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GAME DESIGN: CHARACTER ANIMATION DESIGN Gregory Francis Department of Computing Sciences Villanova University, Villanova, PA 19085 CSC 3990 – Computing Research Topics Gregory.email@example.com ABSTRACT invention of computers. However, these older and highly manual techniques first introduced by Walt Disney and Recent advances in technology and work in artificial other early animated filmmakers set the stage for intelligence have lead to the development of more character animation as we know it with motion pictures cognitive virtual characters in video games today. Thus, like Snow White and the Seven Dwarves. More recently, video games that are released upon the gaming industry companies such as Pixar have further developed the today tend to take place in a very realistic and fluid virtual technology used for animation with animated features environment. With procedures like rotoscoping in the such as Toy Story. Each of these very different movies past helping to pave the way for modern methodologies used a form of motion capture to aid in the animation of including cognitive modeling languages, character skin their onscreen characters. animation, and finite state machines the animation and cognition that video game characters are endowed are When Walt Disney Studios released Snow White in 1937, improving exponentially. This paper aims to explain the it used what is known today as rotoscoping to give actions most modern methods in which developers animate their to the characters in the motion picture. The first characters in order to create the realistic environments rotoscope was invented by Max Fleischer in 1915. “The that gamers immerse themselves in during game play. device projected live-action film, a frame at a time, onto a light table, allowing cartoonists to trace the frame’s image KEY WORDS onto paper” . Rotoscoping is still used today in the Character animation, game design, animation design entertainment field in order to copy realistic motion from actor to cartoon. It is actually the ancestor of motion capture. 1. INTRODUCTION In its day, rotoscoping was cutting edge motion capture design. But as time progressed, technology became more In households today, video games are a very common advanced. As such, more advanced motion capture sight. Video games such as Bungie’s Halo 3 and systems were designed. One such system was that of Op- Ubisoft’s Assassin’s Creed boast intricately designed Eye. This system relied on blinking LEDs and a series of character animations and intelligence . But how do cameras. “They wired a body suit with the LEDs […] these characters inherit the actions that they perform? Two cameras with special photo detectors returned the 2- What is the technology that supports these very lifelike D position of each LED in their fields of view. The behaviors, movements and actions? In this paper, a computer then used the position information from the two historical overview ranging from the original cameras to obtain a 3-D world coordinate for each LED” methodologies for character animation, such as . Op-Eye was used in the Graphical Marionette project rotoscoping through to the current techniques for that was a joint project by MIT and the New York character animation are presented. Particular emphasis is Institute of Technology. The two combined efforts to placed on the technology now used by programmers track the movements of the human body using motion whose jobs it is to implement today’s popular and highly capture. This project, conducted from 1982-1983 helped graphical games, with the capability to use approaches to bring about the advanced motion capture systems we such as motion capture design and cognitive modeling know today. languages. Another early method of motion capture was originally used by Tom Calvert of Simon Fraser University from 2. BACKGROUND/PAST HISTORY 1980-1983. He attached potentiometers at strategic points on the body in order to track human motion for a study in Many now-antiquated approaches to character animation his biomechanics lab. “To track knee flexion, for were not first seen in video games, since they pre-date the instance, they strapped a sort of exoskeleton to each leg, positioning a potentiometer alongside each knee so as to bend in concert with the knee. The analog output was then converted to a digital form and fed to the computer animation system” . In 1992, Brad deGraf introduced a different and very improved type of motion capture. He called his system Alive! and it eventually had the capability to track the motion of the actor’s head, torso, feet, and hands . This system actually helped to create the type of motion capture that aided in the creation of Toy Story. By the time that Toy Story was completed in 1995, character animation had evolved quite substantially since rotoscoping. Toy Story used a method of motion capture very similar to what we know today to enhance the viewing pleasure of the film. Up until now, the human body had never really been used for the animation of characters in movies, let alone video games. For this reason Toy Story was truly a ground breaking movie . Figure 1: Motion Capture example from The Polar Express  3. CURRENT METHODOLOGIES 3.1.1. CAPTURING THE NUANCES OF HUMAN ACTION In today’s gaming world programmers use cutting-edge methods to animate virtual characters. These methods Motion capture has other advantages