Although the interface appears simple, there's a lot of sophisticated math behind it. In order to make the user feel as though she is exploring the space of possible designs, Koltun says, the software has to represent the 100 different parameters governing the appearance of the trees on two-dimensional maps. Those 100 parameters translate mathematically into a space of 100 dimensions. Koltun explains that every choice the user makes causes the program to choose a curved two-dimensional slice of the true space, in a process similar to that used to draw a line of best fit for a set of points in a scatterplot. Naturally, some information is lost in the process, but since the slice curves through multiple dimensions, it allows the user to make changes to far more than two parameters at once.

The researchers also designed the program to intelligently decide which possible trees to display to users. "Showing a set of random designs would be awful," Koltun says, explaining that choosing random values for the 100 possible parameters would likely result in distorted, unappealing trees. So in determining which trees to display, the program is guided by the selections of previous users.

Dryad isn't the only way for users to create 3-D designs. Second Life has a scripting language that can produce anything from a tree to a Zamboni, but it's a language that the user has to learn. Interactive Data Visualization's SpeedTree is a sophisticated tree-design tool that players of Unreal Tournament 3 can use to design trees for customized gaming environments. But company president Michael Sechrest explains that the tool is geared to players with an interest in game development, called modders, and it hasn't been adjusted for ease of use.

Peter Phillips, technical director for Millions of Us, an agency that specializes in creating content for virtual worlds, considers Dryad "a really promising tool to allow inexperienced people to create personalized content." He adds that "one of the things that looks unnatural in MMOs [massively multiplayer online worlds] is when we see repeated identical objects." Phillips thinks that tools such as Dryad can solve that problem by making it easy for people to tailor objects to their personal tastes. While he considers the map interface a good approach, he says that the current version of Dryad has some performance shortcomings.

Right now, Dryad is available for Windows and Macintosh systems, and the Stanford group is working on a Linux version. Koltun notes that Dryad is only a demonstration of a type of design interface, and he says that the group plans to extend the concept to allow users to design human forms and other objects commonly found in virtual worlds.