinas. 



There is a third method of viewing 3-D images, not as well 

 known nor often employed by the film industry. This is the 

 eclipse method of projection, wherein "...shutters are placed in 

 front of both left and right projection lenses and shutters are 

 used in the spectacles worn by the audience. When the left 

 shutter is opened at the projector, the left is opened at the 

 viewing device, and so on. In this way the observer sees only 

 the image meant for the appropriate eye" (Lipton, 1982). The 

 eclipse method is also called Stereo Image Alternation (SIA) and 

 is "...the most successful stereoscopic system used for 

 stereoscopic viewing of projected overlapping diapositives" 

 (Moffitt and Mickhail, 1980). The image to each eye is 

 electronically synchronized with the projector and therefore no 

 optical confusion results. "The main advantage of the SIA system 

 over the anaglyph and polarization systems is the more brilliant 

 image on the viewing platen. The SIA system is also most 

 suitable for viewing color diapositives." (See Moffitt and 

 Mickhail, 1980. ) Eclipse projection has not been widely used in 

 the commercial film industry because the optical shutters 

 previously available were electronically sophisticated and, until 

 quite recently, were too costly to supply to an audience with the 

 propensity to take them home after the show. The newest 

 generation of liquid crystal shutters, however, are relatively 

 cheap, comfortable, and the screen can be viewed from any of a 

 variety of angles by a reasonably large number of people, limited 

 only by the number of optical shutters provided. 



Computer Analysis of 3-D Images 



Once the events that transpired on the dive have been 

 appropriately recorded, the data must be analyzed. Four 

 problems are associated with data analysis: the volume of data, 

 analysis of motion, 3-D spatial reconstruction, and complete 

 image analysis. We quote extensively from Potel and Wassersug 

 (1981): 



"The first problem involves the volume of data . . . Given the 

 large number of [animals observed during a dive] it is 

 necessary to record accurately the positions of all 

 individual organisms [so that their interactions can be 

 accurately evaluated] . Knowledge of spatial distribution of 

 organisms precedes statistical evaluation. Thus, the first 

 intrinsic problem ...is the enormous amount of positional 

 information that has to be recorded." 



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