Monday, September 27, 2010

Creative Human Character Animation: The Incredibles vs. The Polar Express

Isaac Kerlow looks at recent human 3D character animation developments in The Incredibles and The Polar Express.

The Incredibles and The Polar Express are two recent animated features that achieve remarkable creative results using different styles of human character animation. The Incredibles is an example of first-class keyframe cartoon character computer animation that integrates 2D traditional styles with the 3D computer style that we have come to expect from Pixar projects. The Polar Express offers an innovative approach that animates computer-generated virtual characters by applying realtime human performances and keyframe touch-ups.

Comedy and Action Through Squash-and-Stretch
Two aspects of The Incredibles represent significant departures from the topic and style of earlier Pixar movies: the human characters are central to the storyline and they are animated with considerable squash-and-stretch. To make the later possible the technical character team and the animators joined forces to develop new tools for animating squash and stretch humans in real time. The design and ease of use of the character setup rigs was driven by the animators' needs, and keeping the overall look-and-feel similar to earlier versions of Pixar animation software was an important consideration.

One of the main characteristics of the software tools used to animate The Incredibles was the layering of two key stages in the animation process: the bone and muscle calculations, and the squash-and-stretch system. Without the layering of these two stages it would have been difficult to provide animators with realtime feedback. In simple terms, the first stage of this layered process started by running the bone and muscle calculations through all the character's positions in a shot. Using statistical analysis the software determined the most significant changes in the character's skin throughout the shot, and "baked" those deformations into the model so that they could effectively be used as blend shapes. (Without the baking animated models played back at rates of eight to 10 frames per second.) After doing the first pass animation the animators saw the baked geometry and did not have to deal directly with the bone and muscle system. The second stage of the layered animation process consisted of applying the squash-and-stretch to the baked geometry as a post-process, and animators were able to visualize this in realtime.

Geometry decimation was another technique used to keep the playback as close as possible to 24 frames per second. Animators were involved in the process of hand-crafted decimation that brought some meshes to about 25% of the full geometry but kept, for example, the full and decimated silhouette shapes of each character identical. Each character had a decimated version, with most of the savings occurring in the body geometry and not the face. Decimated models did everything that full models did, and the deformation hierarchies remained the same but with much less overhead. Shots got finalized during the animation stage using the decimated version but the final rendering used the full geometry.

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