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Animation Overview Overview Traditional Traditional Animation Animation Keyframe Keyframe Animation Animation Interpolating Interpolating Rotation Rotation 1999 Star Wars: Phantom Menace 2002 LOTR Two Towers 2001 Final Fantasy 1 Computer Animation • Models have parameters – Polygon positions, normals, spline control points, joint angles, camera parameters, lights, color – n parameters define an n-dimensional state space – Values of n parameters = point in state space • Animation defined by path through state space – To produce animation: » » » » 1. 2. 3. 4. start at beginning of state space path set the parameters of your model render the image move to next point along state space path, repeat 2 – Path usually defined by a set of motion curves » one for each parameter • Every animation technique reduces to specifying the state space trajectory—the state space will change with the technique Animation vs. Modeling • Modeling and animation are tightly coupled – Modeling: what are the control knobs and what do they do? – Animation: how to vary them to generate desired motions? 2 Overview • Animation techniques –Traditional animation (frame-by-frame) –Keyframing –Procedural –Behavioral –Performance-based (motion capture) –Physically based (dynamics) • Modeling issues –Rotations –Inverse kinematics Traditional Cel Animation • Film runs at 24 frames per second (fps) – That’s 1440 pictures to draw per minute – 1800 fpm for video (30fps) • Productions issues: – Need to stay organized for efficiency and cost reasons – Need to render the frames systematically (render farms) • Artistic issues: – How to create the desired look and mood while conveying story? – Artistic vision has to be converted into a sequence of still frames – Not enough to get the stills right--must look right at full speed » Hard to “see” the motion given the stills » Hard to “see” the motion at the wrong frame rate 3 Traditional Animation: The Process • Story board –Sequence of drawings with descriptions –Story-based description • Key Frames –Draw a few important frames as line drawings » For example, beginning of stride, end of stride • Inbetweens –Draw the rest of the frames • Painting –Redraw onto acetate Cels, color them in • Hierarchy of jobs (and salary) Layered Motion • It’s often useful to have multiple layers of animation – How to make an object move in front of a background? – Use one layer for background, one for object – Can have multiple animators working simultaneously on different layers, avoid re-drawing and flickering • Transparent acetate allows multiple layers – Draw each separately – Stack them together on a copy stand – Transfer onto film by taking a photograph of the stack 4 Story Boarding (from “A Bug’s Life”) Principles of Traditional Animation [Lasseter, SIGGRAPH 1987] • Stylistic conventions followed by Disney’s animators and others (but this is not the only interesting style, of course) • From experience built up over many years – Squash and stretch -- use distortions to convey flexibility – Timing -- speed conveys mass, personality – Anticipation -- prepare the audience for an action – Followthrough and overlapping action -- continuity with next action – Slow in and out -- speed of transitions conveys subtleties – Arcs -- motion is usually curved – Exaggeration -- emphasize emotional content – Secondary Action -- motion occurring as a consequence – Appeal -- audience must enjoy watching it 5 Principles of Traditional Animation Squash and Stretch 6 Squash and Stretch Anticipation 7 Follow Through Secondary Action 8 Computer Assisted Animation • Computerized Cel painting – Digitize the line drawing, color it using seed fill – Eliminates cel painters (low rung on totem pole) – Widely used in production (little hand painting any more) – e.g. Lion King • Cartoon Inbetweening – Automatically interpolate between two drawings to produce inbetweens (a la morphing) – Hard to get right » inbetweens often don’t look natural » what are the parameters to interpolate? Not clear... » not used very often 3D Computer Animation • Generate the images by rendering a 3-D model • Vary the parameters to produce the animation • Brute force – Manually set the parameters for each and every frame – For an n parameter model: 1440n values per minute • Traditional keyframing – Lead animators draw the important frames – Underpaid drones draw the inbetweens • Computer keyframing – Lead animators create the important frames with 3-D computer models – Unpaid computers draw the inbetweens – The dominant production method 9 Interpolation • Hard to interpolate hand-drawn keyframes – Computers don’t help much • The situation is different in 3D computer animation: – Each keyframe is a defined by a bunch of parameters (state) – Sequence of keyframes = points in high-dimensional state space • Computer inbetweening interpolates these points • How? You guessed it: splines Keyframing Basics params • Despite the name, there aren’t really keyframes, per se. • For each variable, specify its value at the “important” frames. Not all variables need agree about which frames are important. • Hence, key values rather than key frames • Create path for each parameter by interpolating key values frames key values interpolated values 10