that tend to be over include a more advanced version of motion capture, looked. While programmers have the ability to animate character skins, cognitive modeling languages and finite characters with complicated actions including evasive state machines. maneuvers in certain situations, they also have the ability to portray many simple actions. For instance, a certain 3.1. MOTION CAPTURE DESIGN character may be designed with subtle motions that indicate some sort of emotion. For instance, oftentimes One method that consistently has been used since the gamers find their characters shrugging to indicate early days of character animation is “motion capture.” ambivalence; and while this may seem insignificant, it is Motion capture first involves the portrayal of deliberate these minor actions that truly make a virtual character actions by a human actor. These portrayed actions are seem real. In his book, Michael Kipp addresses the idea captured by a system such that they may be translated to that simple actions that humans perform on a daily basis on-screen animation. Modern day systems need a series are the ones that are often ruled out and that without these of cameras as well as an actor outfitted in a suit designed actions, a character cannot be realistic. He separates the for motion capture. This suit is equipped with either actions of humans and ultimately virtual characters into 6 blinking LED’s or reflective balls. Regardless of which classes . are used, the camera system detects the position of them as the actor moves. 3.1.2. CLASSES OF ANIMATION By using this method to aid in the design of a character’s motions and behaviors, programmers can create such The six classes that Kipp mentions are adaptor, iconic, animations maintaining a very high degree of realism. emblem, deictic, metaphoric, and beat. Each of these The motions portrayed by a human are capture and classes has their own specific actions that fall into place translated to the virtual character. These motions are under them. For instance the adaptor class of actions recorded and captured using a system that is very similar involves the touching of objects or one’s self. This may to that of deGraf’s Alive! Series of motion capture clips include the scratching of one’s cheek during a normal may be linked together and characterized as high-level conversation . Once these classes of animation have behaviors . Figure 1 shows and example of an actors been captured, it is important that designers have motion being portrayed in a virtual character. characters to perform them. Character skins aid in the degree of realism conveyed by characters. 3.2. CHARACTER SKINS Though designers may be able to capture the motions via motion capture with a strikingly high degree of realism, there must be a realistic character that performs them. While many may not realize it, in order for a character to seem human, a great deal of detail is required. This includes the animation of tiny nuances like the skin of a virtual character. Character skins, or meshes, are typically laid over a hierarchical skeleton. Designers then implement an action that the character may perform in the sequence game play by using what is called motion compensation and deformation. Both of these are then applied to the skeleton that is laid beneath the character mesh such that as the skeleton moves, so too does the character skin. With gaming industry’s interaction systems today, computation methods tend to dominate the means by which motion is derived . 3.2.1. LINEAR SKIN BLENDING Figure 3: Top Row: Desired outcome. Middle Row: Outcome via Linear Skin Blending. Bottom Row: Outcome via Extended Linear Skin Blending. This is with the addition of one hinge joint to prevent Many character skins today are implemented by using limb twisting.  linear skin blending. Though this method is very accurate and advantageous for designers, it also has its pitfalls and 3.3. FINITE STATE MACHINES flaws. The most prominent downfall of using linear skin blending is the twisting and stretching of limbs. Figure 2 After an actor’s motions have been captured, designers shows an example of limbs twisting and stretching at key need a way to store them so that a character can later areas in the character skin . search through them when determining which action to perform based on a particular situation. This can be done by using what is known as a Finite State Machine, or FSM. Once a character’s motions have been captured and organized into high-level behaviors, they are then intertwined with one another into the FSM . Figure 4 shows an example of a simple FSM . Figure 2: Example of the stretching and twisting of limbs  3.2.2. EXTENSION OF LINEAR SKIN BLENDING Alex Mohr and Michael Gleicher have introduced an extension to linear skin blending that fixes its original Figure 4: A simple FSM  pitfalls while maintaining efficiency. Along with remaining efficient, this method allows for the depiction of more detailed skins; this includes the bulge of muscles 3.3.1. SEARCHING THROUGH THE FSM of characters. Mohr and Gleicher show that skin deformations in virtual characters tend to collapse around Using search algorithms, designers give the character the hinge joints; these include the wrist, knee, and elbow. ability to search through the FSM and plan a sequence of While this collapse is inevitable with original skin events and motions that are designed to meet a user- blending, using their extension, it is possible to prevent defined end. In today’s gaming industry, move-trees are this by simply adding another hinge joint. These joints used to generate these sequences of events. Move trees are added in the locations where previous linear skin are designed solely on a reactive basis such that blending fails in order to prevent the original pitfall . characters choose their actions based on the current Figure 3 shows a desired design of a character skin, the environment . While it seems that a specific FSM is pitfall of twisting limbs that comes along with linear skin required to animate each character in an environment, blending, and the manner that this is fixed with the certain FSM’s may be applied to a range of characters. extension of linear skin blending . 