Notes Animation Principles The newsgroup should be working now: - PDF document

Notes Animation Principles � The newsgroup should be working now: � Disney and co. developed certain principles (starting in the 1930 � s) for making good ubc.courses.cpsc.426 animation � Textbook: • Fluid, natural, realistic motion • Not really required, but recommended • Effective in telling the story - it � s a good second opinion, a great overview • Attractive to look at from a different point of view, and has a lot � Developed for traditional 2d cel animation, but more material on the artistic and modeling equally applicable to any animation side which we won � t touch � This course is mostly about the underlying • I don � t know what � s up at the bookstore… technology for computer animation, but these � TA (Stelian) office hour: Friday 1-2? are still important to have in mind cs426-winter-2008 1 cs426-winter-2008 2 Classic Principles Squash and Stretch � Squash and Stretch � Slow In and Slow Out � Rigid objects look robotic---let them deform to make the motion more natural and fluid � Timing � Arcs � Accounts for physics of � Anticipation � Exaggeration deformation • Think tennis ball… � Staging � Appeal • Communicates to viewer � Follow-Through and � Straight-Ahead and what the object is made of, Secondary Motion Pose-to-Pose how heavy it is, … • Usually large deformations conserve volume: if you squash one � Overlapping Action dimension, stretch in another to keep mass constant and Asymmetry � Also accounts for persistence of vision • Fast moving objects leave an elongated streak on our retinas cs426-winter-2008 3 cs426-winter-2008 4 (squash and stretch cont’d) Timing � Pay careful attention to how long an action takes -- how many frames � How something moves --- not how it looks --- defines its weight and mood to the audience � Also think dramatically: give the audience time to understand one event before going to the next, but don � t bore them cs426-winter-2008 5 cs426-winter-2008 6

Anticipation Staging � The preparation before a motion � Make the action clear • E.g. crouching before jumping, pitcher � Avoid confusing the audience by having winding up to throw a ball two or more things happen at the same � Often physically necessary, and indicates time how much effort a character is making � Select a camera viewpoint, and pose the � Also essential for controlling the characters, so that visually you can � t audience � s attention, to make sure they mistake what is going on don � t miss the action • Clear enough you can tell what � s happening • Signals something is about to happen, and just from the silhouettes (highest contrast) where it is going to happen cs426-winter-2008 7 cs426-winter-2008 8 Follow-Through and Overlapping Action Secondary Motion and Asymmetry � Again, physics demands follow-through -- the inertia � Overlapping action: start the next action before that � s carried over after an action the current one finishes • E.g. knees bending after a jump • Otherwise looks scripted and robotic instead of • Also helps define weight, rigidity, etc. natural and fluid � Secondary motion is movement that � s not part of the � Asymmetry: natural motion is rarely exactly the main action, but is physically necessary to support it same on both sides of the body, or for 2+ • E.g. arms swinging in jump characters � Just about everything should always be in motion - • People very good at spotting “twins”, synchronization, “moving hold” etc. � Animator has to give the audience an impression of • Break up symmetries to avoid scripted or robotic feel reality, or things look stilted and rigid cs426-winter-2008 9 cs426-winter-2008 10 Slow In and Out Arcs � Also called “easing in” and “easing out” � Natural motions tend not to be in straight lines, instead should be curved arcs � More physics: objects generally smoothly accelerate and decelerate, depending on mass • Just doing straight-line interpolation gives and forces robotic, weird movement � Just how gradual it is helps define weight, mood, � Also part of physics etc. • gravity causes parabolic trajectories � Also helpful in emphasizing the key frames, the • joints cause circular motions most important or “extreme” poses • etc. • Character spends more time near those poses, and less time in the transition � Keep motion smooth and interesting • Audience gets better understanding of what � s going on cs426-winter-2008 11 cs426-winter-2008 12

Exaggeration Appeal � Obvious in the old Loony Tunes cartoons � Make animations that people enjoy watching � Not so obvious but necessary ingredient in photo-realistic special effects � Appealing characters aren � t necessarily � If you � re too subtle, even if that is accurate, the attractive, just well designed and rendered audience will miss it: confusing and boring • All the principles of art still apply to each still � Think of stage make-up, movie lighting, and frame other “photo surrealistic” techniques • E.g. controlling symmetry - avoid “twins”, � Don � t worry about being physically accurate: avoid needless complexity convey the correct psychological impression as � Present scenes that are clear and effectively as possible communicate the story effectively cs426-winter-2008 13 cs426-winter-2008 14 Straight Ahead vs. Extremes Pose-to-Pose � The two basic methods for animating � Keyframes are also called extremes, since they usually define the extreme positions of a � Straight Ahead means making one frame after character the other • E.g. for a jump: • Especially suited for rapid, unpredictable motion � the start � Pose-to-Pose means planning it out, making � the lowest crouch “key frames” of the most important poses, then � the lift-off interpolating the frames in between later � the highest part • The typical approach for most scenes � the touch-down � the lowest follow-through • The frames in between (“inbetweens”) introduce nothing new---watching the keyframes shows it all • May add additional keyframes to add some interest, better control the interpolated motion cs426-winter-2008 15 cs426-winter-2008 16 Computer Animation Layering � The task boils down to setting various animation � Work out the big picture first parameters (e.g. positions, angles, sizes, …) in • E.g. where the characters need to be when each frame � Then layer by layer add more details � Straight-ahead: set all variables in frame 0, then • Which way the characters face frame 1, frame 2, … in order • Move their limbs and head � Pose-to-pose: set the variables at keyframes, let • Move their fingers and face the computer smoothly interpolate values for frames in between • Add small details like wrinkles in clothing, hair, … � Can mix the methods: • Keyframe some variables (maybe at different frames), do others straight-ahead cs426-winter-2008 17 cs426-winter-2008 18

Splines and Motion Curves Motion Curves � The most basic capability of an animation package is to let the user set animation variables in each frame • Not so easy --- major HCI challenges for designing an effective user interface • We � ll ignore these issues though � The next is to support keyframing: computer automatically interpolates in-between frames � A motion curve is what you get when you plot an animation variable against time • Computer has to come up with motion curves that interpolate your keyframe values cs426-winter-2008 19 cs426-winter-2008 20 Splines Knots and Control Points � The ends of the intervals, where one � Splines are the standard way to generate a smooth curve which interpolates given values polynomial ends and another one starts, are called “knots” � A spline curve (sometimes just called spline) is just a piecewise-polynomial function � A control point is a knot together with a • Split up the real line into intervals value • Over each interval, pick a different polynomial � The spline is supposed to either interpolate (go through) or approximate � If the polynomials are small degree (typically at most cubics) it � s very fast and easy to compute (go near) the control points with • Interpolation almost always required for user controlled motion curves • Approximation allows other good properties… cs426-winter-2008 21 cs426-winter-2008 22 Hermite Splines Smoothness � Hermite splines have even richer control � Each polynomial in a spline is infinitely differentiable (very smooth) points: as well as a function value, a slope � But at the junction between two polynomials, the (derivative) is specified spline isn � t necessarily even continuous! • So the Hermite spline interpolates the control � We need to enforce constraints on the values and must match the control slopes at polynomials to get the degree of smoothness we the knots want � Particularly useful for animation---more • Polynomial values match: continuous (C 0 ) control over slow in/out, etc. • Slopes (first derivatives) match: C 1 • Second derivatives match: C 2 • Etc. cs426-winter-2008 23 cs426-winter-2008 24