Character Animation Scripting Environment

Synonyms

Animation scripting; Scripting environment; Scripting language

Definition

Scripting languages for character animation can be characterized as the earliest type of motion control systems. In scripting environments, the required animations are generated by writing a script in the animation language. This means that a user must learn the animation scripting language. Such systems typically allow scenes and objects to be described, along with their relationship with each other. Among other advantages, they require no user interface and can therefore be implemented as cross-platform solutions.

Introduction

Character animation can be characterized as a complex and time-consuming process. This is especially true when animating virtual characters based on key-frame techniques, as this requires prior knowledge of software solutions. Moreover, artistic skills are also required since the virtual character should animate as naturally as possible.

In order to avoid time-consuming processes in animating virtual characters, motion capture technologies now provide high-quality and realistic animated sequences. This is possible because the ability to capture real humans in the act of performing is achieved through the provided required motions. The advantages of motion capture techniques are numerous, especially in the entertainment industry. However, the captured motion data, itself, is not always usable, since virtual characters should be able to perform tasks in which the required constraints are not always fulfilled. Thus, methodologies that retarget (Gleicher et al. 1998), wrap (Witkin et al. 1995), blend (Kovar et al. 2003; Park et al. 2002), splice (Van Basten and Egges 2012), interpolate (Kovar et al. 2002; Mukai and Kuriyama 2005), etc., the motion data have become available to help the animators to create the required motion sequences. In addition to the motion synthesis techniques that are based on software solutions, animating a virtual character through programming is also difficult. This is especially true in cases where animators, artists, and students do not have the required programming skills. Hence, animating virtual characters in order to visualize ideas and generate simple scenarios in which virtual characters evolve can be a very complex process.

Based on the aforementioned difficulties that inexperienced programmers can face, this paper introduces a simple, easy-to-use, scripting environment for animating virtual characters, which is based on a small number of scripting commands. The scripting environment presented (see Fig. 1), which is called CHASE, provides a user with the ability to script the action of a character as well as to script possible interaction between a character and objects that are located within the virtual environment.

Character Animation Scripting Environment, Fig. 1

The interface of CHASE

In order to implement CHASE, the following parts were developed. Firstly, identify the basic actions that a character should be able to perform and also generate the basic scripting commands. Secondly, a number of parameters that should allow the user not only to synthesize the required motion of a character but also to gain a higher level of control of each action of the character were defined. By using a reach number of motions that a character can perform, as well as by associating these actions with specified keywords, a motion dataset is created. The input commands are handled by a number of developed background algorithms, which are responsible for retrieving the desired motions and synthesizing the requested actions of the character. During the application’s runtime, CHASE synthesizes the requested motion of the character and displays the final animated sequence.

The remainder of this paper is organized as follows. The section “Related Work” covers related work in character animation by presenting previous solutions for animating virtual characters that are based on interactive or automatic techniques. Previously developed scripting environments for the animation of virtual characters are also presented and discussed. A system overview of CHASE is presented in section “System Overview.” The script commands, possible parameters, and additional functionalities that have been developed for CHASE are presented in section “Scripting Character Animation.” Finally, conclusions are drawn and potential future work is discussed in section “Conclusions and Future Work.”

Related Work

This section presents work that is related to the solution presented. Specifically, the following paragraphs present methodologies that use different input devices or easily specified constraints for animating virtual characters, systems that provides to a user the ability to synthesize task-based or scenario-related animated sequences, and previously proposed scripting environments for character animation. Finally, the advantages provided by CHASE comparing by previous solutions are presented.

Interactive character control can be classified according to the input device that is used for the character animation process (Sarris and Strintzis 2003). In general, the character controller can be a standard input device, such as a keyboard and a joystick (McCann et al. 2007). Alternatively, it can be more specialized, such as text input (Oshita 2010), prosodic features of speech (Levine et al. 2009), drag and drop systems where the motion sequences are placed into a time-line (Oshita 2008), sketch-based interfaces (Davis et al. 2003), or the body of a user (Chai and Hodgins 2005), while the motion is captured by motion capture technologies. Each of the previously mentioned methodologies has advantages and disadvantages. The choice of the most appropriate input device depends on the actual control of the character’s motion that the user requires.

A variety of methodologies for the animation of a virtual character based on easily specified constraints have also been examined. These solutions are based on motion graphs (Kovar et al. 2002); literature such as Safonova and Hodgins (2007); simple footprints (Van De Panne 1997) that a character should follow, on space-time constraints as proposed in Cohen (1992); or statistical models (Min et al. 2012) that are responsible for retrieving and synthesizing a character’s motion. However, even if easily specified constraints enable a user to animate a character, different frameworks that permit either the interactive or automatic animation of a character have been developed. In Feng et al. (2012), which is a task-based character animation system, by using a number of screen buttons, the user is able to animate a character and make it interact with objects that are located within the virtual environment. Other methods (Thiebaux et al. 2008; Kapadia et al. 2011; Shoulson et al. 2013), which can be characterized as scenario-based character animation systems, provide automatic synthesizing of a character’s motion based on AI techniques.

