Is ready for interpolation.

Since the display processing operates faster

than the interpolations, the critical path includes

all of the interpolations, but no other

processes, as the diagram indicates. If by chance

the work required to display a single frame is

completed ahead of schedule, an internal clock

delays the program for the proper interval of

time. The work will never fall behind schedule,

since before the playback process begins, each

sketch has been subdivided only into a number of

points that can be handled at 12 frames per second.

For our present equipment configuration, this

limit has been measured at about 2800 points,

which allows sketches of far greater complexity

than can be previewed using the pre-calculation

method, due to disk-to-core transfer speed limitations.

Of course, sketches of considerable

complexity take on a slightly coarser appearance

due to this filtering/subdivision algorithm, but

the results so far have been very satisfactory.

For sketches containing an initial point count

of under 5000, the filter-down is almost impossible

to detect. Very seldom has any sketch produced

by the artists working on the Cornell system

exceeded 7000 points, which makes the technique

viable for use in a production environment.

In addition to the real-time playback capability,

individual frames in the animated sequence

can be transferred to the color display

for stop-frame examination. The image thus presented

to the artist looks exactly as will the

final film, including multiple-cel opaquing and

full color shading.

IV. CONCLUSION

This paper has presented an overview of the

key-frame computer animation system currently

under development at the Cornell Program of

Computer Graphics.

The basic algorithm employed is a linear interpolation

between successive pairs of key frames.

These key frames are composed of artwork input by

the animator on a graphic tablet and displayed on

either a black and white vector scope or a color

halftone CRT. The attributes of this animation

package are:

1) The initial working environment is twodimensional,

and any of the following

graphic media may be used:

a. Free-form line sketches.

b. Line sketches with color infilling.

c. Color paintings.

d. Two-dimensional projections of complex

three-dimensional scenes.

2) These two-dimensional images are combined

using a multiplane cel animation technique

to produce depth and motion illusions.

3) Real-time film previewing, utilizing an

on-the-fly interpolation algorithm, provides

the artist with instant playback

of animated sequences.

4) All interaction with the program is pictorial

in order to facilitate the use of

this system by the non-programming artist.

It is evident that numerous approaches have

been taken to computer animation. This can be

ascribed,to a large extent, to variations in artistic

taste. There are, however, problems common to

all animation systems which must be solved before

they can be viably used for production work. These

involve increasing the resolution, allowing for

real time playback at this increased resolution,

and providing a more efficient input system.

Future research in computer animation must address

these issues, as described below:

1) Most currently available color frame buffers

are designed for the 512 x 512 nominal

resolution of a video monitor. In

order to obtain a satisfactory image for

theater projection, a resolution of at

least 2000 x 2000 is required. The enormous

data storage problems this implies,

and the lack of any viable device for

previewing such images, pose formidable

problems for computer animation systems.

2) The complex pixel-by-pixel computations

required to generate a single frame of

color animation make real-time color previewing

very difficult to achieve. The

only viable solution yet evolved for this

limitation is to compute each frame beforehand

and store it on a video disk or stopframe

video tape-recorder for subsequent

playback in real-time.

3) Most animation systems currently require

that the artist input his sketches manually,

using a graphic tablet and pen. This

time-consuming extra step can be eliminated

through the use of a high-precision

optical scanner. The edge-finding and

stroke identification algorithms necessary

for such a procedure pose complex problems

in themselves, but if solved, promise to

further ease the labor burden on the

animator.

V. REFERENCES

1. Catmull, Edward, (Personal conference at New

York Institute of Technology), New York,(1976).

2. Defanti, Thomas, "The Graphics Symbiosis System",

Real-Time Film Animation, annual report

to the NSF, submitted by the Computer Graphics

Research Group at Ohio State University,

(1973), pp. 5-116.

3. Finch, Christopher, The Art of Walt Disney,

Walt Disney Productions, (1975).

4. Gracer and Blasgen, "Karma: A System for Storyboard

Animation", UAIDE,(1970), pp.210-224.

5. Greenberg, Donald P., "An Interdisciplinary

Laboratory for Graphics Research and Applications",

Proceedings of the Fourth Annual

Conference on Computer Graphics, Interactive

Techniques, and Image Processing (July, 1977).

6. Shoup, Richard, (Presentation of the SHAZAM

animation system at the Third Annual SIGGRAPH

Conference), Phila. Pa., (1976).

7. Wein, Marceli, "Interactive Skeleton Techniques

for Enhanced Motion Dynamics in Key-Frame

Animation", Communications of the ACM,

Oct. 1976).

8. Wein, Marceli, and Lestor Burtnyk, "Computer

Generated Key-frame Animation", Journal SMPTE,

(March 1971), pp. 149-153.

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