American Cinematographer - April 2008 - 58

edge of monocular depth cues, and your perception that the object is floating in front of the screen will diminish to the point of inconsequence. Your brain has learned that when one object is occluded by another, the occluded object must be farther away. In spite of all the stereoscopic depth cues, your brain knows that if that object is occluded by the edge of the screen, then it must be at or behind the screen plane. This scenario will be very noticeable to viewers as their brains attempt to sort out these contradic- tory depth cues. Monocular depth perception overrules stereo depth cues because we are hard-wired to protect ourselves from danger. Because most danger (such as an approaching lion, bear or saber-toothed tiger) starts from outside our stereoscopic depth zone, it’s easy to understand how the brain defaults to the depth cues that govern most of our life. The 3-D axiom to remember is that offscreen objects should never touch the edge of the screen, because if they do, the illusion will be disrupt- ed. The illusion is most effective with objects that can float or be thrust toward the audience. You will also notice that when you experience these illusions, filmmakers are keeping the offscreen objects closer to the center of the screen in order to avoid the proscenium arch. As with many axioms, however, there is sometimes an exception. There is a scenario in which an occluded object can still appear as though it’s coming off the screen. Imagine a medium shot of a man who walks from behind the screen plane toward the screen plane, and then continues toward the audience until he is in front of the screen. Surprisingly, this shot will still work with the character apparently coming off the screen, even though the lower half of his body is cut off by the bottom of the projected image. The requirement for it to work, contrary to our earlier axiom, is that the viewer must have other audience members in front of him, with the bottom of the screen occluded by their heads. When the bottom of an object is occluded by people very close to you, your brain will still believe the object is getting closer. However, even a clear view of the bottom of the screen will result in a fairly good effect of the man coming off the screen; because we’re programmed to look straight ahead and often don’t see or focus on the lower half of the person coming toward us, obscuration of that person’s lower half usually won’t entirely ruin the offscreen effect. One must also be aware of the constraints on editing in 3-D. This concept is relatively simple to grasp but is often disregarded to the detriment of a 3-D presentation. When editing for 3-D, it is important to consider the convergence extremes that the audience will experience in order to realize the stereoscopic illusion. For example, if the audience is viewing action that occurs behind the screen plane, it is inadvisable to 58 April 2008

American Cinematographer - April 2008

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