Increasing frame rates can produce sharper apparent motion but may introduce other problems. The key to smoother motion images may lie in what has been called a ‘soft shutter’. By capturing media at a high frame rate, the visual ‘look’ can be resampled in software and output at appropriate frame rates for distribution in different formats.
Tony Davis is the senior scientist for RealD, which acquired Tessive, the company he founded that created Time Filter software to avoid temporal aliasing in motion pictures. The system is now marketed as RealD TrueMotion.
He presented some examples to a packed audience in the Ray Dolby Ballroom at the SMPTE technical conference in Hollywood.
Many of the problems associated with low frame rates are easily recognised. Car wheels can seem to spin backwards, fences appear to jump across the screen, and helicopter rotors look jerky.
Technically there are two main effects: judder and strobing. Judder, or temporal sampling aliasing, is related to how the camera samples motion in the scene. Strobing, or temporal reconstruction aliasing, is related to how the motion is represented in display.
Various shutter systems have been used in cameras to reduce judder but mechanical and electronic systems are limited in that they can only affect a single frame.
A soft or synthetic shutter can mathematically consider adjacent frames based on different weights, which can include both positive and negative components.
The concept is based on capturing at high frame rate and then temporally resampling with a digital filter or synthetic shutter.
Material shot at 120 frames per second can be converted to 24 frames per second by sampling five adjacent frames with equal weights.
Different filters or soft shutters can be used to produce different aesthetic effects.
The approach is similar to how spatial resolution can be resampled from a high-resolution source to produce different resolutions for distribution.
It turns out that it is not even necessary for the input rate to be an exact integer multiple of the output frame rate. According to RealD, so long as the capture rate is 120 frames per second or higher, it is possible to produce output at lower frame rates, including fractional frame rates.
The TrueMotion system has been used to create distribution versions of Billy Lynn’s Long Halftime Walk, from director Ang Lee.
The principles of temporal resampling of high frame rate source material could have far wider implications.
The meeting paper “It’s all backwards: Rethinking frame rate and temporal fidelity in a cinema workflow” by Tony Davis of RealD was presented at the SMPTE Annual Technical Conference 2016 in Hollywood.