Computer Animation with Scripts and Actors - Semantic Scholar

C o m p u t e r A n i m a t i o n w i t h Scripts a n d Actors b y C r a i g W. R e y n o l d s Information International Inc.

Abstract

Introduction

A t e c h n i q u e and p h i l o s o p h y for controlling c o m p u t e r animation is discussed. Using the Actor/Scriptor Animation System (ASAS) a s e q u e n c e is described by the a n i m a t o r as a formal w r i t t e n SCRIPT, w h i c h is in fact a p r o g r a m in an animat i o n / g r a p h i c language. Getting the. desired a n i m a t i o n is t h e n equivalent to "debugging" the script. Typical images m a n i p u lated w i t h ASAS are synthetic, 3D perspective, color, s h a d e d images. However, the a n i m a t i o n control t e c h n i q u e s are indep e n d e n t of the underlying s o f t w a r e and h a r d w a r e of the display system, so apply to o t h e r types (still, B&W, 2D, line d r a w i n g ...). D y n a m i c (and static) g r a p h i c s are based on a set of geometric object data types and a set of geometric o p e r a t o r s o n these types. Both sets are extensible. The o p e r a t o r s are applied to the objects u n d e r the control of m o d u l a r a n i m a t e d p r o g r a m structures. These s t r u c t u r e s (called a c t o r s ) allow parallelism, independence, and optionally, synchronization, so that they can r e n d e r the full range of the time s e q u e n c i n g of events. Actors are the e m b o d i m e n t of imaginary players in a simulated movie. A type of a n i m a t e d n u m b e r can be used to drive geometric expressions (nested geometrical operators) w i t h d y n a m i c p a r a m e t e r s to p r o d u c e a n i m a t e d objects. Ideas f r o m p r o g r a m m i n g styles used in c u r r e n t Artificial Intelligence research inspired the design of ASAS, w h i c h is in fact an extension to the Lisp p r o g r a m m i n g e n v i r o n m e n t . ASAS w a s developed in an academic r e s e a r c h e n v i r o n m e n t and m a d e the transition to the "real world" of c o m m e r c i a l m o t i o n g r a p h i c s production.

This p a p e r describes the Actor/Scriptor Animation System (ASAS), w h i c h is a w a y of thinking about and describing c o m p u t e r g r a p h i c animation. ASAS is basically a notation for a n i m a t e d graphics. The n o t a t i o n for an a n i m a t e d s e q u e n c e (the script) can be automatically read and converted into a n i m a t e d images by an ASAS interpreter. As in the case of musical notation being interpreted by a g r o u p of m u s i c i a n s - - o r the script of a video p r o d u c t i o n being executed by a host of actors, camera, audio, lighting and video technicians--ASAS allows the creation a n d use of any n u m b e r of simulated particpants, "actors" each of w h i c h can control one or m o r e aspects of the animation. The ability of ASAS a c t o r s to operate i n d e p e n d e n t l y or (by c o m m u n i c a t i n g w i t h each other) to act in s y n c h r o n i z a t i o n allows a simple and u n a m b i g u o u s description of the f u n c t i o n of each actor. ASAS differs f r o m "performance" based real time c o m p u t e r g r a p h i c s s y s t e m s as well as f r o m c o m m a n d o r "menu" based systems. Writing the ASAS n o t a t i o n for an a n i m a t e d s e q u e n c e will p r o b a b l y take longer t h a n the final r u n n i n g time of the sequence. On the o t h e r hand, an ASAS script is typically m o r e c o m p a c t t h a n a simple listing of the value of all relevant p a r a m e t e r s for each frame, as m i g h t be r e q u i r e d in a c o m m a n d m e n u system. This results f r o m the fact that ASAS is a p r o c e d u r a l notation, a p r o g r a m m i n g language for a n i m a t i o n and graphics. I n fact ASAS is a "full" p r o g r a m m i n g language a n d includes all of the typical m o d e r n s t r u c t u r e d p r o g r a m m i n g features (procedures (recursive), local variables, "if t h e n else"s loops, typed data s t r u c t u r e s a n d generic operators). Additionally ASAS s u p p o r t s i n d e p e n d e n t , parallel, "animated" p r o g r a m s t r u c t u r e s (actors), and includes a rich set of geometric and p h o t o m e t r i c objects a n d generic o p e r a t o r s o n these objects.

CR Categories and Subject Descriptors: 1.3 [Computer Graphics]; !.3.5 [CG]: Computational Geometry and Object Modeling; 1.3.6 [CG]: Methodology and Techniques--Languages; 1.3.7 [CG]: Three-Dimensional Graphics and Realism--Animation' General Terms: Design, Languages Additional Key Words and Phrases: Lisp, Procedural Animation Languages, Motion Picture Production Author's addresses; US Mail: III, 5933 Slauson Ave., Culver City, CA 90230 ARPAnet mail: Reynolds@Rand-AI Uocp network mail: ucbvax!randvax!reynolds

The existence of a formal n o t a t i o n for a field of e n d e a v o r leads to a w o r k a b l e p r o c e d u r e for the d e v e l o p m e n t of an idea. Like an algorithm being debugged by a c o m p u t e r p r o g r a m m e r , o r a musical score being revised, an ASAS script being developed is b o t h u n a m b i g u o u s a n d precisely modifiable. It is possible to c h a n g e just o n e small aspect w h i l e keeping everything else exactly the same. This p r o p e r t y of notation allows the p r o c e s s of progressive refinement ("tweaking") to be u s e d to converge o n the desired algorithm, m u s i c or animation.

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© 1982

ACM 0-89791-076-1/82/007/0289

History ASAS w a s developed at the Architecture Machine G r o u p at MIT as t w o thesis projects b e t w e e n 1975 and 1978 [24,24]. "ASAS 0" w a s not a full i m p l e m e n t a t i o n , b u t "ASAS 1" did actually work,

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giants" b e c a u s e of its relationship to the p r o g r a m m i n g language Lisp. ASAS can be c o n s i d e r e d to be either i m p l e m e n t e d in, or an extension to, Lisp. The rich p r o g r a m m i n g e n v i r o n m e n t of ASAS is d u e largely to Lisp, since all Lisp primitives a n d utilities are usable f r o m ASAS (and vice versa). A Lisp i n t e r p r e t e r p l u s the ASAS s o f t w a r e yields an ASAS interpreter. ASAS w a s developed under MagicSixLisp at The Architecture Mac h i n e G r o u p at MIT a n d w a s painlessly t r a n s p l a n t e d to r u n u n d e r MIT MacLisp at III. For a long time Lisp h a s h a d a r e p u t a t i o n as s o m e w h a t of a "toy" language, a p o w e r f u l b u t q u a i n t tool u s e d by g n o m i s h academic artificial intelligence researchers, b u t not a language really suited for c o m m e r c i a l use. These critisisms are misdirected, Lisp is o n e of the m o s t elegant and useable algorithmic n o t a t i o n s ever devised. The bad r e p u t a t i o n is d u e m a i n l y to p o o r i m p l e m e n t a t i o n s o r u n d e r p o w e r e d c o m p u t e r s . Because Lisp trades off r a w c o m p u t a t i o n a l efficiency for expressive p o w e r and usability, a well designed i n t e r p r e t e r and a fast m a c h i n e are r e q u i r e d for a p r o d u c t i o n e n v i r o n m e n t . Today there are several good Lisp s y s t e m s for various types of general p u r p o s e c o m p u t e r s (MacLisp [20], InterLISP [29], ...) and currently three firms are selling specially designed Lisp m a c h i n e s (Lisp Machine Inc., Symbolics Inc., a n d Xerox)

