What do a simulation of swimming fish, a video of a brown-haired girl, alternating layers of solid colors and a multilayer version of the classic puzzle video game of Tetris have in common? Not much, except that researchers at Carnegie Mellon University's Robotics Institute are projecting each onto layers of falling water droplets to create a multidimensional display.

The seemingly random images the researchers have been projecting were chosen "mostly for fun" and to show the range of possibilities, said Peter Barnum, a Ph.D. student in robotics.

"The point of this is not that we made a new video game," he added, "The point is what shows on the display."

With the new display technology, named AquaLux 3D, "there is no limit to the type of content you can project," said Srinivasa Narasimhan, associate professor of robotics, who worked on the project along with Mr. Barnum and Takeo Kanade, professor of computer science and robotics.

What's great about the project is that a wide range of people -- from a 5-year-old who likes looking at the projected images to a physics professor who likes thinking about the science behind them -- can enjoy it, Mr. Barnum said.

AquaLux 3D differs from existing water displays -- such as Disney's Fantasmic! -- in that it can show very high-resolution 3-D images by using multiple layers of water droplets and, unlike many other displays, can continuously stream video content, researchers said.

To achieve such high resolution, researchers first precisely controlled the generation of water droplets. A series of synchronized, computer-controlled manifolds, which consist of fine steel needles attached to pipes filled with water, dictate the rate at which the drops fall. Using the principles of fluid mechanics, researchers ensured that no falling drop would block any other, Mr. Narasimhan said.

In the prototype system, a single camera records where the drops are falling. A projector then targets the drops, illuminating each with the correct color.

"The drops are like spherical lenses," Mr. Narasimhan said. Each serves a voxel, or three-dimensional pixel, that doesn't absorb projected light but instead refracts it across a wide range of angles. That means the display can be viewed from any angle, he said. And since water isn't solid, the display can be touched from both the inside and the outside, opening up interactive possibilities.

The water can fall at a rate of between 10 and 60 drops per second. At 5 drops per second, the image flickers and seems broken instead of solid, but by 10 drops per second the brain is able to integrate the image, researchers said. Above that, it is difficult for the human eye to perceive much difference in clarity, Mr. Narasimhan added.

The prototype system has up to four layers of water. "We used multiple layers to get the depth feel," Mr. Narasimhan said.

The more layers, the greater the sense of depth, researchers said. The prototype system uses a 60-frame-per-second projector, but a faster projector could illuminate more layers of water droplets.

A computer simulation of the technology with 17 layers of water depicts a globe that is clearly made of planes but still appears truly three-dimensional.

The researchers were scheduled to discuss AquaLux3D yesterday at SIGGRAPH, the 37th International Conference and Exhibition on Computer Graphics and Interactive Techniques, in Los Angeles.

They are hoping that others will take their technology to the next level.

"We are geeks, but I'm sure Disney people will be much more innovative," Mr. Narasimhan said, adding that the display could be used for marketing and advertising, in theme parks and museums, or for teaching children about scientific principles in an enjoyable way.

AquaLux 3D began as an offshoot of work on a car headlight "that can see through rain," Mr. Narasimhan said. Because raindrops can reflect light from existing car headlights, making it hard to see, the researchers were trying to create a headlight with beams that would avoid them. As they worked to develop such a headlight -- a project that is still ongoing -- they realized it was a lot easier to hit the raindrops than to miss them.

Both technical and non-technical video demonstrations of the technology are available on the project website: www.cs.cmu.edu/~ILIM/projects/IL/waterDisplay2/index.html. Together, the two YouTube videos have received more than 100,000 hits.

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