An image can be canceled out by a projection of its exact negative. Libsparrow consists of two projectors set up to compete over a single screen: each tries to cancel the output of the other and overlay its own images.
Both projections tend to show pictures of sparrows and Lower Hutt scenery, but they very rarely want to show the same pictures at the same time. Out of this conflict arise complex patterns of video feedback, which are sometimes catastrophically altered by people getting close and blocking a projector or camera.
Libsparrow was first shown at the Dowse Art Gallery in July 2010.
Libgstsparrow is the software behind Libsparrow. It is a Gstreamer plug-in that performs the necessary calibration, decides what it wants to show, and negates what it doesn't like. It is free software.
Source code
There is a git repository at http://repo.or.cz/w/sparrow.git. It is licensed under the LGPL, like Gstreamer itself.
Examples
Here are some videos the work took of itself being installed at the Dowse. It wasn't quite finished. These pictures are of course taken from the work's own camera: this is what each projections uses to decide what to negate.