The Polar Express Diary: Part 3 -- The MoCap/Anim Process
July 17-October 24, 2003 A tweak cluster is a selection of points on a surface grouped together under one controller. The controller allows you to push, pull, rotate or scale many predefined points on a surface. The weighting of each point (how much it is influenced by the movement of the controller) is accomplished by “painting” the amount of effect that the controller has on the selected points. In general, the weighting is painted such that the central region of control has the greatest affect, graduating to minimal control toward the outer edges. This allows you to essentially sculpt the model, and animate that sculpt on the fly.
John creates a single control on the tip of Chris’ chin that would realistically bend and give way as he put the weight of his head on the banister. It was easy to set up, easy to use and the end result was very effective.
Train Arrives Sequence (TA) October 9, 2003 June 19-December 8, 2003 Unfortunately, stand-alone props with animation curves applied could not make it through our pipeline as-is. An animatable rig is required in order to survive the upgrade that occurs when a shot moves forward to the next department. To solve this problem, Carolyn Oros (pipeline TD) scripted a procedure that was able to take those rudimentary props and use them to drive a rigged component so that it would follow the pipeline of our component assembly system. This became yet one more of our standard tools.
Our next milestone occurred when our boy sneaks down the stairs, and when he realizes that Santa hasn’t visited yet, rests his chin on the banister as he ponders that thought. The problem was that his chin was rigid as it rested on the rail, and not deforming as you would expect. All of our facial controls were designed to imitate muscles affecting the skin from underneath. It was never considered how we would deform the skin and underlying tissue with an external force. John Matthews did some investigation into how we might do this and came up with tweak clusters as the solution.
Note We started using tweak clusters everywhere, especially on our main characters. We were able to sculpt facial expressions that were simply unattainable with the muscle system alone. We could sculpt cheek creases, brow furrows (skin bunching between the brows) and convincing “swallowing” animation with tweak clusters in the jaw and neck. This was a huge relief, because whatever was not possible with the animation rig could surely be handled with tweak clusters if it came down to it. However, this kind of control can easily get out of hand. We ended up using tweak clusters on the train, the ticket, and at one point all the facial animation on several elf shots with tweak clusters alone. Rather than devising new animation controls for a specific action, our setup group would often defer us to the tweak cluster solution because it was so simple and effective.
Number of Shots: 36
July 17, 2003-February 16, 2004
A tricky shot of the boy pulling a number of deforming props from a drawer has been underway for quite some time. Several elaborate rigs were built early on to manipulate the props — but the results were only moderately successful. Upon discovering the tweak cluster solution, the shot was retro-fitted with tweak clusters and animator Marco Marenghi was able to produce the desired results almost immediately.
When the train arrives, the boy’s room starts shaking violently, which means that all of the props in the room need to shake also. Animator Stephane Couture begins animating each prop by hand, which seemed to be the most logical approach at first. As the shaking gets progressively more violent he begins experimenting with dynamic simulation and is able to achieve excellent results in very little time. A good example is the jar of pencils that falls over, and the pencils spill onto the desk and some roll off the edge. The lamps and lampshades were also set up for dynamics, and some very natural results were achieved.
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