Three-dimensional (3D) photographs are made using two images of the same scene taken from slightly different angles (parallaxes). The image obtained on the right is called R, and on the left, L. The stereoscope, an object made with lenses or mirrors, directs the right eye to see just the R image and the left eye the L image. Thus, the brain fuses the two images, producing the 3D sensation.
The same principle applies to 3D videos, like the two types of auto-stereoscopic screens that generate 3D without the viewers having to use special glasses. In the “parallax barrier” system, used in games and laptops, the R and L images are cut up into narrow vertical columns of pixels and placed alternately on the screen. In front of them, there is a mask of clear and dark slits (the parallax barrier). The dark line of the parallax barrier hides the L image from the right eye and the R image from the left eye, so that each eye only receives the image directed at it.
In the “lenticular screen” system, used in televisions, the R and L images are also cut up into vertical columns of pixels and placed alternately. But, in place of the parallax barrier, there are cylindrical lenses that project the images of the R and L columns of pixels in different directions (see infographics). In this case in particular there are problems, such as the position of the viewers. Depending on where they are, they may see the R and L images simultaneously or even inverted.
José Henrique Vuolo, University of São Paulo (USP)
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