Figure 1: An Arch Fractal despite a very s l o w a n d u n i n t e r e s t i n g display package. In 1979 ASAS w a s integrated into the Digital Scene Simulation s y s t e m of I n f o r m a t i o n International Inc. c I I r ' , "tripled"] in Culver City, California. In this instance, ASAS is not u s e d to m a k e images directly, b u t serves as a p r e p r o c e s s o r for tit's existing 3D, h i d d e n surface and s h a d e d g r a p h i c s system. Hence "ASAS 2" f u n c t i o n e d as a true language compiler, translating f r o m the a n i m a t o r ' s s c r i p t to the c o m m a n d s e q u e n c e for the display software. The i n c o n v e n i e n c e of having the display s u p p o r t in a s e p a r a t e s o f t w a r e package is offset by the m u c h w i d e r range of g r a p h i c features m a d e available to the ASAS u s e r t h r o u g h the very advanced III software. After t h r e e y e a r s of c o m m e r c i a l use the s y s t e m w a s refined and b e c a m e "ASAS 3". The c u r r e n t reference for the language is the u n f i n i s h e d ASAS U s e r ' s

ASAS E x p r e s s i o n s ASAS a n d Lisp use a simple, if u n u s u a l , notation. A "parenthesized prefix notation" is u s e d for operators, control s t r u c t u r e s a n d data. An ASAS e x p r e s s i o n is either: (1) a n u m b e r (2) a s y m b o l ("variable") (3) a p a r e n t h e s i z e d list of e x p r e s s i o n s . If the e x p r e s s i o n is a list, the first (leftmost) thing is the n a m e of an o p e r a t o r (or "function") a n d any o t h e r e x p r e s s i o n s in the list are p a r a m e t e r s for the operator. W h e n an e x p r e s s i o n is evaluated (or "executed"), n u m b e r s evaluate to themselves, s y m b o l s to their currently defined value, and a list (eg: "(plus a (abfl b))" ) evaluates to the result of a p p l y i n g the o p e r a t o r ("plus ~) to the recursively evaluated p a r a m e t e r s (values of "a" a n d " ( a b s b)"). For example, to define the s y m b o l ~wheels" to be the n u m b e r of tricycles times 3, w e w o u l d write:

Manual 3.0. [26] The design of ASAS w a s influenced by s o m e c o n c e p t s f r o m r e s e a r c h in the Artificial Intelligence field. The basic c o n c e p t of g r a p h i c databases a n d a n i m a t i o n scripts as p r o g r a m s (procedural e m b e d d i n g of knowledge) w a s i n s p i r e d by Terry W i n o g r a d ' s p i o n e e r i n g w o r k in c o m p u t e r linguistics. In Winograd's s y s t e m n a t u r a l language w a s r e p r e s e n t e d by a p r o c e d u r a l data structure. [30] The c o n c e p t of m e s s a g e p a s s i n g a c t o r s w a s f r o m Carl Hewitt's b o d y of w o r k in ~actor s y s t e m s ° s u c h as PLASMA. [12,13,14] (Similar c o n c e p t s exist in Smalltalk [11], Simula [5] a n d Modula [32].)

(define wheels

(times tricycles 3))

An a n i m a t i o n s y s t e m in d e v e l o p m e n t at a b o u t the s a m e time as ASAS by Ken Kahn s h a r e d s o m e c o n c e p t s w i t h ASAS. [16,17] K a h n ' s s y s t e m h a d w h a t Hewitt calls a " u n i f o r m actor basis" a n d so p e r h a p s a theoretically "cleaner" structure. K a h n ' s w o r k placed m o r e e m p h a s i s o n e m b e d d i n g c o m m o n - s e n s e a n d theatrical k n o w l e d g e in a n i m a t i o n characters, and less e m p h a sis o n c o m p l e x graphics.

Normal ASAS o p e r a t o r s (like times) evaluate each of their p a r a m e t e r s , w h i l e certain o p e r a t o r s have special evaluation p a t t e r n s (like define, w h i c h does not evaluate its first p a r a m e ter, these are called "macros"). To define a s i m p l e o p e r a t o r (call it "thrice") w h i c h multiplies its single p a r a m e t e r ('x") by three:

(defop thrice (param: x) (times x 3))

ASAS and Lisp As a p r o g r a m m i n g language, ASAS "stands o n the s h o u l d e r s of

290

(define houses (group red-house yellow-house brown-house))

an equivalent definition in MacLisp w o u l d be:

(defun thrice (x) (times x 3))

The "point of view" object (pov) is used to define the p o i n t of v i e w of an o b s e r v e r (for example the ASAS camera) or of an object. That is, a p o v describes the three coordinate axis basis vectors and the position of the origin of an arbitrary coordinate space. We refer to s u c h spaces by n a m e s like "eye space" and "an object's local coordinate space". Note: a pov plays a role very similar to a "4X4 h o m o g e n e o u s t r a n s f o r m matrix" in o t h e r 3D g r a p h i c s s y s t e m s (there is a s i m p l e t r a n s f o r m a t i o n f r o m a pov to a 4X4 matrix) b u t a p o v is a geometrical object c o m p o s e d of vectors and can be m a n i p u l a t e d just like any o t h e r object.

The first e x a m p l e could t h e n be rewritten:

(define wheels (thrice tricycles)) Special Symbols Within a script certain aspects of the p r o d u c t i o n are controlled by the values given to s o m e special symbols. None of these s y m b o l s are actually "reserved words", but it is best to use the script symbols background and camera only for the p u r p o s e of defining the c u r r e n t c o l o r of the graphical b a c k g r o u n d of the image and the c u r r e n t c a m e r a description (as a pov, see the section on geometric objects). The initial ASAS e n v i r o n m e n t h a s o t h e r s y m b o l s defined to various frequently u s e d objects (axes, colors, basic solids), it is good practice to k n o w these and avoid redefining them.

A s u b w o r l d is an object associated w i t h a p o v . This allows ASAS to m a n i p u l a t e a c o m p l e x object by modifying only the pov, h e n c e various "instances" of an object m a y s h a r e the s a m e underlying data. S u b w o r l d s also allow ASAS to w o r k w i t h "levels of abstraction" in a g r a p h i c database, w h e n a subworld is f o r m e d it notes the "overall size" and "typical color" of its contents. At display time this allows efficient tree s t r u c t u r e d clipping ( w h e n an entire s u b w o r l d is offscreen) and h a n d l i n g of detail too small to see ( w h e n an entire s u b w o r l d lies w i t h i n a single pixel). [4] Hence the u s e r can build levels of abstraction

Geometric Objects In addition to the data types f o u n d in m o s t p r o g r a m m i n g languages, ASAS provides a set of geometric (and photometric)

objects: vector, color, polygon, solid, group, pov, subworld, and light. The vector r e p r e s e n t s a position in three d i m e n s i o n a l Cartesian space. It allows three parameters, the X, Y and Z coordinates. Trailing zero coordinates m a y be omitted. A color object m a y be specified either by its Red, Green and Blue c o m p o n e n t s , o r by Intensity, Hue, Saturation. The t w o o p e r a t o r s are called rgb a n d ihs, each of w h i c h accept three n u m b e r s b e t w e e n 0 and 1. A s i m p l e polygon contains a color and a list of v e c t o r s , the "boundary". The cut-hole o p e r a t o r allows the c o n s t r u c t i o n of polygons w i t h "holes and islands" (that is, multiple boundaries). The color can be a group of a front color and a back color. The b o u n d a r y p o i n t s m a y be listed separately or as a group of vectors. Here is a p o l y g o n e x p r e s s i o n for a certain blue triangle:

(polygon blue (vector 1 0 O) (vector 0 1 O) (vector 0 0 1)) A solid r e p r e s e n t s a b o u n d e d region of space, a closed p o l y h e d r o n . It is c o m p o s e d of vertices and faces (as v e c t o r s and p o l y g o n s ) in addition to topological c o n n e c t i o n information. Several geometric objects can be "glued together" into a g r o u p object, w h i c h is t h e n m a n i p u l a t e d as a w h o l e by the geometric operators. A group e x p r e s s i o n allows any n u m b e r of p a r a m e t e r s -geometrical objects to be grouped together, including o t h e r groups.

