Tuesday, January 15, 2013

HFR and The Hobbit: There and Back Again

This entry appears on the Oxford University Press blog site: http://blog.oup.com 

     Is it the sense of experiencing reality that makes movies so compelling? Technological advances in film, such as sound, color, widescreen, 3-D, and now high frame rate (HFR), have offered ever increasing semblances of realism on the screen. In The Hobbit: An Unexpected Journey, we are introduced to the world of 48 frames per second (fps), which presents much sharper moving images than what we've seen in movies produced at the standard 24 fps. Yet many viewers, including myself, have come away with a less-than-satisfying experience as the sharp rendering of the characters portrayed is reminiscent of either old videotaped TV programs (soap operas, BBC productions) or recent CGI video games. What features of HFR create this new sensory experience and why does it appear so unsettlingly similar to the experience of watching a low budget TV program?

      One factor that can be ruled out is the potential difference in flicker rate. Moving images are of course created by the rapid succession of still frames, and thus the flicker or on-and-off rate must be fast enough so that we do not perceive any change in illumination between frames. With early silent films, the flicker rate was less than 16 fps, and a noticeable flashing or flickering was apparent (hence the term "flicks" to refer to these early movies). Since the advent of sound, the standard has been 24 fps, though the flicker rate is increased with the use of a propeller-like shutter that spins rapidly in a movie projector so that a movie running at 24 fps actually presents each frame two or three times, thereby increasing the flicker rate to 48 or 72 fps. Thus, with respect to flicker rate we have always watched movies at HFR.
         Two factors have motivated the current interest in HFR. The obvious one is that actions recorded at more rapid frame rates, such as a car chase shot at 48 fps vs 24 fps, would reduce by half the distance objects move across successive frames. With HFR we are presented shorter increments of movement, and our brains need not work as hard to extrapolate apparent motion across frames, which may result in a smoother sense of motion. I, however, do not think that it is this between-frame difference that is driving our sensory experience as we watch The Hobbit. A second, less known factor, is that the movie was shot at a faster shutter speed than movies shot at 24 fps. Filmmakers have a rule that states that the shutter speed at which each frame is shot should be half as long as the frame duration. Thus, most movies we've seen have been shot at 24 fps with a shutter speed of 1/48 sec for each frame. Those of you who have played with photography know that this shutter speed would produce rather blurry images when the camera is hand held. On a tripod, a movie filmed with this shutter speed would show fast moving objects (e.g., cars) with a noticeable blur. When movies filmed at 24 fps are shot with a faster shutter speed and less motion blur, actions appear jerky and unnatural.
         The Hobbit was filmed with a shutter speed of 1/64 sec, which produced less motion blur and thus sharper images compared to movies shot at 24 fps. At the faster frame rate, the jerkiness associated with presenting sharp images at 24 fps is largely reduced, though I did notice that on some occasions large camera movements and fast movements of actors appeared stilted and unnatural. A psychological study by Kuroki and colleagues showed that in order to perceive naturalistic movements with sharp moving images (i.e., no motion blur) it is necessary to use frame rates of 250 fps or faster. Interestingly, the shutter speed used for The Hobbit closely matches that used for old videotaped TV programs, which were filmed at 30 fps with a shutter speed of 1/60 sec. I suspect that it is this close match in shutter speed (and thus similarity in image sharpness) that creates the impression of viewing a soap opera when we watch Bilbo Baggins and company.
         In the future, after years of experiencing HFR movies, will we be able to appreciate the more realistic renderings garnered by this new technology? Will a younger generation without prior associations to videotaped TV programs be enamored by the sharper images? Time will tell, though I'm skeptical. HFR does offer a more realistic rendering than what we've previously encountered at the movies, and further advances may help to refine its use. Yet do we really want to have an entirely realistic portrayal? In most cases that would mean having the experience of sitting next to the director watching actors on a sound stage with artificial lighting, which is exactly the impression I had while watching Bilbo backlit by what was supposed to be moonlight. Instead, we may end up preferring a softer image which maintains the illusion of being engaged in an adventure with our favorite fictional characters and partaking in a wonderfully unexpected journey.

 Watch this video for details on motion blur and shutter speed: