Beyond Belief

The Curse of Clarity

Effects pioneer Willis O'Brien made the giant-creature shots in King Kong using a technique called stop-motion animation. Stop-motion animators pose flexible model creatures (apes, dinosaurs, what have you) in miniature settings and expose a single frame of film. Then they move the creatures into the positions they would have assumed 1/24-second later if actually in motion, expose another frame, and begin the dreary round again. It's laborious, but for decades it was the only way to bring truly outrageous objects "to life" in the cinema.

The thing to remember about traditional stop-motion is that none of the "moving" subjects are actually in motion when they are photographed. But an illusion of motion arises when the human eye views these still pictures, twenty-four of them each second, on a motion picture screen. (The numbers differ for video, but the same principles apply.)

Stop-motion is far from perfect. To see why, we must consider how motion picture images are created--and how the brain interprets them to produce the illusion of motion. A motion picture camera operating at sound speed exposes twenty- four frames per second (FPS). It's a purely mechanical process-- advance the film a frame, stop it, open the shutter, close the shutter, advance to the next frame--so less than half of the time between frames is available for exposing the film. The actual exposure time for a frame of motion picture film is about 1/60- second. If you've used a 35mm still camera that lets you set your own shutter speeds, you know 1/60-second isn't very fast. If you photographed a basketball game at that shutter speed, you'd get blurry images of the players. To get stills that freeze the action, you'd want a faster shutter speed -- say, 1/500-second.

Why do moving objects blur with exposures in the 1/60- second range? Imagine photographing a passing car. At 60 mph, the car moves 88 feet per second, almost a foot and a half during the 1/60-second that your shutter is open. How will the car look when the prints come back? It will blur, of course, reflecting the fact that the car was not in the same place throughout the exposure. Blur severity varies not with an object's absolute speed, but with how much of the image area it crosses during an exposure. Set your shutter at 1/60-second, stand 200 feet from a superhighway, and snap a picture. The cars will only blur a little. Stand on the shoulder and snap that same traffic, and you won't be able to tell Fords from Toyotas.

When it comes to blur, Hollywood movie cameras work just like your still camera at 1/60-second. They just cost more. Pop your favorite movie in the VCR. Freeze a single frame of an action scene. You'll see blurring you never imagined was there. But your brain notices it. More, it expects moving objects to be blurry.

That brings us to the question of how the brain interprets the projected motion picture image. Everyone knows the basic principle: Successive still images are flashed on a screen, and a phenomenon called "persistence of vision" keeps us from seeing the intervals of darkness between frames. We view the succession of stills as a continuous image. When an object changes position from frame to frame, we perceive that the object is in motion. But this isn't the only cue that can fool the visual system into perceiving movement. 3-D movies exploited stereoscopic vision to create vivid impressions of movement. Conventional movies don't take advantage of stereopsis. But they do take advantage of other assumptions the brain seems to make about moving objects. One such assumption is that the image of a fast-moving object will be degraded as a consequence of its movement. In other words, if an object is moving quickly enough across the visual field, the brain expects it to blur.

Keeping that in mind, we can reconstruct why the pilot's- eye view shot in King Kong looked so good, and why the smash- the-airplane shot looked so bad. Since subjects in classical stop- motion do not move during exposures, they do not blur. For the pilot's-eye view shot, Willis O'Brien rigged a stop-motion camera to roll down a track over a huge New York skyline diorama toward a model Empire State Building. Though the camera seemed to move at 150 mph, since the objects in motion relative to the camera (the skyline) were distant, nothing moved very far across the image area between any two frames. If the shot had been staged for real, you wouldn't expect much blur. So the fact that the stop-motion sequence had no blur did not detract from the illusion of motion it created.

The smash-the-airplane shot was a fairly close, static shot. The airplane sped across the screen; Kong's arm lashed out and struck it. With each frame, they crossed large fractions of the image area--normally a recipe for severe blurring. But stop- motion can't blur! When viewing this scene, we experience conflict between two modes of visual interpretation. Objects change positions drastically from frame to frame, which tells the brain that they are moving quickly. But the absence of blur tells the brain that everything is stationary. Result: interpretive conflict. The illusion of motion is compromised.

Stop-motion shots that don't have the blurring they need give viewers a "strobing" sensation. Objects jerk-jerk-jerk like dancers under an old disco strobe light. Three famous scenes created by stop-motion master Ray Harryhausen in the 1950s and 1960s exemplify the problem. Next time you find Jason and the Argonauts on late-night cable, watch how the swordfighting skeletons "strobe," especially in close shots. Check out the jerky movements of the giant crab in The Mysterious Island. In One Million Years B.C., a stop-motion pterodactyl carries off Raquel Welch. Its wings flap in and out of frame at high speed--but with no blur. Even to untrained eyes, it looks profoundly wrong.

In the next installment, we'll see how moviemakers since the time of Star Wars have applied high technology--and sometimes, startlingly low technology--to inject blur into animated footage. Understanding how and why Hollywood professionals use blur to make their illusions more effective can help us all understand the myriad ways the eye can be fooled.

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