a.k.a. "active stereo"
Glasses have liquid crystal lenses which can be darkened & cleared rapidly.
At any time, one lens is clear and one is dark.
Glasses are synchronized with video display - one eye sees odd frames, other eye sees even frames.
Requires fast video rate (> 90 Hz) to prevent flicker.
LCD projectors are not capable of high frequencies.
Affordable DLP projectors not programmed to support high frequencies.