Figure 2:

(defop

H o w to m a k e an A r c h Fractal

(~iven an arch element).

arch-fractalizer

(param: (local:

arch-element top-color bot-color levels fractal-ratio height width leg-width) (total-levels (offset-dist (sub-tower-offset-1 (sub-tower-offset-2

(arch-tower

levels) (half (dif width leg-width))) (vector offset-dist 0 0)) (mirror x-axis sub-tower-offset-I)))

levels))

(defop arch-tower (param:

levels)

(if (zerop

levels)

(then nothing) (else (add-arch-level

(arch-tower (dif

levels 1))))))

(defop add-arch-level (param:

sub-tower)

(grasp sub-tower (scale fractal-ratio) (move (vector 0 height 0)) (rotate 0.25 y-axis)) (grasp arch-element (recolor (interp (quo levels total-levels) bot-color top-color))) (subworld

291

(group arch-element (move subtower-offset-1 sub-tower) (move subtower-offset-2 sub-tower))))

in to a g e o m e t r i c d a t a b a s e by the n e s t i n g of subworld objects.

tortoise) w a s first u s e d by J i m Stansfield a n d t h e n r e f i n e d b y

Objects to be s e e n in s h a d e d i m a g e s are i l l u m i n a t e d by light objects. L i g h t s are c o m p o s e d of a p o s i t i o n v e c t o r a n d a color.

g o o d t r e a t m e n t of t h i s s u b j e c t c a n be f o u n d i n [1]. U s u ally

H e n r y L i e b e r m a n in a 3D l i ne d r a w i n g e x t e n s i o n to LOGO. A objects w i l l be d e f i n e d so t h a t t h e o r i g i n of t h e i r local c o o r d i n a t e

Geometric O p e r a t o r s

s p a c e is at t h e c e n t e r of t h e object. For t h i s r e a s o n w e w i l l i n f o r m a l l y re fe r to t h e "origin of t h e local c o o r d i n a t e s y s tem "

ASAS's g e o m e t r i c o p e r a t o r s are t h e tools t h e a n i m a t o r u s e s to s h a p e , m o v e a n d o r i e n t objects. An o b j e c t ' s s h a p e m a y c o m e d i r e c t l y f r o m t h e a c t i o n of o p e r a t o r s , o r p a r t s e n c o d e d by h a n d w i t h a d i g i t i z e r c a n be a s s e m b l e d w i t h t h e o p e r a t o r s . T h e s a m e o p e r a t o r s are u s e d b o t h for static a r r a n g e m e n t s , or to c r e a t e a n i m a t e d m o t i o n , by o p e r a t i n g f r a m e by f r a m e u n d e r t h e c o n t r o l of an actor.

as t h e "center". Local o p e r a t o r s a re p r o v i d e d for m o v i n g , rotating, s c a l i n g a n d "zooming" r e l a t i v e to t h e local c o o r d i n a t e s y s t e m . All of t h e s e o p e r a t o r s a c c e p t o n e or t w o p a r a m e t e r s , t h e s e c o n d o p t i o n a l p a r a m e t e r is t h e object to o p e r a t e on, if n o n e is specified, t h e c u r r e n t l y g r a s p e d object is r e d e f i n e d . Note: t h e r e l a t i o n s h i p b e t w e e n g l o b a l a n d local o p e r a t o r s is

In m a n y c o m m a n d / m e n u b a s e d g r a p h i c s s y s t e m s it is difficult to p r e c i s e l y s p e c i f y t h e c o r r e c t o r d e r i n g of g e o m e t r i c transform a t i o n . For e x a m p l e , t h e r e w i l l be a "rotate" c o m m a n d w h i c h a c c e p t s t h r e e n u m b e r s , t h e a n g l e s of r o t a t i o n for e a c h axis. Often t h e r e is no m e n t i o n of in w h a t o r d e r t h e r o t a t i o n s a r e a p p l i e d , let a l o n e a w a y to s p e c i f y t h e d e s i r e d order. In ASAS, t h e a n i m a t o r e x p l i c t l y d e t e r m i n e s t h e o r d e r i n g of o p e r a t i o n s by th e s t r u c t u r e of t h e n e s t i n g of t h e e x p r e s s i o n s w r i t t e n in t h e

s i m i l a r to t h e t h a t of pre- a n d p o s t - m u l t i p l i c a t i o n of t r a n s f o r m m a t r i c e s . Also note: w h e n obj e c t s o t h e r t h a n f l u b w o r l d s o r p o v s a re p a s s e d to self r e l a t i v e o p e r a t o r s t h e y a re first p u t in to a n i d e n t i t y ("home") s u b w o r l d , t h e n o p e r a t e d on. Local o p e r a t o r s :

g row shrink forward backward left right up down cw ccw zoom-in zoom-out local-move local-stretch home

script. T h e b a s i c o p e r a t o r s are ~generic", t h e y c a n be g i v e n a n y t y p e of g e o m e t r i c object a n d o p e r a t e on it as is a p p r o p r i a t e for t h a t o b j e c r s type. ASAS o p e r a t o r s NEVER m o d i f y t h e object t h e y a re o p e r a t i n g on. T h e v a l u e r e t u r n e d by a n o p e r a t o r is a g e o m e t r i cally m o d i f i e d c o p y of the o r i g i n a l object w i t h o t h e r w i s e t h e same type and structure. A n o t a t i o n a l s h o r t h a n d is p r o v i d e d for t h e c o m m o n o c c u r e n c e of a s e r i e s of o p e r a t i o n s to be p e r f o r m e d o n a s i n g l e object. The object to be o p e r a t e d u p o n c a n be m a d e t h e "current" object (using th e grasp operator). The "grasped" object w i l l t h e n be r e d e f i n e d by calls to o p e r a t o r s w h i c h do n o t e x p l i c i t l y s p e c i f y an object to o p e r a t e on.

s c a l e u p a b o u t local c e n t e r s c a l e d o w n a b o u t local c e n t e r m o v e a l o n g local +Z axis m o v e a l o n g local -Z axis r o t a t e to left a b o u t local Y axis r o t a t e to r i g h t a b o u t local Y axis r o t a t e u p w a r d a b o u t local X axis r o t a t e d o w n w a r d a b o u t local X axis r o t a t e c l o c k w i s e a b o u t local Z axis r o t a t e c o u n t e r - c l o c k w i s e a b o u t local Z axis s c a l e u p local Z axis s c a l e d o w n local Z axis m o v e a l o n g a r b i t r a r y local v e c t o r s c a l e e a c h local axis i n d e p e n d e n t l y r e s e t s b a c k to o r i g i n a l d e f i n i t i o n s p a c e