3.3.2. APPLICABILITY OF THE FSM One of the major advantages of using an FSM to aid in the animation of a character in a gaming environment is that in certain situations, one FSM can be used to animate 3.3.3. USING A GLOBAL APPROACH many characters. This holds true for characters that are not even of the same species. For instance, Manfred Lau Manfred Lau and James Kuffner introduce a new way to and James Kuffner used the same FSM to animate both a search through an FSM that increases applicability as well skateboarder and a horse placed in dynamic as artificial intelligence. Kuffner and Lau’s approach environments. This is shown in figure 5 . creates a search tree of states of motion within the FSM and then decides between a group of possible sequences based on global characteristics before finally deciding the final motion. While avoiding a great deal of pitfalls, this approach also increases the applicability of the FSM. For example, an FSM designing a skateboarder can also be used to design the motion of a horse. This algorithm may also be interrupted at anytime to return a best-fit motion series if constraints call for it. The motions that are generated, however, may need to be smoothed out accordingly. 3.4. COGNITIVE MODELING LANGUAGES Animation designers are constantly looking for ways to make their characters more intelligent. One way to do this is to give them the ability to learn on their own. This Figure 5: Both the skateboarder and the horse are animated using the same FSM even though they are of different character species and in a can be done by means of cognitive modeling. By using different environment.  cognitive modeling, programmers give their characters The applicability of FSM’s also aids in efficiency of the ability to learn and plan their own actions. This developing character animation. For example, one FSM method involves the animated character learning as it can be applied to groups of characters that are all progresses. They then store the information, remember designed to perform similar actions. This can be seen in certain scenarios and how the knowledge was obtained so games today in many different situations. Take for that it can later be used to create its own actions in a new example, a first person shooter in which groups of situation. Cognitive modeling may be broken down into enemies must attack and eliminate an opposer. They may the subcategories of domain knowledge specification and each be designed using the same FSM such that they character direction . follow the same actions in order to seek out and destroy a certain target. Figure 6 shows a group of one-hundred 3.4.1. DOMAIN KNOWLEDGE characters animated using a single FSM along with a SPECIFICATION AND CHARACTER possible design for that FSM . DIRECTION Domain knowledge specification involves the character’s understanding of its surrounds as a global aspect along with the means by which these surroundings are subject to change. The aspects of a character’s global computer generated domain that are subject to change are known as “fluents.” Character direction, on the other hand, involves the planning and choosing of a sequence of events for the character to perform in certain situations in order to obtain a user specified end. For this reason, the high level programming language called Cognitive Modeling Language was developed, or CML . 3.4.2. ANIMATING CHARACTERS BASED ON “FLUENTS” CML is based off of the idea that “knowledge + directives Figure 6: Top image depicts one-hundred characters animated with the = intelligent behavior.” Since characters are armed with a same FSM. Bottom image shows a possible design of that FSM.  number of actions, both primitive and complex, CML own actions based upon the abilities that they are give them the ability to choose between both types of endowed with via CML. This would allow for a more actions to characters based on the “fluents” of the global realistic and fluid environment. computer generated domain. When an action is performed and a “fluent” changes, CML allows that fluent 4.1.2 LET THEM SEARCH ON THEIR to take on a new specified value. This can be seen in the OWN following CML pseudo code: occurrence drop(x) results in !Holding(x); Since characters designed using CML have the ability to occurrence pickup(x) results in Holding(x); learn actions on their own, why not allow them to search  through the FSM independently as well? When it comes to characters designed with CML their ability to learn is With CML, designers have the ability to give intelligence endowed to them by their developers. After being placed to characters as well as cameras to