Dana Vrajitoru
I310 Multimedia Arts and Technology
Animation
Introduction to Animation
- Def. Animation is a sequence of images projected fast
enough for the eye not to process them independently.
- In computer graphics, it describes the evolution of a scene
through time.
- Things that can evolve in the animation: objects, camera, light
sources
- Types of animation:
- flip-book or frame-by-frame
- key-frame or interpolated
Frame
- The effect of motion is obtained by displaying still images
(frames) fast enough that the eye perceives them as a continuous
motion.
- Standard frame rate: 24 frames / second.
- Computer screen: 30 - 60 frames / second.
- Real-time animation: the program is capable of displaying the new
frames so fast that the viewer sees the action as it happens (games,
scientific visualization).
- Pre-rendered animation: the image is rendered, recorded, and the
compiled animation can be projected afterwards (motion picture,
cartoons).
Object Animation
- Things that can change for an object:
- position (canon bullet)
- orientation (motorcycle on the road)
- size (grow or shrink)
- form (morphing)
- color (blushing)
- transparency (fluids)
- For a light source:
- position, orientation, intensity
- For the camera:
- position, look-at point (point of interest), view angle
Traditional Animation
Cel animation - comes from celluloid transparent sheets used to
create every frame.
Camera Animation
- Still camera: the camera position and orientation are
fixed, the image is moving. Especially used with dialog scenes.
- Dolly shots: the camera moves on a straight line but
focuses on one object in the scene. Several types depending on the
movement with respect to the object of focus: character dolly (or
push-in), pull-back reveal, depth, expand, contract.
- Pan shots: in which the camera rotates around a given position.
- Fly-through animation: usually for landscape scenes, filmed
form the pov of a bird/plane flying through in a smooth motion above
the scene.
- Subjective camera: the camera shows the scene from the
point of view of the object performing the action (actor). Typical
application: the roller coaster animations, first person games.
- Generally animations are produced with a combination of the these,
in particular if they are created digitally.
Examples
Mathematical Definition
- Geometrical scene defined as the state of the system
- state_of_the_system = F(t)
- state_of_the_system(t + D t) = F(state_of_the_system(t), D t)
- Types of animation:
- frame-by-frame (flip-book)
- key-frame (interpolated)
Frame-by-Frame Animation
- Frame-by-Frame (flip-book): compute each frame separately.
- Traditional animation technique.
- Common method in scientific visualization.
- Simulates the behavior of dynamic systems.
- Dt small enough to pass from one distinct frame to the next.
Key-Frame Animation
- Key-frame animation: compute a number of key frames, then
interpolate in between.
- Computer-aided animation.
- Interpolation:
- between object position, achieved by interpolating the
object-space transformations, includes interpolating the camera
position,
- between the object form: 3D morphing,
- between resulting 2D images : 2D morphing.
- Dt much bigger
Examples of Animation Function
- Moving an object in a straight line at constant speed:
- x = oldx + Dt speedx ,
y = oldy + Dt speedy
- z = oldz(t) + Dt speedz
- Rotating an object around the origin and around the y axis:
- a = Dt w, w = angular speed (rotation speed)
- x = oldx cos a - oldz sin a, y = oldy
- z = oldx sin a + oldz cos a
- Rotating a wheel: rotation (local) + translation (position of the
wheel relative to the bike) + rotation (bike orientation) +
translation (bike position)
Interpolation
- Consider one key frame scene (t1), and the next one scene (
t2). We want to generatenframes in between the two key frames. Let
ai= i / n, where i is the frame number .
- Linear interpolation (Objects move in straight lines):
- scene (ti) = (1 - ai) scene (t1) + ai scene ( t2)
- Ck interpolation: we want the derivative of order k of
the interpolated function to be continuous. Object movements appear
smooth. Usually C1 or C2.
- Holding a pose: slow down the animation as it approaches each of
the key frames and hold the pose for a while.
Video File Formats
- Gif - animated gif, can only hold visual information.
- Windows avi - equivalent to the bmp format for still
images. Uncompressed.
- Mpeg - Motion Picture Expert Group. The equivalent of the jpeg
compression for audio and video (audio - MP3). Mpeg-2 is the DVD
format, MPEG-4 is used for streamed media, broadcast tv.
- Application-specific: .mov (Apple QuickTime), rm (RealPlayer),
DivX avi (using a special compression), .wmv (Windows Media Video,
Windows Media Player).
- Codec (code/decode): algorithm used to understand a particular
file format. In comes as a plug-in for the players.