Choosing Values
What is a Value? Choosing Vlues
character move sowly or swiftly? Should it move very deliberately or uncertainly"
Should it be fa5-ter than some other character? Should it tend to avoid others?
Should it ove very directly or in graceful curves? Should it appear large or small, rounded or pointed? All these questions can be answered by choosing among a few permissible V F- I Lies for the element in question. Currently AN knows a number of eiements for the typical or default dynamics of a character. The Ani's mechanisms
for coosing values for tese elements should work well for other elements such as the size, qhape, color, and texture of the characters. There are no theoretical reasons why AN cannot handle these other elements.1
The process of choosing a value for an element differs greatly from the choosing of events and activities (which is described in the next chapter). An element has only one value which is selected from a small set of possible values.
This contracts ith the coosing of activities where there are very many different possible activities and Ani sometimes chooses more than one for the same task.
Another major difference is that there usually exists relative values for elements while nothing corresponding exists for methods or activities. For instance, Ani decided tat te typical speed of Cinderella should be slower relative to that of her stepmother.
I. Becawe ne hould oniv devote so much effort for demonstration purposes in the implementatioii of pototype, i decided to concentrate on the essence of aimation. namely motion ad ee the other aspects of -In mation underdeveloped.
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Choosin," Values What Is Value?
This breaking of the dynamics (or appearance) of a character into elemental parts builds certain prejudices into Ani. Something as complex as the dynamics of a chcaracter can be broken up 'in many different ways. Ani divides the dynamics into speed (and acceleration), path (length, curvature, smoothness), style (e.g.
repetiveness, deliberateness, level of activity) and tendencies (e.g. to avoid strangers, to be attracted to strangers). A different way of breaking up the dynamics would result in a different style of animation.1
Ani creates a typical dynamics of a character to gve it some character or identity. The personality, mood, desires and activities of a character' are all conveyed by te motion of that character in the pure animation that Ani makes.2 The typical dynamics 'Interact in a complex manner with display methods to produce
tire actual motions of a character. A simple example is when a character typically moves slowly but during a chase scene moves much faster than usual. The way this Happens is described in detail in Section C "How a Method i's Elaborated if on page
1 22.
1, There r of cours mny other factors that contribute to the style of an animator as is dscussed in 'Section C 'Ani nd Style" on page 29.
Of course in more conventional animation one can also use the appearance, facial express1011, dialog'. or rolor to (to this.
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How Vlue is Chosen Choosing Valurs
Section B How a Value is Chosen
The process of filling out a detailed description of the typical dynamics of a character consists mainly of repeatedly choosing a value for a different element of the dynamics from a small set of permissible values. AN goes about this by gathering LIP suggestions (as described in Capter IV "Suggestions" on page 9), combining them, resolving any conflicts, and finally choosing a value,
To illustrate this, a trace of the process of choosing of a typical speed for the stepmother is presented in this section, Speed 'is an especially simple element because it hs only three values, "low", "medium", and "high".
Ani begins by creating a choice point which is an actor that keeps findings, partial reSURS a history of work on this choice, and a ustification of the final choice for te stepmother's speed. Suggestions are then gathered from the description of the stepmother and the suggestion of a high speed from "strong" is the only one found, Ani oes not consider tis a sufficiently good reason to choose a high speed.
The focus of the film includes a sample of a minimally strong suggestion and this is the standard used to decide 'if a particular group of suggestions constitutes a sufficiently good reason" to choose a particular value. AN proceeds to gather up SUggeStions from the relationships and comparisons of the stepmother and the other characters. Choice points for the relative speed of the stepmother and Cinderella and for te stepmother and the fairy godmother are created and are asked for
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Writes How Ve is Chosen
suggestions. They are found to be 'in agreement with a high speed for the stepmother. The first one suggests that the stepmother be faster than Cinderella which is compatible since Cinderella is slow. The decision that the stepmother be faster than Cinderella is nearly as complex as this choice of the stepmother's speed and is based Upon their relationship and a comparison of the two characters. The choice point for te relative speeds of the stepmother and the fairy godmother suggests that the speeds of the two be different (since AN recognizes that the characters are so different). Since the fairy godmother's speed is medium this suggestion is compatible with a high speed for the stepmother.
