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

• Zoom and pan:
  http://www.youtube.com/watch?v=AfqDVP_0O0c
• Fly through:
  http://www.youtube.com/watch?v=Fgs5mopTu_Y
• Morphing:
  http://www.youtube.com/watch?v=-SK7nLDu_iA
• Dolly shots:
  http://www.youtube.com/watch?v=xTVLDuH9juI
• Change in perspective:
  http://www.youtube.com/watch?v=Y48R6-iIYHs

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 = old_x + Dt speed_x , y = old_y + Dt speed_y
• z = old_z(t) + Dt speed_z
• Rotating an object around the origin and around the y axis:
• a = Dt w, w = angular speed (rotation speed)
• x = old_x cos a - old_z sin a, y = old_y
• z = old_x sin a + old_z 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 a_i= i / n, where i is the frame number .
• Linear interpolation (Objects move in straight lines):
• scene (t_i) = (1 - a_i) scene (t₁) + a_i scene ( t₂)
• C_k 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.