The German fairy tale Rapunzel is nearly two centuries old, but generations of children all over the world know its most famous line by heart, “Rapunzel, Rapunzel, let down your hair so that I may climb the golden stair.”
A new generation of children is seeing that golden stair, in all its computer-generated glory, in “Tangled,” the new Disney movie adapted from that old story.
It features Rapunzel, the young woman shut away in a tower in the middle of the woods, along with the handsome prince who uttered the famous line.
But it also features the golden stair, 70 feet of computer-generated hair that Carolina alumnus Kelly Ward began imagining into being five years ago when Walt Disney Animation Studios told her about the project during her job interview.
That interview, Ward said, came about thanks to Ming Lin, the professor in the computer science department who served as her graduate adviser. Not only did Lin give her the freedom and support to conduct research work on animated hair modeling, Ward said, but Lin also gave a lecture at Disney studios highlighting her tremendous progress.
Ward started working on hair modeling as a class project in which her goal was to try to make a single hair move.
After she did it, she realized there was much more to learn, such as figuring out the complex computations it would take to create all the stands of hair on the human head – 100,000 – and get them to move as they would in real life from the stroke of a hand, a toss of the head or a gust of wind.
But the feat of rendering this flowing mound of 700 feet of hair into virtual being – in a form that appears both realistic, and at times, magical – is the kind of challenge software engineers have never before attempted.
“We often looked at Rapunzel’s hair as a character on its own,” Ward said. “But Rapunzel is such a beautiful character herself – so young and expressive and full of life – we also wanted her hair to embody those same qualities.”
Ward also had to get the hair to defy the laws of physics by reducing the effects of gravity and friction without anyone noticing.
“Rapunzel is a petite girl,” she said. “In real life, 70 feet of hair would weigh about 60 pounds, more weight than a real person would be able to move around as effortlessly as we allow Rapunzel to do in the movie.
“Throughout the movie, Rapunzel does a lot of running and jumping. She does cartwheels. The hair is everywhere and characters are always in her hair. Rolling in it. Combing it. Climbing it.”
It is the golden stair, after all.