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Most
theatrical screens, of all types, tend to be perforated. In order
that sound from speakers located behind the screen will emanate with
the least amount of attenuation, these "sound perfs" cover
the entire area of the screen. When analog photographic motion picture film is projected on these screens, there is little problem. Film grain tends to be both homogenous and random, varying in position from one frame to the next. With digital projection, each pixel strikes the screen at exactly the same point. If a pixel lands on a hole, it will disappear. For planar 2-D, this is still no problem, as both eyes will perceive the loss of the same pixel identically. Stereoscopic 3-D, however, creates a different impression. Since the particular pixel representing a point in one eye-view may land in a hole, and the pixel representing the corresponding homologous point for the other eye-view may land on reflective screen material, that particular point will be perceived in only one eye-view, as compared to the other ones surrounding it. This results in a condition known as "retinal rivalry". A stereoscopic image with image parts apparently "moving" across the screen will appear to scintillate or sparkle wherever this retinal rivalry occurs. Recognizing this problem, screen manufacturers now make aluminized screens with much smaller perforations, called "micro-perfs". These help, but do not entirely eliminate the problem. Also, not all auditoriums are equipped with these. From the standpoint of the producer, in moving parts of the image, it is good to avoid small detail, especially repetitive or geometric patterns. Costumes and set dressings should be designed carefully, so that individual elements, or groups of elements, are not generated such that they appear identical. A randomizing program not only makes these small detail parts appear more realistic, but also minimizes adjacencies that aggravate the retinal rivalry problem in relation to screen perfs. |