points within each photograph. Knowledge of the distances 

 between the two cameras and from camera lens to film plate and 

 measurement of the size and horizontal displacement in the 

 paired images of a reference object of known size and distance 

 from the cameras permit accurate measurement of any 3-D object 

 in space. If the cameras are stationary, accurate speeds in 3-D 

 space, etc. can be recorded; if the cameras track a given 

 individual, this animal becomes the reference point and 

 movements of other individuals are calculated in relation to the 

 moving reference point. Recalibration of the cameras is not 

 necessary as long as the cameras are not moved in relation to 

 one another. 



3-D video projection 



The motion picture industry has used two methods to display 

 3-D images to audiences of more than one person. The anaglyphic 

 method uses complementary colored filters over the left and 

 right projector lenses to superimpose both images on a screen. 

 The viewer wears one red and one green lens to isolate the 

 appropriate image for each eye. But proper anaglyphic projection 

 is extremely difficult because it is technically tricky to 

 balance the color densities of the red and green. The result is 

 invariably a binocular asymmetry in color, resulting in eye 

 strain. Further, two-color mixing processes produce an image 

 with unnatural color. Several years ago the television industry 

 enjoyed a brief flurry of interest in 3-D anaglyphic cable 

 television, but problems of viewing were not resolved and 

 interest in that format quickly waned (Lipton, 1982). 



The second common method of viewing 3-D images uses linearly 

 polarized images for the right and left eyes, with the planes of 

 polarization for the two eyes at right angles. Polarizing lenses 

 worn by the viewer occlude the image of the contralateral 

 projection lens, producing one discrete image for each eye, 

 perceived by the brain and processed as are our own retinal 

 images. The optics work fine if the viewer is seated properly in 

 relation to the screen and the head is not tilted sideways. 

 Lateral movement of the head moves the glasses out of alignment 

 with the fixed axes of polarization of the projected image, 

 resulting in distorted 3-D images and rapid eye fatigue due to 

 unnatural tracking patterns of accomodation and convergence. 



During the last several years circular polarizing filters 

 have become available, and these "...provide excellent 

 extinction over a wide range of angular rotation, thus providing 

 the opportunity to view 3-D comfortably with great freedom of 

 head motion and position." (Walworth et al., 1984). We utilize 

 this projection technique to view in 3-D the paired video tapes 

 recorded with our shallow water system. The paired images are 

 projected stereoscopically in the laboratory by two video 

 projectors with left and right circular polarizers fitted over 



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