T w o b a s i c t y p e s of g e o m e t r i c o p e r a t o r s are p r o v i d e d by ASAS, "global" a n d "local" ( s o m e t i m e s called "self relative").The ASAS g lobal g e o m e t r i c o p e r a t o r s are called: scale, move, rotate,

E x a m p l e s : a n o p e r a t o r s e q u e n c e w h i c h (if e v a l u a t e d e a c h frame)

stretch and mirror.

w i l l c a u s e t h e AIRPLANE (that is, t h e s e q u e n c e of obj ects w h i c h f o r m t h e a n i m a t e d v a l u e of t h e v a r i a b l e AIRPLANE) to p e r f o r m

G e n e r a l l y t h e s e o p e r a t o r s a p p l y t h e n a m e d g e o m e t r i c a l t ra ns f o r m to a n y g i v e n g e o m e t r i c a l object T h e t r a n s f o r m s a re r e l a t i v e to t h e o r i g i n a n d m a j o r axes of t h e g l o b a l c o o r d i n a t e space. T h e p a r a m e t e r t y p e s to e a c h are n u m b e r s a n d v e c t o r s as a p p r o p r i ate. ("Stretch" is a d i f f e r e n t i a l s c a l i n g for e a c h axis, s p e c i f i e d by a v e c t o r of scale factors).

"barrel rolls", a n d o n e to c a u s e t h e CAMERA to p a n a r o u n d w h i l e z o o m i n g out:

(grasp airplane) (forward 0.1) (cw 0.02) (up 0.02)

As an e x a m p l e of t h e u s a g e of t h e ASAS g l o b a l o p e r a t o r s see F i g u r e s 1 a n d 2, t h e s e s h o w h o w last y e a r ' s SIGGRAPH c o v e r was constructed.

(grasp camera) (right pan-speed) (zoom-out 1.01)

T h e "local" o p e r a t o r s are s i m i l a r in effect to t h e global o p e r a t o r s , e x c e p t that t h e y a r e b a s e d on an o b j e c t ' s OWN c o o r d i n a t e s y s t e m r a t h e r t h a n t h e global c o o r d i n a t e s y s t e m . A subworid c a r r i e s a l o n g its o w n little c o o r d i n a t e s y s t e m , its p o v . Not o n l y d o e s this a l l o w efficient m o d i f i c a t i o n of t h e subworld b u t it also p r o v i d e s a r e f e r e n c e for o p e r a t i o n s in t h e objec t ' s local c o o r d i n a t e space. T h e local o p e r a t o r s w e r e i n s p i r e d by t h e "turtle" of the LOGO g r a p h i c s l a n g u a g e [2], a n d are i n t e n d e d to be a t h r e e d i m e n s i o n a l a n a l o g of the t u r t l e o p e r a t i o n s (w a l k f o r w a r d s or b a c k w a r d s , t u r n right or left). T h i s n o t i o n of a 3D t u r t l e (more of a d e e p sea s w i m m i n g t u r t l e t h a n a l a n d c r a w l i n g

V a ri ous o t h e r ASAS o p e r a t o r s a r e a v a i l a b l e b u t w i l l n o t b e

discussed here. There are recolor and cut-hole, and interp the general purpose interpolater, r o w and ring w h i c h make regular g r o u p s of objects, a n d prism w h i c h m a k e s s o l i d s by p r o j e c t i n g a polygon. Here a r e s o m e e x a m p l e s of s o m e of t h e m , an o p e r a t o r to m a k e a n - s i d e d r e g u l a r p o l y g o n ( i n s c r i b e d w i t h i n a u n i t r a d i u s circle), a n d a n o p e r a t o r to m a k e a p r i s m w i t h r e g u l a r "ends":

292

(defop regular-polygon (param: color sides)

s t r u c t u r e of t h e action. W h i l e a n ASAS "cue" is i n fact s i m p l y a n u m b e r (a f r a m e n u m b e r r e l a t i v e to t he c u r r e n t animate block) it s h o u l d n o t be t h o u g h t of as a c o n s t a n t . B e c a u s e of t h e c o m p u t a t i o n a l n a t u r e of a script it c a n be q u i t e e a s y to m o v e c u e s a r o u n d , s i n c e all c u e p o i n t s c a n be h a n d l e d s y m b o l i c a l l y (by n a m e r a t h e r t h a n by a literal n u m b e r ) . For e x a m p l e it is a s i m p l e rrlatter to c h a n g e t h e o v er all r u n t i m e of a s c r i p t (for a " qui c k r u n t h r o u g h " test) if all c u e p o i n t s a re d e f i n e d r e l a t i v e to o n e v a r i a b l e (e.g. "runtime"). Li bra ry m a c r o s exist to facilitate just s u c h a s c h e m e . T h e a n i m a t o r m a y find it n e c e s s a r y for a r t i s t i c r e a s o n s to m o v e a c u e p o i n t w i t h i n a s c r i p t , a g a i n t h i s w i l l be q u i t e p a i n l e s s if e v e r y t h i n g w h i c h is supposed to b e g i n or e n d at t h a t c u e p o i n t refers to it o n l y s y m b o l i c a l l y .

(polygon color (ring sides (vector 0 1 O) z-axis))) (defop regular-prism (param: color sides thickness) (prism color (vector 0 0 thickness) (regular-polygon color sides))) This s u m m a r y of ASAS o p e r a t o r s suffers b e c a u s e of t he l a n g u a g e ' s extensibility; t h e full list is e n d l e s s s i n c e t h e u s e r i n v e n t s n e w o n e s as n e e d e d . B e y o n d s i m p l e c o m b i n a t i o n s of b a s i c l i n e a r o p e r a t i o n s , t h e r e is a large class of n o n l i n e a r "bending" o p e r a t o r s . For e x a m p l e c o n s i d e r "curl-up" w h i c h t a k e s a l o n g t h i n object a n d c u r l s it i n t o a s p i r a l (Escher fans w i l l k n o w t h e a p p l i c a t i o n for that).

Actors The c o n t r o l s t r u c t u r e of a n a n i m a t i o n s y s t e m w o u l d be v e r y s i m p l e if w e c o u l d a s s u m e t h a t all s e q u e n c e s to be p r o d u c e d h a d at m o s t o n e i n d e p e n d e n t l y a n i m a t e d f e a t u r e at a n y o n e t i me . On t he o t h e r h a n d , if w e a s s u m e t h a t t h e r e m a y b e a n y n u m b e r of fully i n d e p e n d e n t a n i m a t e d f e a t u r e s (starting a n d s t o p p i n g at r a n d o m times, h a p p e n i n g at di ffe re nt rates, r u n n i n g in s y n e or not) t h e n c o n v e n t i o n a l c o n t r o l s t r u c t u r e s ar e n o longer the most appropriate.

Scripts and Animate B l o c k s The m a i n p r o g r a m an ASAS u s e r w r i t e s is called a s c r i p t , w h i c h is a s p e c i a l t y p e of defop. A script h a n d l e s t h e s e t t i n g u p a n d s e t t i n g d o w n n e e d e d to p r o d u c e an a n i m a t e d s e q u e n c e (or w r i t e a file for l a t e r p r o d u c t i o n by a n o t h e r system). The script e x p r e s s i o n i n c l u d e s a n a m e a n d a n y n u m b e r of s u b e x p r e s s i o n s . The effect is to d e f i n e a n o p e r a t o r w i t h t h a t n a m e w h i c h o p e n s p r o d u c t i o n , e v a l u a t e s e a c h e x p r e s s i o n , in t h e body, a n d closes p r o d u c t i o n . T h e r e is no restriction, b u t t h e t h i n g s in t h e b o d y are u s u a l l y e i t h e r animate e x p r e s s i o n s ("animate blocks") or p r o d u c t i o n u t i l i t i e s (such as " m a k e N b l a n k frames", "put t h i s slate text", or " m a k e an N s e c o n d c o u n t d o w n " ) .