implement a more in an environment filled with “fluents,” they must make fluid and cognitive environment. Ultimately, unlike a decisions as to what they should do in specific scenarios. behavioral model which is reactive, when it comes to With characters having the ability to learn on their own, it designing character animations, a cognitive model is seems feasible for them to be able to search through an deliberative. With the help of the high-level language FSM on their own as well. Take for example a character CML, characters can analyze preconditions and whose sole purpose is to hide. The actions within the consequences of actions following for a more realistic FSM may be for them to run in all directions as well as computer generated world of game play. The following crouch behind objects to take cover. When the situation CML pseudo code determines how long it has been since calls for it, let the character search through the FSM the character last spoke . independently and make a decision on their own what they should do. occurrence tick results in silenceCount = n − 1 With the ability to search through an FSM independently when silenceCount = n && !Talking(A,B); coupled with the intelligence to learn via CML, characters occurrence stopTalk (A,B) results in silenceCount = ka; seem more realistic. This ultimately would bring occurrence setCount results in silenceCount = ka; developers one step closer to what seems to be the end  goal of creating a human-like virtual being. Unfortunately, these characters are still virtual and oftentimes they do not make the smartest of decisions. 4. PROPOSED WORK  No matter how advanced our technology becomes in all 4.2. MISTAKE RECOGNITION fields, there is always room to improve. This holds true for character animation design as well. Virtual evolution of characters brings them ever closer to the goal of a perfectly human complex. However, there is 4.1. COMBINGING THEM ALL still one aspect that these entities lack; the ability to reflect upon a certain decision, change, and improve it. With the introduction, use and advancement of CML, That is not to say that this is not possible. Using an FSM FSM, and motion capture developers now have the means already enables developers to interrupt the algorithm and tools by which to endow their characters with searching through it in order to output a best-fit algorithm. intelligent actions and animations. However, what if a This brings across the idea of interruption of a virtual developer were to combine these techniques in order to character’s action if they get stuck in a loop known to animate their characters? In order to do this, it would gamers as a “glitch.” only be necessary to integrate CML into the process. A glitch is when a virtual personification encounters an Since an FSM is already a combination of high-level internal problem and gets “stuck” performing some behaviors designed and captured with the help of motion miscellaneous action. Since an FSM can interrupt a capture it would only be redundant to attempt to combine character’s actions in order to make a best-fit animation, it these two. Thus, the only combination necessary is is plausible then to posit that if a “glitch” occurs the between FSM and CML. animation can be stopped entirely. Upon stopping the animation developers could tell the character to restart 4.1.1. INTEGRATING CML their entire process; reanalyze their surroundings, including “fluents,” restart the search through the FSM, Though integrating CML into an FSM may be and finally restart their process of action. With the ability cumbersome, the process could potentially bring about an to reflect upon and correct decisions, virtual characters abundance of improvements. Not only would characters again step towards the realm of reality.  have the realistic actions given to them by motion capture design, they would also have the intelligence to learn their harms way. Ultimately, with the advancement of finite 4.3. QUALIFICATIONS state machines, character skins, cognitive modeling languages and motion capture, we have the ability to With the experience that I have gained from programming benefit and improve our own well being. classes such as Computer Systems and Algorithms and Data Structures, a solid groundwork has been set such that REFERENCES I would be able to step up to learning something as complex as a CML. Coupled with the understanding of  Alberto Menache. Understanding Motion Capture for tree structures from extensive use in Java, I am able to Computer Animation and Video Games. 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With the use of animation design we are able to “Cognitive modeling: knowledge, reasoning and planning figure out ways to improve ourselves physically. This for intelligent characters.” International Conference on subject also pertains to the military. With the use of Computer Graphics and Interactive Techniques virtual characters designed to be lifelike, we are able to Proceedings of the 26th annual conference on Computer simulate combat situations without putting humans in graphics and interactive techniques, 1999. Pages 29-38.  Rob McKaughan. “Motion Capture: Acting with Balls”. February 22, 2006. Last Accessed December 9, 2009. Image of motion capture used for The Polar Express. http://www.artisticwhim.com/blog/archives/2006/02/moti on_capture_acting_with_bal.html.
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