Custom Controls Speed Graphics Production

You may not be able to fool Mother Nature, but you can change theseasons — if you have the right 3D tools. At Rhinoceros VisualEffects, New York, we discovered this firsthand while using Houdini 5.5from Side Effects Software on “Snowman,” a commercial spotfor GM's Cadillac Division.

This project was under a tight deadline. The client needed to shooton location in Vancouver in September even though the final,postproduced spot was to take place in a full-blown winter landscape.The challenge, then, was to shoot the material and winterize it throughseamless visual effects.

Every single shot in the digitally enhanced spot would requirereplacing the existing environment with digital winter counterparts.Some shots would be completely digital and would feature everythingfrom CG snow, trees, and cars to digital matte paintings andcompositing. And, of course, all the work would have to be turnedaround in about a month's time.

Given the extremely short production schedule, we looked forsoftware solutions that would provide us with the most flexibility andpower. Since the project involved extensive snowscapes as well asphotorealistic water, mist, and tires interacting with puddles, we feltwe needed the flexibility and custom control that Houdini gives theartist.

Version 5.5 provides a number of advancements, particularly forworkflow productivity. This allowed our boutique-based team to deliverthe high-end work — 3D backgrounds, elements, and effects —required for this spot on deadline.

New modeling, animation, and user interface tools within version 5.5make it even easier to access Houdini's trademark proceduralism. Byadding a set of VEX Operators (VOPs) that enable users to buildproduction-ready shaders without writing code, Side Effects hassubstantially enhanced the ways in which artists can harness the powerof Houdini's unique and powerful scripting language.

Our two-person visual effects team was able to establish a highlyefficient and creative workflow based around Houdini 5.5. Because wewere using similar effects with varying degrees of complexitythroughout, we were able to generate our effects procedurally to keepthings very flexible. Scenes were set up in Houdini and whenever thedirector, Richard Blair, wanted to try something different, we simplyadded, subtracted, or moved objects over various shots on the fly. Thishelped keep the creative dialogue open at all times, and it was also ahuge timesaver. For example, our technical director, Jim Callahan, wasable to procedurally generate more than a hundred trees by changingjust one number.

We created several types of trees using a combination of customParticle Operators (POPs) and a simulation model that closelyapproximated the forms of real-world trees. We attached most of thesimulation input parameters, which ranged from trunk radius and branchradius to rate of branching and deflection forces to spare channels.These were controlled by keyframed splines. Once we chose a set ofparameters for a type of tree, changing a single, random seed parametercould produce a variety of solutions. This allowed us to create a largeset of trees with minimal user intervention.

Another interesting challenge was to create heavy snow buildup onthe conifer trees. Because the snow spanned many small branches andpine needle tufts, it could not be accomplished by displacing the treegeometry. We rendered depth maps using orthogonal cameras pointed downat modified tree geometry to define where the snow would fall. We thenused these depth maps in a custom Surface Operator (SOP) to displace agrid with the same resolution as the depth map. If the depth mapindicated that a point on the grid would miss the tree, it wasdeleted.

Similarly, we also deleted points with excessive surface curvatureand those that were too distant from their neighbors. This resulted inseveral islands of connected triangles that we extruded upwards,smoothed, and then rendered as subdivision surfaces along with theoriginal tree geometry. We added all of these actions procedurally inHoudini and could update them as the positions of the treeschanged.

Tire interaction with slush and puddles was another problem solvedwith Houdini 5.5. Starting with a computer-generated camera thatmatched the live-action motion and four track points that representedthe location and motion of the car on the live-action plates, we set upanother procedural workflow.

Working from the trackers, geometry was constrained to the points,which were used as tire markers and locations from which to emitparticles. To create the pseudo-random effect of the tires splashingwater, we used a carve operation to trim the geometry where the tiremeets the surface and the top of the street. This effect was broughtinto Channel Operators (CHOPs), where curves based on the speed of thecar were generated. Fast motion would mean more geometry for particlegeneration because the tire would be hotter and pick up more moisture.Traveling more slowly would result in the reverse. On this basis, somechoppy motion was added by pre-multiplying a noise function to give amore realistic effect.

For water droplets that shoot from under the tires, we took geometrythat was being trimmed, added UV coordinates, and mapped the normals tothe UV coordinates, again adjusting the amplitude and projectileaccording to the velocity of the car. This allowed us to create a pieceof geometry that could do exactly what we needed it to do, no matterwhat the shot was.

Another challenge in simulating reality was synthetically achievingthe soft global lighting condition of a winter atmosphere. For this, weagain looked to VEX to generate shaders, surface deformation tools,particle operations, and lighting techniques, such as area lighting, tocreate a natural look.

Speeding up things were numerous features within Houdini 5.5: theability to drag and drop one channel to another, to design myriadcustom attributes, and to create custom GUIs to automate manual tasks.Another great capability within version 5.5 is the ability to setaliases to change an entire scene with the stroke of one key. This wasparticularly efficient in working with and reviewing multiple scenessimultaneously, with parameters, strings, and networks all switching toa new scene upon command. Houdini's open architecture additionally madeexchanging camera data, custom VEX, SOPs, POPs, and geometry veryeasy.

Whether we were developing organic trees and water spray orrendering out the final shots, Houdini helped make it all happen. Evenduring the bidding process Houdini helped us prove our ideas wereworkable. At the end, we were able to impress our client withsophisticated digital visual effects on a very short deadline. Withseven scenes realized for “Snowman” by two Houdini artistsin just four weeks, our decision to go with Houdini 5.5 let us achievea high level of complexity while keeping a flexible and artist-friendlyproduction approach.

Vincent Serritella is visual effects artist with RhinocerosVisual Effects and Design, a full-service digital studio in New Yorkfor commercials, music videos, and broadcast and televisionprogramming. The studio has used Houdini-based tools from Side EffectsSoftware on projects for clients ranging from AT&T to M&Ms. Fora link to view “Snowman” online, log on to www.rhinofx.com. Formore information about Houdini from Side Effects, visit www.sidefx.com.