Back in March I had an opportunity to see a production called Kindur put on by the Italian Company Compagnia TPO. One of the most beautiful and compelling effects that they utilized during the show was to use a live-camera to create a mask that revealed a hidden color field. The technique of using a live feed in this way allows a programmer to work with smaller resolution input video while still achieving a very fluid and beautiful effect.
This effect is relatively easy to generate by using just a few actors. To star this process we'll start with a Video-In Watcher actor. The video-in will be the live feed from our camera, and will ultimately be the mask that we're using to obscure and reveal our underlying layer of imagery. This video-in actor connects to a Difference actor which looks for the difference between two sequencial frames in the video stream. This is then in turn passed to a Motion Blur actor. The motion blur actor will allow you to specify the amount of accumulated blur effect as well as the decay (disappearance rate) of the effect. To soften the edges of this the image stream is next passed to a Gaussian Blur actor. Finally this stream is passed to an Add Alpha Channel actor by passing the live feed into the mask inlet on the actor. The underlying geometry is then passed in through the video inlet in the Add Alpha Channel actor. Finally, the outlet of the Add Alpha actor is passed out to a projector.
As a matter of best-practice I like to use a Performance Monitor actor when I'm creating a scene in order to keep an eye on the FPS count. This can also be useful when trying to diagnose what's causing a system to slow down during playback.
This effect works equally well over still images or video, and is certainly something that's fun to experiment with. Like all things in live systems, your milage may vary - motion blur and gaussian blur can quickly become resource expensive, and it's worth turning down your capture settings to help combat a system slow-down.
You can see my full documentation set here: