The phrase “bullet time” first entered the popular lexicon with the launch of the 1999 film The Matrix. This showed the hero dodging bullets in a slow motion as the camera angle moved lazily around the action. Indeed, in “bullet time” it is possible to freeze the action entirely and still move the camera angle.

This is possible by filming the action using a number of different cameras closely spaced around the scene. It is then possible to change the point of view by switching from one camera angle to the next, even when the action is frozen.

Since then, bullet time has become common in big budget films. More recently, hobbyists have begun to experiment with it, albeit at considerable cost because the technique requires a large number of cameras. That makes bullet time an expensive and time consuming luxury for filmmakers.

Today, all that changes thanks to the work of Yan Wang at Columbia University and a couple of pals who have developed a smartphone app call CamSwarm that allows a group of them to record bullet time action. As a result, bullet time is set to become a low cost feature that more or less anyone can shoot.

The idea is simple in principle. Any array of cameras can record bullet time provided they meet certain standards of positioning and synchronization.

First, the cameras must be linked and controlled using a suitable communications protocol. They must be oriented and spaced in a way that provides good coverage of the action with all pointing toward and focused on the same spot. And finally, their shutters must be synchronized when they start capturing footage.

Wang and co’s CamSwarm app takes on these tasks using a local Wi-Fi network to coӧrdinate the cameras. Each smartphone in the swarm must run the app, with one designated as a leader. This app generates a QR code that the others photograph to join the group. The video from them all is then streamed to a cloud server that stores the data and helps direct the swarm.

The app works by showing each user his or her own footage along with the footage from cameras next to them in space. This allows them to adjust their spacing and orientation so that each camera has a slightly overlapping field of view and has the target occupying a similar fraction of each camera’s screen and so on. All this ensures a smooth transition from one point of view to another during bullet time.

In particular, the app provides on screen guidance to help people achieve the right kind of spacing and orientation, which is otherwise hard to gauge visually. It does this using the smartphone’s built in gyroscopes and digital compass to determine their direction and any changes in orientation.

And when the action begins, the app ensures that the shutters of all cameras are synchronized.

Finally, the app replays all the footage and allows the user to choose which sequences of camera angles to use to create the bullet-time effect.

Wang and co say this allows a bullet time sequence to be set up and ready for shooting in under a minute, significantly less than with any other set up.

The team goes on to evaluate its app by asking 20 people in five groups to use the app to create a bullet-time movie and to fill in a questionnaire about their experiences.

These evaluations seem to be successful (although the team does provide a link to any bullet time examples produced using the app). “Preliminary user study results suggest that the system can help users achieve higher quality output than simpler alternatives, and is easy and fun to use,” say Wang and co.

That’s an interesting approach to collaborative photography. Since it was invented more than 150 years ago, photography has largely been a solitary hobby.

Sure, the photographer will interact with his or her subjects and may even have assistants to help with lighting, make up, and so on. But the act of picture taking, the holding of the camera or smartphone, the framing of the image and the pressing of the shutter release are almost always under the sole control of a single individual.

With CamSwarm and a related app called PanoSwarm for taking collaborative panoramas, all this changes. These apps turn photography into a collaborative activity allowing many people to contribute to the creation of an image or movie.

That’s something that could have a profound effect on the nature of photography and the kind of creativity that it produces although the team has yet to announce a date for the apps’ release into the wild.

Ref: http://arxiv.org/abs/1507.01148 : CamSwarm: Instantaneous Smartphone Camera Arrays for Collaborative Photography