An ASAS actor c a n be t h o u g h t of s e ve ra l w a y s . Most b a s i c a l l y a n a c t o r is a "chunk" of c o d e w h i c h w i l l be e x e c u t e d o n c e e a c h frame. Usually a n actor (or a t e a m of t he m) is r e s p o n s i b l e for o n e vi s i bl e e l e m e n t in a n a n i m a t i o n s e q u e n c e , h e n c e it c o n t a i n s all v a l u e s a n d c o m p u t a t i o n s w h i c h re l a t e to t h a t object. In t h i s s e n s e a n actor s e r v e s to m o d u l a r i z e a n d l oc a l i z e t h e co d e r e l a t e d to o n e aspect, i s o l a t i n g it f r o m u n r e l a t e d code. F r o m a f o r m a l p o i n t of vi e w , a n a c t o r is a n i n d e p e n d e n t c o m p u t i n g p r o c e s s in a n o n - h i e r a r c h i c a l s y s t e m w i t h s y n c h r o n i z e d activat i on a n d able to c o m m u n i c a t e w i t h o t h e r actors by m e s s a g e passing.

An animate b l o c k is a s p e c i a l t y p e of loop. E a c h t i m e a r o u n d t h e loop, after it e v a l u a t e s its body, a f l a m e of a n i m a t i o n is p r o d u c e d a u t o m a t i c a l l y . Usually t h e b o d y c o n t a i n s c u e express i o n s ("cue at f r a m e N .... "L T h e s e c a u s e objects to be m a d e visib le (with t h e s e e o p e r a t o r ) or start, s t o p or d i r e c t actors. Animate b l o c k s are e x i t e d w h e n a c u t o p e r a t o r is e v a l u a t e d .

W h e n a n actor is "on" ( b e t w e e n b e i n g started a n d stOped), it w i l l be a w a k e n e d o n c e e a c h f l a m e , its local v a r i a b l e s restored, its b o d y e va l ua t e d, its v a r i a b l e s saved, t h e n p u t b a c k to sleep. (Hence a n actor h a s p r o p e r t i e s b e t w e e n a "closure" a n d a "process" in r e c e n t Lisp i m p l e m e n t a t i o n s . ) A c t o r s are p u t in to a c t i o n w i t h t h e start o p e r a t o r , w h i c h t a k e s a n actor a n d r e t u r n s t h e "actor i n s t a n c e id", a u n i q u e n u m b e r for e a c h active actor. An actor c a n d e a c t i v a t e itself, or c a n be g u n n e d d o w n by t h e script or a n o t h e r actor, t h i s is d o n e w i t h t h e StOp o p e r a t o r w h i c h a c c e p t s a n "actor i n s t a n c e id". R u n is a c o m b i n a t i o n of start and stop, it s t a r t s a n actor w i t h a p r e d e t e r m i n e d s t o p cue. Thi s is t h e d e f i n i t i o n of t h e o p e r a t o r "spin-cube-actor" u s e d i n t h e s c r i p t " s pi nni ng-c ube s " :

T h i s e x a m p l e script c o n t a i n s o n e animate block, w h i c h s t a rt s t w o s i m i l a r a c t o r s at different times. Both a c t o r s t h e n r u n u n t i l t h e e n d of t h e block.

(script spinning-cubes (local: (runtime 96) (midpoint (half runtime)))

(animate (cue (at O) (start (spin-cube-actor green)))

(defop spin-cube-actor (param: color)

(cue (at midpoint) (start (spin-cube-actor blue)))

(actor (local: (angle O)

(cue (at runtime) (cut))))

(d-angle (quo 3 runtime)) (my-cube (recolor color cube)))

(see (rotate angle y-axis my-cube))

(Note: cut a c c e p t s an o p t i o n a l f r a m e n u m b e r , a n d w i l l c ut o n l y if that is the c u r r e n t frame, so t h a t t h i r d c u e c o u l d h a v e b e e n w r i t t e n as "(cut runtime)") W h e n a n animate b l o c k is exited, all of th e actors a s s o c i a t e d w i t h it are s t o p p e d . Hence animate blo cks are s o m e w h a t like t h e "scenes" of a movie, the c o a r s e

(define angle (plus angle d-angle)))) It e x p e c t s o n e parameter, a color, a n d r e t u r n s a n a c t o r object.

293

Computer Graphics

V o l u m e 16, N u m b e r 3

The actor itself h a s t h r e e local variables, e a c h of w h i c h is a s s i g n e d a n i n i t i a l v a l u e in t h i s case: "angle" is t h e c u r r e n t a n g l e o f r o t a t i o n for t h i s a c t o r s cube, "d-angle" is t h e i n c r e m e n t a l v e l o c i t y of t h e angle, "my-cube" is a r e c o l o r e d v e r s i o n of ASAS's p r e d e f i n e d "cube" s o l i d . Each f r a m e t h e actor c o n s t r u c t s t h e r o t a t e d v e r s i o n of "my-cube" a n d p a s s e s it to t h e d i s p l a y e r , t h e c u r r e n t "angle" is u p d a t e d for t h e next frame.

July 1982

i n d e p e n d e n t "instances" of a g i v e n "class" of

actors.

T h e o t h e r ke y f e a t u r e of actors w h i c h m a k e s t h e m s u i t a b l e for b e h a v i o r s i m u l a t i o n is t h e a b i l i t y to p a s s m e s s a g e s . Clearly t h e b i r d s in t h e flock a re e x c h a n g i n g i n f o r m a t i o n , t h r o u g h t h e a c t i o n of l i ght a n d s o u n d on t h e b i r d ' s s e n s e s e a c h o n e is a w a r e of w h e r e t h e o t h e r s a r e a n d w h e r e t h e y a re g o i n g (in a n i n t r i n s i c a l l y d e p t h s o r t e d order!) In a n a c t o r s i m u l a t i o n of t h e flock w e w o u l d n o t go to t h e e x t e n t of m o d e l i n g l i ght a n d s o u n d , b u t w e c o u l d r e a l i s t i c a l l y h a v e e a c h b i r d b r o a d c a s t i n g to e v e r y o n e t h e m e s s a g e "I a m h e r e (x y z) a n d I ' m h e a d i n g (dx dy dz)". In t h a t i m p l e m e n t a t i o n , e a c h b i r d w o u l d h a v e to p u t t h e o t h e r s in o r d e r of i m p o r t a n c e , p r o b a b l y u s i n g a ~hidden b i r d algorithm".

