Introduction to Digital Image Creation and Manipulation (C 102)

UCLA Department of Design | Media Arts
Spring Quarter 2002

Visiting professor Machiko Kusahara


May 17, 2002

Animation

Moving computer graphics images are called animation regardless of their purposes to serve (such as for scientific visualization). This is because "animation" means giving movements, or "anima", to objects that are not alive and thus would not move.
"Animation" does not necessarily mean what was produced for entertainment or art.

I. Some techical issues on display technology

"Persistence of vision"

We see moving images on TV screens, cinema screens or computer screens.
The images shown on these screens are shown as frames, but the persistence of vision of our eyes create the illusion of continuous movements.

Frames and Fields -Film: 24 frames/sec
-TV(NTSC): 30 frames/sec
-CRT display for computer: depends on the number of pixels (For example, a monitor which can provide 72Hz refresh rate at 800x600 may only be capable of 60Hz refresh at 1024x768. Because there is no "interlace" on computer monitors, their reflesh rate should be at least equivalent of x2 of TV monitors - i.e. 60Hz at the minimum)
Low frame rate causes flickers. LCD displays do not have frame rates, as they do not need to refresh.

TV/video standards:(for more detail see http://www.siggraph.org/education/materials/HyperGraph/animation/rick_parent/Rcrd.html)
NTSC: 30 frames/sec= 60 fields/sec, 525 scanlines (since 1953. US, Canada, Central and South America, Japan, etc.)
Number of effective scanlines on the screen in NTSC is 480. With aspect, a NTSC screen is equivalent to 640x480 pixels.
PAL: 25 frames/sec=50 fields/sec, 625 scanlines (since 1967, professional use in Europe, China, etc.) Pal standard provides with sharper images compared to those given by NTSC. SECAM: 25 frames/sec=50 fields/sec, 625 scanlines (home use in France and other countries in Europe, Arab and Africa) HDTV: approx. twice horizontal and twice vertical resolution

Each frame on TV (or video) consists of 2 fields. By "interlacing" a frame, flickering effect is practically avoided. Because of such feature of the video signal, "field animation" is possible, and it enables smooth motion. Especially in Europe, field animation is more commonly used to cover the less frame rate of PAL.

As can be understood from these numbers, converting between film and video, or between different video standards means adding or removing frames for each second.


II. How to animate?

Digital images in 2D or 3D can be animated with many animation tehniques including the followings. While some techniques are based on traditional animation, many others are based on computational features, which brings many advantages.
In creating animation in 3D, modeling and rendering methods should be considered from that aspect.

1. Keyframe animation (Keyframe interpolation)

By preparing keyframes and interpolating between the keyframes, animators can create moving images. Computer can calculate the in-between frames, either in 2D or 3D, which used to be done manually in traditional animation.
Inbetweening can be achieved either by linear interpolation or curved interpolation.

2. Morphing, Free-form shape animation, Deformation

Animation can be created by interpolating between two images or objects, or deforming an object using lattices (boundary boxes).



3. Model animation

Keyframe animation can be done effectively when movement of 3D models are calculated by the computer. Especially in case of character animation, a model is given a hierarchical structure to effectively use computational features and to create realistic movements.
Model animation includes following elements among others.
-skelton animation
-hierarchical animation
-forward kinematics
-inverse kinematics
-motion path
-camera animation
-light animation



4. Procedural animation (rule-based animation)

-Dynamic simulation
Dynamic simulation enables creating realistic motion of objects, interaction between objects, and natural phenomena. Physically-based modeling is used to model flexible objects.
-Particle systems
Particle systems are widely used not only representing motions of particles such as water or flame, but also in animating irregular or organic shapes, and controling movements or creation of many small objects, or growing things.
-Graftal, fractal, growth systems and others
These systems can be used both for modeling and animation. Systems such as L-System and AMAP are based on simulation of plants growing.
-Flock animation and crowd animation
Flocks, schools, crowds of birds, fish, other animals including human beings can be animated by setting relatively simple rules among the members. Flock algorithms are now widely used in films.
-Autonomous behavior

5. Motion Capture

By using either optical or electromagnetic systems, movements of actors/actresses can be captured in real time. Based on skelton animation system and kinematics, computer apply the motion data onto computer models. Facial motion and lip movement can be also captured in real time.

6. Interactive animation

While motion capture is a direct mapping of human motion data onto computer models, other kinds of information related to human emotion can be used for motion data. For example, body movement, gesture, voice pitch, text, temperature, and other elements can be used in creating animation.

7. Real time animation

The range of possibilities in real time animation has been expanding rapidly. Real time animation is expected to play an important role in supporting telecommunication and in entertainment business.