There are suggestions other than these three suggestions, however, which conflict with te idea tat her speed be fast. For example, the comparison of the stepmother and te Prince suggested that their values be different and yet his speed is also igh. This conflict eventually gets resolved and the suggestion for the relative speed of the stepmother and the Prince i's ignored because 'it is much less strongly suggested.
1. This is de to a vry arge difference n the strength of the suggestions. If this hd not been the cas( A i %vould hve ore work to do to resolve this conflict. See the following section for a disciissioll of hw conflicts tire esolved.
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-How a Vflue is Chosen Choosing VaILKS
Ant is still not very happy with this state-of affairs and gets suggestions from the film"s global descriptors, More conflicts are discovered. A compromise
between SLIfygeStions of a high speed from the variety level and energy level and a suggestion for a low speed from the desired amount of flashiness is worked out.
This is ten cornbined with the previous suggestions and AN chooses a high speed for the stepmother.
Section C How Conflicts are Resolved
Upon finding no conflicting suggestions for a value Ani 'ust picks the value Suggested If in the rare event that no suggestions could be found, Ani just uses the default value associated with each element. More typically there are many suggestions and they don't all agree. It is important that she resolve these conflicts as sensibiy as possible if there is to be much coherence to a character.
Ani proceeds to resolve conflicts by taking each pair of conflicting suggestions and applying a succession of heuristics upon the conflict. If either of the suggestions was rejected because of their role in an already resolved conflict,
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-Choosing V lors How Conflicts are Resolved
then it is rejected gain,' Failing this Ani considers the relative strength of the two suggestions. Recall that associated with every suggestion is the strength given to it by its- ,ource and tat s suggestions are combined their strength accumulates If the strengths differ very much, Ani picks the stronger one.
Ani next finds out the relationship of each of the conflicting suggestions with the most favored suggestions so far only if neither of the two conflicting ones are among the favored suggestions). If one is compatible and the other not, then the compatible one is joined with the favored ones. Similarly if neither are compatible but one can be easily compromised with the favored suggestions it is compromised with the favored suggestions. The last part of this heuristic checks whether the favored suggestions are much more strongly suggested than either of the conflicting suggestions and if so rejects both of them.
Finally, AN tries to force a compromise between the two conflicting suggestions and failing that 'ust picks the more strongly suggested one. The strencyth of the suggestions are a function of the number of sources, the original strengths of the suggestions, and the relative importance of the different suggestion
I. If h f. Ani calls some never fnished procedures. If they had been completed they ould hve tried to rolve the conflict by ung cticism eher from the user or self gnerated) Fling h nd
if tells 1101CP AI mde before (as when A ni is being called upon to make variations of h same f ne) tir A l will ether f avor or disf avor the peviously chosen values depending pon the desired drgree of rigmality.
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How Conflicts are Resolved Choosing Values
sources as indicated by the film's focus,
Section D Postponement and the Focus
The account of the selection of the stepmother's speed is accurate from the point of view of te choice point in question. From a more global point of vew the choice point is often considered, postponed for other choices to be made, and then resumed aaain. Ani asks the choice points for the dynamics elements of each character to come up with values concurrently, As they run into trouble, especially if the'trouble is te lack of values of other choice points, then a choice poi.nt is postponed. This scheme minimizes the arbitrariness of any choices that might result
from the order in which te choices were made. It 'Introduces two new problems to AN tough: when to postpone the efforts of a choice point and when to resume them. The pilosophy behind this part of Ani 'is to order tasks so that the ones that are likely to be the easiest and most straight-forward run first, followed by those which are difficult but will not be helped much by the resolution of other choice points, and only when there are no other tasks to work on are the difficult ones that are lacking information run. This last situation is common and the focus is used to determine their ordering,
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choo'Ilo Values Postimienient ad the Focus
The section of the stepmother's speed is an extreme example of this process since it was postponed ten times before finishing. When first asked to choose a value the coice point for the stepmother's speed found the suggestion from "strong" but nothing else and so postponed itself saying it was "not happy enowyli" Next time it was awakened, it looked for relative values and'soon gave up be;,-1,,-LJse the choice points for the speeds of the Prince, Cinderella and the fairy godmother were not making any better progress.