Animated N u m b e r s In t h e last e x a m p l e t h e s y m b o l "angle" t o o k on a s e r i e s of n u m e r i c values, f r a m e b y frame, f o r m i n g a n a r i t h m e t i c series. But for m o r e c o m p l e x t i m e b e h a v i o r ( q u a d r a t i c o r c u b i c curves) t h e i n l i n e c o d e to h a n d l e a n d u p d a t e all t h o s e l i n e a r d i f f e r e n c e t e r m s b e c o m e s a b u r d e n . To a v o i d this, ASAS s u p p o r t s a n a n i m a t e d n u m e r i c object called a n e w t o n (as in N e w t o n i a n m e c h a n i c s ) . N e w t o n s c a n be u s e d a n y p l a c e a n u m b e r w o u l d b e used, s u c h as a c o o r d i n a t e in a v e c t o r o r t h e a n g l e p a r a m e t e r for rotate. B e t w e e n f r a m e s h o w e v e r , newtons are a u t o m a t i cally u p d a t e d to t h e next v a l u e in t h e i r p r e d e f i n e d s e q u e n c e . T h e n e w t o n d a t a s t r u c t u r e h o l d s its f u t u r e as a c h a i n of p i e c e w i s e c u b i c c u r v e s w i t h s e l e c t a b l e d e g r e e of c o n t i n u i t y at t h e joints.

Message P a s s i n g ASAS messages a re h a n d l e d by t w o s p e c i a l o p e r a t o r s : send a n d receive. Send c o m p o s e s m e s s a g e s a n d p o s t s t h e m at th e r e c i p i e n t ' s ma i l box. R e c e i v e r e a d s e a c h m e s s a g e in t h e m ailbox, r e s p o n d i n g to e a c h in a m a n n e r d e p e n d i n g on t h e t y p e of m e s s a g e . (ASAS a c t o r s act o n c e e a c h frame, n o t w h e n e v e r a m e s s a g e comes, h e n c e m a i l m a y p i l e u p b e t w e e n f r a m e s so t h e m a i l b o x is i m p l e m e n t e d as a FIFO queue.)

A n e w t o n c a n be s p e c i f i e d in t e r m s of p o s i t i o n , velocity, a c c e l e r a t i o n a n d delta a c c e l e r a t i o n ("jerk" or "jerkiness") w h e n t h o s e v a l u e s are k n o w n . But m o r e t y p i c a l l y n e w t o n s a re defined with utilities which produce curves with certain p r o p e r t i e s . A n i m a t o r s are f a m i l i a r w i t h t e r m s like "slow in" or "slow out" m e a n i n g t h a t a n a c t i o n s h o u l d start (or end) w i t h z e r o v e l o c i t y (first derivative). T h e five m o s t c o m m o n c u r v e s (or p i e c e s of curves) u s e d in ASAS are: hold, linear, slowin, s l o w o u t a n d s l o w i o . H o l d a c c e p t s a v a l u e a n d a l e n g t h of time, e a c h of t h e o t h e r s t a k e s a s t a r t i n g a n d e n d i n g v a l u e a n d a time. SIowio (slow in a n d s l o w out) h a s z e r o d e r i v a t i v e s at b o t h ends. W h e n n o n e of t h e s t a n d a r d c u r v e s are a p p r o p r i a t e a n i n t e r p o l a t i n g c u b i c s p l i n e fit is used.

S e n d t a k e s a n a ddre s s , a " m e s s a g e type" (optional), a n d a n y o t h e r s p e c i f i c m e s s a g e d a t a ( n u m b e r s , g e o m e t r i c objects, s y m bols). T h e a d d r e s s is e i t h e r a n a c t o r id o r a s p e c i a l s y m b o l ; uall" m e a n s s e n d to all a c t o r s a n d ~script" and "animate" c a n b e u s e d to s e n d m e s s a g e s to t h e s u r r o u n d i n g script or animate block. T h e " m e s s a g e type" is a n y s y m b o l u s e d to d e s c r i b e t h e t y p e of m e s s a g e , it m u s t m a t c h t h e m e s s a g e t y p e i n t h e r e c i p i e n t ' s receive c o n s t r u c t . For e x a m p l e , t h e s e s e n d s (1) tell "bouncer" to s p e e d u p by 10 p e r c e n t a n d (2) a n n o u n c e to all b i r d s w h e r e w e are:

(send bouncer speedup 1.10) (send all bird-state cur-position cur-velocity)

Actors and Behavior Simulation

T h e receive c o n s t r u c t h a s a b o d y m u c h like a case c o n s t r u c t . E a c h m e s s a g e in t h e m a i l b o x is e x a m i n e d i n turn, t h e " m e s s a g e type" of e a c h is c o m p a r e d w i t h t h e t y p e of t h e v a r i o u s clauses. If o n e of t h e c l a u s e s in t h e b o d y of t h e receive m a t c h e s t h e i n c o m i n g m e s s a g e type, t h e b o d y of t h e c l a u s e is e v a l u a t e d in r e s p o n s e to t h e m e s s a g e . M e s s a g e t y p e "any" in a c l a u s e w i l l m a t c h a n y i n c o m i n g m e s s a g e . T h e c o n t e n t s of a m e s s a g e (past t h e type) m a y be a c c e s s e d by s p e c i f y i n g p a r a m e t e r b i n d i n g s for t h e c l a u s e type. For e x a m p l e , t h i s actor k n o w s h o w to r e c e i v e only "speedup" and "slowdown" message:

So me a n i m a t i o n is m a d e to m a t c h a p r e c o n c e i v e d i ma ge , e s p e c i a l l y in c o m m e r c i a l p r o d u c t i o n . O t h e r times, a n i m a t i o n is p r o d u c e d as an e x p e r i m e n t , t h e a n s w e r to " w h a t w o u l d h a p p e n if ...". In t h e s e c o n d type, w h i c h m i g h t be c a l l e d " b e h a v i o r s i m u l a t i o n " , the a n i m a t o r sets u p a little w o r l d by d e f i n i n g t h e r u l e s of b e h a v i o r a n d s e l e c t i n g t h e c a s t of c h a r a c t e r s . W h e n t h e b e h a v i o r s i m u l a t i o n is r u n w e o b t a i n i m a g e s of w h a t w e n t on in t h e little w o r l d . A classic e x a m p l e of t h i s sort of t h i n g is to try to b u i l d a c o m p u t e r g r a p h i c s i m u l a t i o n of a flock of birds. We m u s t d e f i n e t h e b e h a v i o r o f a s i n g l e b i r d so t h a t w h e n a lot of i n s t a n c e s of t h e b i r d are s i m u l a t e d , t h e y flock c o n v i n c i n g l y . T h e flock s e e m s to b e f o l l o w i n g a leader, b u t e a c h t i m e t h e y turn, a n e w b i r d b e c o m e s t h e leader. T h e flock c h a n g e s d i r e c t i o n like a s i n g l e unit, y e t it is just a n a s s e m b l y of i n d i v i d u a l s . T h e flock is a d e n s e cluster, b u t t h e b i r d s do not often collide.

(receive ((speedup f)

(define speed (times speed f)))

((slowdown f)

(define speed (quo speed f)))

(any

(print 'What?)))

T h e m e s s a g e p a s s i n g m e c h a n i s m d e s c r i b e d a b o v e is b a s e d o n a m o r e p r i m a t i v e o p e r a t o r c a l l e d in. The in o p e r a t o r a l l o w s t h e e v a l u a t i o n of a n y e x p r e s s i o n "inside" t h e local v a r i a b l e s p a c e of a n actor, t h u s a l l o w i n g e x a m i n i n g a n d s e t t i n g t h e local v a r i a b l e s of t h e actor. T h i s is a u s e f u l b u t d a n g e r o u s tool w h i c h s h o u l d be u s e d o n l y i n a w e l l d e s i g n e d prot oc ol .

