July 6, 2010 at 7:34 PM ET"Help me, Obi-Wan Kenobi. You're my only hope."It's been 33 years since Princess Leia's tiny hologram made that 3-D plea in the "Star Wars" saga - and ever since, researchers have been working on image projection systems that could turn that science-fiction special effect into reality.Today, Carnegie Mellon University is highlighting a projection system called AquaLux 3D that takes one more small step toward that virtual-reality dream - but the system is just one of many approaches that's being tried.The grand goal is to project three-dimensional, moving images in what seems to be thin air, and in such a way that you could interact with them. This might involve projecting the images onto fast-spinning mirrors in an enclosed space, like the groovy gizmo offered up by graphics geeks at the University of Southern California's Institute for Creative Technologies.Or you could use video projectors to throw pictures onto a spray of mist or fog. That's the kind of thing you can see at a Disney "Fantasmic" show, or a Heliodisplay demonstration, or a FogScreen presentation. A few years back, a Mitsubishi experiment known as Submerging Technologies showed how light projections and sensors could be used to add some interactive twists to water sculptures.Most of these systems made their debut at the annual SIGGRAPH showcase for interactive graphics, and AquaLux 3D will have its own turn in the SIGGRAPH spotlight later this month in Los Angeles.If you're at all familiar with Latin, you know already that the AquaLux system relies on water plus light. The Carnegie Mellon researchers - including robotics professors Srinivasa Narasimhan and Ph.D. student Peter Barnum - started out trying to develop LED automobile headlights that were optimized for driving through rain at night. They were aiming to control the light beam dynamically so it could actually shine between the raindrops, rather than reflecting off the droplets."What we realized is that it was much easier to shine light on the drops themselves," Narasimhan said in today's news release."The beauty of water drops is that they refract most incident light, so they serve as excellent wide-angle lenses that can be among the brightest elements of an environment," he said. "By carefully generating several layers of drops so that no two drops occupy the same line of sight from the projector, we can use each drop as a voxel that can be illuminated to create a 3-D image."Voxel? That's a fancy word for a 3-D pixel. The AquaLux system is designed to control a single video projector as well as a high-speed water dripper to build a precisely calibrated image in three dimensions, much as the pixels on a TV or computer screen build up a two-dimensional image. In the SIGGRAPH presentation, the researchers demonstrate what they call a 2.5-D system: Images are projected on five sheets of water droplets, created by emitters capable of putting out 60 drops per second from each valve. Even 10 drops a second is enough to produce a continuous image for the human eye, the researchers say.The team's YouTube video shows how the system can project text and video images ... even a multilayered Tetris video game and a virtual aquarium. The system could be used for the usual applications, such as displays for theme parks and trade shows. They could also open the way for video games or virtual-reality interfaces that float in three dimensions, without any need for clunky 3-D specs or VR helmets."One unique aspect of AquaLux 3D is the potential for physical interaction," Narasimhan said. "People can touch the water drops and alter the appearance of images, which could lead to interactive experiences we can't begin to predict. We look forward to the day when creative people can fully explore the potential of this display."The next step would be to increase the density of the droplets for a more realistic 3-D effect. "The main limitation of this work is that it only has a few layers. ... A faster projector could allow for a display with many more layers. But to create a true 3-D drop display would also require more precise drop control," the researchers say.The fastest projectors that are currently available could produce a 17-layer, 3-D display - as opposed to the five-layer, 2.5-D display to be demonstrated at SIGGRAPH. That might be an interesting experiment. But if there's anyone out there who wants to give it a try, remember Yoda's 30-year-old advice from "The Empire Strikes Back":"Do, or do not. There is no 'try.'"For more about "Star Wars" science fact and fiction, including 3-D projections, check out this report from 2005. Join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter. And if you really want to be friendly, ask me about "The Case for Pluto."