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The Grammar of Growth: Digital Domain Brings Life to What Dreams May Come

Oct 1, 1998 12:00 PM, Ellen Wolff

A perennial challenge for computer animators is the creation of vegetation that feels "real." While they have moved beyond the "popsicle plants" of years past, it is still daunting to create complex trees that sway believably in the breeze. For Polygram Pictures' What Dreams May Come, the animators at Venice, California-based Digital Domain faced this challenge in spades. They had to build a towering tree covered with hundreds of thousands of flowers and then blow them away in swirling, colorful clouds. As actors Robin Williams and Cuba Gooding Jr. look on, the wind strips the huge tree bare. "It's an emotional climax of the film," says DD's visual effects supervisor Kevin Mack.

As a starting place, Director Vincent Ward presented a stylized drawing of a tree with strands like a weeping willow, flowers like a jacaranda, and has the giant limbs of an oak. Not only did this composite tree have a unique look, but DD decided to build it in an innovative way. They generated it using principles of artificial life, which, in this case, meant simulating the tree's growth via procedural software. "Rather than model it frame by frame," notes Mack, "we GREW it."

This 3-D modeling approach involved the use of L-systems, "which are a formal grammar for defining plant growth and plant branching structures," Mack explains. "It's sort of like writing a genetic code for the tree, which defines the rules for how it will grow. The computer executes this code and grows the tree according to those rules. We could place key branches where we wanted them. But for the most part, it was up to the random nature of the gene. It modeled this beautiful complex tree that we could never have actually modeled ourselves."

Mack admits that some people worried about this approach at first, but says: "Once Vincent realized that we could modify things explicitly at his whim, he was pretty into the idea. The turnaround time on changes was actually faster because we weren't hand-modeling anything-to re-grow parts we just changed the rules." DD implemented the L-system in SideEffects' Prisms/Houdini software running on a network of SGIs, Mack explains. But he adds, "We did have to write code in order to utilize that functionality." He credits DD's Matthew Butler and David Prescott for making the approach work. "David grew the tree, and the two got it to blow in the wind and make the branches sway."

While they worked on the tree, lead matte painter Martha Mack used Amazon Paint to create the multi-layered, "2-1/2 D" landscape in which the tree sits. Since the tree's flowers had to fall convincingly to the ground, DD modeled a 3-D terrain in Prisms/Houdini to match Martha's composition. This virtual ground plane also provided a surface on which to cast accurate shadows from the 3-D tree.

"To animate the flowers blowing off," Mack continues, "Matthew used multiple particle systems and actual physical simulation of the wind on tiny leaves that have mass and inertia. They blow off and form swirling masses and sweep along the terrain into little vortices-it's a beautiful model of turbulence." Prisms' collision detection software insured that the flowers touched the 3-D ground plane in a believable way. "As the flowers are blowing off," adds Mack, "we also had the smaller branches sway more than the bigger branches, which was a pretty elaborate part of getting this all to look realistic."

Meanwhile, DD animated the background "in 2-1/2 D so that we have a lot of multi-planing," explains Mack. "We did 2-D warps to Martha's multiple sky paintings so that the clouds roil and swirl and light streams through them. Rain elements move across the sky way off in the distance, and red color lights up the horizon because the director wanted to see the lighting change during the sequence."

DD then applied all of these lighting changes to the tree, its leaves, and its flowers and rendered each as a separate element. Since DD had to apply motion blur to thousands of flowers, the team chose RenderMan software because it offered the fastest speed. Blowing off all the flowers took only about 12 seconds, notes Mack, "but our renders were harsh-this shot used huge amounts of RAM. We brought the whole facility to its knees a number of times."

DD used its proprietary compositing software, "Nuke," to combine the tree and background elements with multiple passes of flowers and shadows, along with footage of the actors shot against green screen. Mack recalls: "We did quite a bit of color correction to get the lighting just right and get the tree matched into the painting perfectly. The compositing done by Darren Poe really made the shot sing."

Looking back on the process, which took about six weeks, Mack believes that "there was definitely an economy gained by doing it procedurally. It would have been suicide to try it any other way."

In the end, the director got his otherworldly tree, and Mack kept a unique souvenir of the process taped to his wall-a print-out of what the computer did, step by step, at each branching of the tree. "It's the actual execution of the genome typed out," he laughs. "It's 11 densely filled, single-spaced typewritten pages of the most intense gibberish you ever saw...and it's really cool."

Director: Vincent Ward; For Digital Domain: Visual Effects Supervisor: Kevin Mack; Modeler/Animator: David Prescott; Particle System Animator: Matthew Butler; Matte Painter: Martha Mack; Compositor: Darren Poe

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