ASAS a c t o r s p r o v i d e a c o n v e n i e n t w a y of i m p l e m e n t i n g s u c h b e h a v i o r s i m u l a t i o n s . As m e n t i o n e d before, o n e of t h e f e a t u r e s of a c t o r s is t h e w a y t h e y p r o m o t e " s e p a r a t i o n of p o w e r s " , i n d e p e n d e n t m o d u l e s of c o d e w h i c h do n o t i n t e r f e r e w i t h e a c h other. T h i s a l l o w s an a c t o r to t a k e t h e p a r t of o n e c h a r a c t e r s in t h e s i m u l a t i o n . If t h e s a m e sort of c h a r a c t e r o c c u r s m a n y t i m e s in t h e s i m u l a t i o n (e.g. m a n y c o p i e s of BIRD) w e c a n u s e

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Commercial

Production

at III

Often design constraints are stated in s u c h an indirect fashion ("have the c a m e r a p o i n t i n g s u c h that the logo is positioned here and oriented like in this sketch") that the only w o r k a b l e w a y to find the desired n u m e r i c a l p a r a m e t e r s is experimentally w i t h graphical feedback. After specific "key" f r a m e s have b e e n c o m p o s e d , and the transitions b e t w e e n t h e m defined, a m o t i o n test is made. Usually this test is m a d e in either line ("vector") or l o w resolution s h a d e d image mode. The symbolic n a t u r e of ASAS scripts m a k e it easy to adjust the r u n t i m e of a sequence, m a k i n g p r e l i m i n a r y tests at 10:1 s p e e d ratios allows faster t u r n a r o u n d . Also the script can be simplified for these tests, by d r o p p i n g out certain elements, replacing o t h e r s w i t h "standins", all these c h a n g e s can be m a d e u n d e r the control of ASAS script flags. Often the m o t i o n test will reveal p r o b l e m s in the "feel" of the d y n a m i c s of the a n i m a t i o n o r u n e x p e c t e d b e h a v i o r b e t w e e n the key frames.

ASAS frequently plays a central role in commercial a n i m a t i o n p r o d u c t i o n at III, a l t h o u g h o t h e r t e c h n i q u e s of a n i m a t i o n control are used. Projects m a d e w i t h ASAS include, "MICROMA" a n i m a t e d logo, "LBS" a n i m a t e d logo, "NEWS CENTER 2" TV n e w s s h o w intro, t w o TV c o m m e r c i a l s for "TORNADO", v a r i o u s m a g a z i n e ads, all of the t h e m e a n i m a t i o n for the III 1981 Sample Reel (~The Juggler"), a b o u t half of the special effects for the Ladd C o m p a n y ' s feature m o t i o n picture ~LOOKER", a n d all of the a n i m a t i o n a n d still images III is m a k i n g for the recently released Disney feature "TRON".

A n o t h e r p a s s o r t w o is m a d e to finalize the m o t i o n and t h e n attention is shifted to the color, lighting, s h a d i n g and o t h e r "photometeric" p a r a m e t e r s of the animation. Key f r a m e s are e x a m i n e d on a high resolution video display and color tests are m a d e onto the type of color film w h i c h will be used for the final image. Often a p a r a m e t e r will be d e t e r m i n e d w i t h a "wedge test", m a k i n g a series of f r a m e s w h i c h differ only by a single p a r a m e t e r value (e.g. the a m o u n t of a m b i e n t light in the scene), w i t h the g a m u t of values laid out, the final value can be easily be selected.

Copyright 0 1982 by Walt Disney Productions and Information International, Inc.

Figure 3:

Solar Sailer Escape Sequence f r o m TRON

The Digital Scene Simulation G r o u p usually w o r k s o n a contract basis w i t h clients s u c h as film and video p r o d u c e r s and advertising agencies. Some projects c o m e to us carefully p l a n n e d out by the client, w h i l e o t h e r s c o m e in a very vague foITn. If w e do not get specific artistic directions (timings, s t o r y b o a r d s a n d renderings) f r o m the client, o u r Art Departm e n t creates these materials in consultation w i t h the client. A t e a m of at least three staff m e m b e r s (art director, d e s i g n e r / e n coder, technical director) is f o r m e d to w o r k on the job.

W h e n all has been decided, a high resolution final filming is made. Typically w h e n the result is screened, the client will find at least one r e a s o n to reject the w o r k a n d the w h o l e p r o c e s s goes back to the beginning. Conclusion

This p a p e r h a s p r e s e n t e d ASAS, a general p u r p 6 s e p r o g r a m m i n g language w h i c h has b e e n extended to include geometric objects and operators, parallel control s t r u c t u r e s and o t h e r features to m a k e it useful for a n i m a t e d c o m p u t e r g r a p h i c applications. ASAS m a k e s use of an abstract c o m p u t i n g element called an actor, w e have seen h o w actors p r o m o t e modularity and h o w they can simulate a w i d e range of b e h a v i o r by exchanging m e s s a g e s w i t h each other. In three y e a r s of c o m m e r c i a l use ASAS has p r o v e d itself a w o r k a b l e a n d practical tool. While the specific feature a u s e r w a n t s m a y not already be a part of the language, the extensibility of ASAS allows it to g r o w w i t h its users. ASAS has e x p a n d e d o u r "complexity barrier" a n o t h e r notch, allowing us to a t t e m p t w o r k w i t h m o r e i n d e p e n d e n t l y a n i m a t e d elements t h a n before.

W h e n the artistic c o n c e p t is s o m e w h a t settled, w o r k is started on its c o m p u t e r g r a p h i c realization. The first step is to create a geometrical model of the s h a p e s of the objects to be used in the animation. Unless the g e o m e t r y of the object is regular e n o u g h to allow it to be c o n s t r u c t e d u n d e r p r o g r a m control, the s h a p e definition is d o n e by h a n d in a laborious p r o c e s s similar to technnical drafting w e call "encoding". Often s o m e mix of m a n u a l data entry and p r o c e s s i n g by various ~geometrical tool" p r o g r a m s is used to obtain a finished object s h a p e description. As the objects are being finalized, the technical director begins to w r i t e the ASAS scripts a n d related p r o g r a m s . W h e n the g r o u p is w o r k i n g m a n y closely related scenes, for instance d u r i n g p r o d u c t i o n o n a feature film, m a n y of the "sets" or e n v i r o n m e n t s will be s h a r e d b e t w e e n several scenes. In s u c h cases it b e c o m e s convenient to set u p "libraries" of c o m m o n ASAS f u n c t i o n and object definitions. Contributions are m a d e to these "public libraries" by all a n i m a t o r s w o r k i n g on a project. Usually the m o t i o n s in an a n i m a t e d s e q u e n c e are so specifically p l a n n e d out in advance that an outline of the script can be w r i t t e n before any of the objects are available for test pictures. At this stage the ASAS script is very abstract, references are m a d e to symbolic c o n s t a n t s w h o s e values are not yet k n o w n . W h e n object files are ready a n d the script is r o u g h e d out the "graphical debugging" begins.

The a u t h o r is not p r e p a r e d to state that w h e n the ULTIMATE c o m p u t e r a n i m a t i o n s y s t e m is built, it will be p r o g r a m m i n g language based. But it is h a r d to visualize a s y s t e m w h i c h allows arbitrary extensions into u n e x p e c t e d realms w i t h o u t being fully p r o g r a m m a b l e . However, p r o g r a m m i n g and m a k i n g aesthetic j u d g e m e n t s s e e m to be disjoint in m o s t p e o p l e ' s thinking processes. The u s e r of a g r a p h i c s p r o g r a m m i n g system m u s t always be on g u a r d against c o m p r o m i s i n g aesthetic j u d g e m e n t s to simplify the p r o g r a m m i n g ! The solution u s e d in o u r c o m m e r c i a l w o r k is to m a k e the p r o d u c t i o n a joint effort of several people, s o m e responsible for artistic issues and o t h e r s responsible for technical issues.

