UC Berkeley Main Campus
Department of Art Practice, Center for New Media, Fall 2004
We implement Hierarchy Animation principles with skeletons and learn how to rig a character with inverse kinematics.
Walk cycles are the foundation of every character: many of a character's features are apparent in the way it walks. For example, we can tell a character's age, health and weight by observing the speed and ryhthm of its gait. We can also observe its attitude: an ambitious walk is very distinct from a lazy walk. Finally, we can make deeper observations about a character's history: be it noble, harried, stoic, urban, rural, detached, bouncy, dancerly, light, somber, the gait is honed by life-long habit. Naturally, animators exploit this perceptive sensitivity for expression.
For animation, we parametrize a gait in the following terms:
Gait > Sequence of Strides > Stride Cycle > Consists of two Steps > Step consists of Stance and Swing > Stance consists of Heel, Foot and Toe Phase and Swing consists of Lift and Pass Phase. The complicated part of the walk cycle is that the left and right foot steps are not exactly opposed, but slightly overlapping. This means that a walk cycle generally starts and ends at the beginning of the right swing, but begins and ends in the middle of the left step stance phase. Here is a graph showing the step phases, and here is a more detailed illustration. Finally here is an indispensable film clip of a fast walk cycle from CMU (8 MB).
Gait is measured in stride frequency and stride distance. Stride frequency is the number of steps per minute, and stride distance is the distance covered in one minute. For animation purposes, we can convert these measurements into frames per cycle and units per cycle. The average gait stride frequency is about 1 second, or 30 frames. The average stride distance is about one third of the body height, or, two thirds of the length of a character's legs.
Keyframes are set in accordance with the gait phases. It is best to animate the right leg first and then use the timing of the right leg to time the left leg.
We use the same positions and keyframe times for the left leg, but we start in a different position:
Note that the cycle does not begin where it ends exactly, it begins one frame after the frame it ends on. This prevents redundancy at the seam of the loop.
Rig your character from Assignment 4 or a new character using inverse kinematics. Using a walk cycle, animate your character to walk towards a solid obstacle using one walk cycle. The animation should be no longer than 30 seconds. Add sound effects if you like, but no music, please. We will start with the music in the next assignment.
When rigging your character, watch out for groupings and inherit transforms. To move your character forward, create a group which includes all skins, all IK's and of course the skeleton. The skins should be bound to the skeleton before you group them together. The skins Inherit Transform should be turned off. The IK Handle Inherit Transform should be turned on. Test your character for deformations and hierarchy errors before you start animating.
For extra credit, animate your character having a misadventure: let it trip and snub its toe. Show the reaction to the pain in a changed walk cycle.
Output the animation to playblast and post it at art.berkeley.edu/coursework/niemeyer/courses/art172/art_172_wk05/student in a folder named as follows: firstinitial_lastname_firstinitial_lastname_a05. This assignment is due next Wednesday.
A reference file of a basic skeleton is available for downloads here: Body_04.ma . There are more downloadable, advanced characters at the Center for New Media downloads site.
Main Course Website
Gait Metrics Paper
Biped Motion
Last Year's Finals