Potentially, this situation could ead to a deadlock since the choice point for the stepmother i's waiting for values from others who are in turn waiting for the stepmother's speed. Since a choice point explicitly represents the history of its reasons for postponement this is not a very serious problem. A choice point is not allowed to postpone if its postponement reasons are dentical to the reasons for its previous postponements The only exception is 'if the previous postponement reason was tat the values of some other choice points are missing and the most recent postponement reason of at least one of those choice points 'Indicated that it was making some progress. This exception was added so a choice point could repeatedly be postponed because it I's waiting for some other choice points which have not yet concluded so long as at least one of them 'is making progress.
1, Actually choice points don't 1ostpone themselves without asking a postponement malag-er fr pern)),sjon first. It is this Manager that prevents this deadlock.
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Posti)onement nd the Focus choosing Vhws
The third time tat the choice point of the sepmother's speed was CMakened, it found tt it still needed the values of the other three character's speed but tat te choice point for Cinderella's speed had made a little progress (that one went ahe,rid without waiting for the values of the others since Cinderella is part of te film's focus). The stepmother's choice point for her speed was ost 'oned three ore times before the speed of Cinderella was chosen. It took three more awakenings before the other two characters had a speed. At that point the relative suggestions were added to the choice point. Conflicts were found among them and te choice point was postponed. Next time it woke up, it gathered up te global film descriptor's suggestions and tried to resolve conflicts but discovered that in compromising the global suggestions a new conflict was generated and so was postponed one ore time.
In deciding wich choice point to awaken next Ani organizes the yet-to-be-decided coice points in a tree. The tree is organized according to advice from the film's focus. For example, it can be organized by character and then ech caracter is further organized by type of element (e.g. motion descriptors and movement tendencies) or the type of element could be more primary and it is
subdivided by characters. This choice is left up to the focus since a film can focus on different aspects, eg a film can gve more priority to the coherence of the characters or be more concerned with, say, the (aesthetic) balance of the positions
of te caracters.
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choosing Values Postponement nd the Focus
Dynamics
dmother
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r e a _ince Stepm
Cinde P other Fairy-_`goi
Movement Tendencies Path Style itto> <ditto>
Len h Curva ure
'Speed Acceleration ... Deliberateness Rhythm
Avoid-strangers
Dynamics
Movement Tendencies pa Style
Cinderella Prince Ste mother Fairy-go mother <ditto> <ditto> <ditto>
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-Spee( Accelera
Avoid-strangers ... ength Curvature ...
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ve i Derateness Knytnm. -. .
Figure 52 Another Way to Organize the Choice Space
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Figure 5.1 One Way to Organize the Choice Space
Postponement and the Focus Choosing Values
Thi stree of choice points is used by Ani to pick a subset of the undecided choice points to work on, Ani starts at the top of the tree and chooses between tire immediate descendants by following the rankings 'in the film's focus and giving preference to those nodes which have the most choice points under them whose values are needed by others. The node selected is either a choice point, in which case it is awakened, or else is a node. If it is a node the process recurses and selects between the immediate descendants again.
The focus is consulted here since the nodes that are worked on first are more likely to be coherent and locally optimal since they will not be constrained by tire subsequent determination of the values of other choice points. Consider the situation in the previous example where one of the four character's speed had to be determined first to avoid a deadlock. Cnderella went first because the film focuses on ler, As a result she received a speed that 'is consistent with her description nd te film"s global description and was not 'influenced by the speeds of the other characters. The other ordering rule prefers nodes that are holding up the largest number of other choice points. This tends to make AN work first on the tasks that are the greatest bottlenecks to progress.
This part of AN is organized to maximize the amount of parallelism possible and, when not possible, to carefully order the choices. Each choice point can be created and explored almost independently of the others. Only when one needs the results of another tat has yet to finish is there a question of postponing, resuming,
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and deciding who should go first in cases of mutual dependencies. The large number of elements of a character's default dynamics multiplied by the potentially laro-e number of caracters in a film is the number of choice points that could be explored in prallel-' It is interesting to contemplate if human animators conceive their films sequentially or if they exploit this potential for parallelism. Section A
"The Simulation of Ani's Characters" on page 251 and Section B Running Activities in Parallel" on page 253 discuss in detail other roles parallelism has in Ani.
1. The nmber would icrease fUrther if other types of elements of a character's definition esr- the Plements of chiricter's appenance) were considered,
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clloosjll' Values Postponement and the Focus t
Choosing Methods