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Motion Parallax Effects in UAS Aerial Videography

By John Reinhardt
Source: FAA Safety Briefing Nov/Dec 2020

Controlled flight into terrain (CFIT), trees, or towers is a prime concern of mine every time I fly. I fly a three-pound quadcopter to take aerial images at low altitudes and there are so many obstacles to avoid.

Before takeoff, I consider distance estimation and depth perception factors to avoid a CFIT. The FAA’s Pilot’s Handbook of Aeronautical Knowledge (PHAK) explains these factors and identifies them as both monocular and binocular in nature.

Monocular cues include motion parallax, geometric perspective, and aerial perspective. Binocular cues are dependent upon the viewing angle difference between each eye to the object. These visual illusions can be dreadful for flying; however, they are very useful for composing aerial images.

PHAK defines various monocular cues and explains that motion parallax is an “apparent motion of stationary objects as viewed by an observer moving across the landscape.” For example, take driving on the highway — when you look out the window, fences and electrical poles close to the road zoom past, while trees in the distance appear to move at a slower rate. The PHAK further explains that geometric perspective occurs when an object appears to have a different shape when viewed from varying perspectives and distances. It also states that aerial perspective refers to the effect the atmosphere has on the appearance of an object when viewed from a distance; the object has fewer details and its color will become less saturated with blue tones (at daylight) or red/orange tones (during the golden hour).

To prevent CFIT, I take several steps before liftoff to counteract monocular cues, in addition to using the quadcopter’s obstacle avoidance system. The first step is to plan the operation to determine the operation’s objective, airspace, weather, and flight path. Second, I conduct a site survey to identify ground and air risks. Third, I establish a geo-fence and an altitude hard deck that clears all obstacles. I fly low and slow (or hover) to frame and compose the image, adjust camera settings, and track the subject. Flying within visual line of sight, I use the quadcopter’s camera to view a different perspective and ensure I’m clear of any obstacles.

To exploit monocular cues in the digital images, especially the motion parallax effect, I use the camera’s settings and design specific flight sequences. It is extremely rewarding to capture images that show these effects. To record video showcasing the parallax effect, I fly the quadcopter using two different flight sequences:

  1. For the first sequence, I fly really low on top of the ground or water and rapidly move forward toward the skyline. The video captured in this sequence shows the foreground moving faster than the skyline.
  2. The second sequence uses the quadcopter’s ActiveTrack function and is flown laterally. This captures the parallax effect — trees or other static structures in the foreground — that presents the motion differ-ence between the foreground and the skyline. As the quadcopter flies laterally, it also yaws to maintain focus on a tagged object in the skyline. The video captured in this sequence will show the foreground moving faster than the background with a slight yaw.

The other visual cues can also be exploited using frame composition and camera settings to create interesting aerial images.

Understanding both of these concepts will allow you, as a UAS pilot videographer, to better frame the images you desire while maintaining the highest level of safety. Learning to use monocular cues while flying drones can save costs and may inspire you to use different approaches to videography, creating more interesting images.

John Reinhardt is a program manager in the UAS Integration Office’s Operational Program Branch. He currently manages the FAA’s UAS Test Sites Program.

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