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[22] Pfister, G. "A High Level L a n g u a g e E x t e n s i o n for Creating a n d Controlling D y n a m i c Pictures", A C M SIGPLAN/SIGGRAPH S y m p o s i u m on Graphical Languages, April 1976.

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[1] Abelson, H. a n d diDessa, A. Turtle Geometry, MIT Press (Series in Artificial Intelligence), Cambridge, MA, 1981.

[23] Preissler, M. "Multi-processed M u s i c Synthesis", BS T h e s i s MIT EECS D e p a r t m e n t , May 1976.

[2] Austin, H., "The LOGO Primer", MIT A.I. Lab. Logo W o r k i n g P a p e r 19.

[24] Reynolds, C. "A M u l t i p r o e e s s i n g A p p r o a c h to C o m p u t e r Animation", SB thesis, MIT EECS D e p a r t m e n t , A u g u s t 1975.

[3]' C h u r c h , A "The Calculi of L a m b d a Conversions", A n n a l s o f M a t h e m a t i c a l S t u d i e s 6, P r i n c e t o n U n i v e r s i t y Press 1941, Rep r i n t e d by Klaus R e p r i n t Co., 1965.

[25] Reynolds, C. " C o m p u t e r A n i m a t i o n in t h e W o r l d of Actors a n d Scripts", SM thesis, MIT (Architecture M a c h i n e Group), May 1976

[4] Clark, J., ~Hierarchical G e o m e t r i c Models for Visible Surface Algorithms", CACM, O c t o b e r 1976. [5] Dahl, M y h r h a u g , a n d Nygaard T h e SIMULA 67 C o m m o n Base Language, N o r w e g i a n C o m p u t i n g Centre, Oslo, 1968.

[26] Reynolds, C. A c t o r / Scriptor A n i m a t i o n S y s t e m User's M a n u a l 3.0, (ASASUM 3), I n f o r m a t i o n I n t e r n a t i o n a l Inc., in preparation.

[6] DeFanti, T. "The Digital C o m p o n e n t of t h e Circle G r a p h i c s Habitat", Proceedings NCC 1976.

[27] Smoliar, S. "A Parallel P r o c e s s i n g Model of Musical Structures" MIT AI Lab T e c h n i c a l Report 242, S e p t e m b e r 1971.

[7] Dijkstra, E.W. "Notes o n S t r u c t u r e d P r o g r a m m i n g " , A u g u s t 1969

[28] S u s s m a n , G. a n d Steele, G. "SCHEME - An I n t e r p r e t e r for E x t e n d e d L a m b d a Calculus", MIT AI Lab M e m o 349, D e c e m b e r 1975.

[8] E a s t m a n , C. a n d Henrion, M. "GLIDE: A L a n g u a g e for Design I n f o r m a t i o n Systems", SIGGRAPH '77 Proceedings, July 1977, San Jose, CA.

b

[29] Teitelman, W. InterLISP R e f e r e n c e Manual, Xerox Palo Alto R e s e a r c h Center, 1978.

[9] Futrelle, R. P. a n d Barta, G. " T o w a r d s t h e Design of a n Intrinsically Graphical Language", SIGGRAPH '78 Pl~gceedings, A u g u s t 1978, Atlanta, GA.

[30] W i n o g r a d , T. U n d e r s t a n d i n g Natural Language, A c a d e m i c Press, 1974.

[10] Goates, G., Griss, M. a n d Herron, G. "PICTUREBALM: A Lisp-based G r a p h i c s L a n g u a g e S y s t e m w i t h Flexible Syntax a n d Hierarchical Data Structure", SIGGRAPH '80 Proceedings, July 1980, Seattle, WA.

[32] Wirth, N. "MODULA: a L a n g u a g e for M o d u l a r Multip r o g r a m m i n g " , Software, Practice a n d E x p e r i e n c e 7,1; 1977 pp. 3-35.

[31] W i n s t o n , P. a n d Horn, B. Lisp, A d d i s o n Wesley, 1981.

[11] Goldberg, A. a n d Kay, A. SMALLTALK-72 I n s t r u c t i o n M a n u a l L e a r n i n g R e s e a r c h Group, Xerox Palo Alto Research, M a r c h 1976. [12] Gre!f, I. a n d Hewitt, C. "Actor S e m a n t i c s of PLANNER-73", Proc. o f A C M SIGPLAN-SIGACT Conf., Palo Alto, CA, J a n u a r y 1975. [13] Hewitt, C. a n d Smith, B. " T o w a r d s a P r o g r a m m i n g A p p r e n tice", MIT AI Lab W o r k i n g P a p e r 90, J a n u a r y 1975. [14] Hewitt, C. a n d Atkinson, R., "Parallelism a n d S y n c h r o n i z a tion in Actor System", A C M S y m p o s i u m o n Principles o f P r o g r a m m i n g Languages 4, J a n u a r y 1977, L. A. CA. [15] Jones, B. "An e x t e n d e d ALGOL-60 for S h a d e d C o m p u t e r Graphics", A C M SIGPLAN/SIGGRAPH S y m p o s i u m on Graphical Languages, April 1976. [16] Kahn, K. "An Actor-Based C o m p u t e r A n i m a t i o n Language", Prec. o f the ACM-SIGGRAPH W o r k s h o p on User-Oriented Design o f C o m p u t e r Graphics S y s t e m s , Pittsburg, PA, O c t o b e r 1976. [17] Kahn, K., "A C o m p u t a t i o n a l T h e o r y Of A n i m a t i o n " , MIT A.I. Lab. W o r k i n g P a p e r 145, April 1977. [18] Larkin, F. " C o m p u t i n g w i t h T e x t - G r a p h i c Forms", Confere n c e R e c o r d o f t h e 1980 LISP Conference, A u g u s t 1980, Stanford University.

Note T h i s e n t i r e p a p e r is a n e x a m p l e of c o m p u t e r g r a p h i c s . All p i c t u r e s w e r e p r o d u c e d w i t h Digital Scene Simulation, a n d directly (digitally) c o n v e r t e d into f o u r color h a l f t o n e s in t h e Infocolor f o r m a t . T h e text w a s edited a n d c o m p o s e d o n TECS, a n d a s s e m b l e d w i t h a Page M a k e u p System. C a m e r a ready, full p a g e art (including t y p e s e t t i n g a n d h a l f t o n e g e n e r a t i o n ) w a s p r o d u c e d w i t h a COMp80/2-Pagesetter. All of t h e s e s y s t e m s are p r o d u c t s of I n f o r m a t i o n I n t e r n a t i o n a l Inc.

[19] Larkin, F. "A S t r u c t u r e f r o m M a n i p u l a t i o n for Text-Graphic Objects", SIGGRAPH '80 Proceedings, July 1980, Seattle, WA. [20] Moon, D. MacLisp R e f e r e n c e Manual, Revision O, MIT Project MAC, D e c e m b e r 1975. [21] N e w m a n , W. a n d Sproull, R. Principles o f Interactive C o m p u t e r Graphics, McGraw-Hill, 1973 a n d 1979.

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