In the past, researchers developed scripting languages and systems in the field of embodied conversational agents. The XSAMPL3D (Vitzthum et al. 2012), AniLan (Formella et al. 1996), AnimalScript (Rößling and Freisleben 2001), SMIL-Agent (Balci et al. 2007), and many others enable a user to script a character’s actions based only on predefined command. Among the best known markup languages for scripting the animation of virtual characters are the Multimodal Presentation Markup Language (Prendinger et al. 2004), the Character Markup Language (Arafa et al. 2003), the Multimodal Utterance Representation Markup Language (Kranstedt et al. 2002), the Avatar Markup Language (Kshirsagar et al. 2002), the Rich Representation Language (Piwek et al. 2002), the Behavior Markup Language (Vilhjalmsson et al. 2007), and the Player Markup Language (Jung et al. 2008), which were developed for controlling the behavior of virtual characters.

The representation of all previously mentioned languages is based to an XML-style format that allows users to script tasks featuring virtual characters. However, these languages focus more on communicative behavior such as gestures, facial expression, gaze, and speech of virtual reality characters, instead of providing functional characters that can generate scenario-related animated sequences.

Various solutions that are similar to the presented methodology were proposed previously for the animation of virtual characters based on scripting commands. StoryBoard (Gervautz et al. 1994) provides the ability to integrate a scripting language into an interactive character animation framework. Improv (Perlin et al. 1996), another framework with which to create real-time behavior-based animated actors, enables a user to script the specific action of a character based on simple behavior commands. STEP (Huang et al. 2002) framework provides a user the ability to script such actions as gestures and postures. This methodology, which is based on the formal semantics of dynamic logic, provides a solid semantic foundation that enriches the number of actions that a character can perform.

The majority of previously developed scripting environments and markup languages provide only specific actions that a character can perform. An additional limitation is the inability of such systems to enhance a character’s synthesized motion. Therefore, a user always receives a lower level of control of the synthesized motion of a character. Moreover, in cases in which a user must generate an animated sequence where many characters will take part, a great deal of effort will be required due to the difficulty of scripting multiple actions for multiple characters. This is especially true for users who wish to generate a sequence with animated characters, but are inexperienced in programming.

These difficulties are overcome in the presented scripting environment. Firstly, instead of enabling a user to script an animated character based on XML-related formats, a simplified scripting environment with its associated scripted language, which is based only on three commands, is introduced. Secondly, since a character should be able to perform concurrent actions, a simple extension of the basic command handles this. Therefore, the user achieves a higher level of control of a character’s action. Moreover, in cases where a user must animate more than one character simultaneously, one can specify the character that should perform the requested action by adding an additional method to the existing command for a character. Finally, in cases where a user must generate an animated character in a multitask scenario, by simply specifying the row in which the task should appear, the system will synthesize the tasks requested automatically.

We assume that the described unique functionalities that are implemented in CHASE will enable a user to synthesize compelling animated sequences in which a variety of virtual characters are involved. Hence, in view of the simplicity of the developed commands, in conjunction with the associated parameters, the proposed methodology is quite powerful in comparison with the previous solution. In addition, the easy-to-use and easy-to-remember commands make the presented scripting environment effective, especially for users who are inexperienced in programming.

System Overview

This section briefly describes the proposed system. Specifically, a variety of background algorithms are responsible for recognizing the input commands and synthesizing the motion of a character. The developed background algorithms communicate with the animation system, which is responsible for generating a character’s motion, as well as with a path-finding methodology to retrieve the path that the character should follow when a locomotion sequence is required. Finally, CHASE synthesizes and displays the requested motion sequence. Figure 2 represents the procedure.

Character Animation Scripting Environment, Fig. 2

The architecture of CHASE

Interface

The interface of CHASE (see Fig. 1) is characterized by its simplicity. In its current implementation, it consists of a scene panel that displays the resulting animations, an edit mode panel to edit the input objects, a progress bar that shows the progress of the displayed animation, a scripting box, and a few buttons for use in building, playing, and clearing the written scripts. Finally, buttons that save the scripted code and export the generated animated sequences are also provided.

A downloadable version of the presented system, documentation specifying all of its capabilities, and examples of scenes can be found on the CHASE project page.

Third-Party Implementations

A number of techniques and libraries are used to construct CHASE. CHASE uses the Recast/Detour library (Mononen 2014) for the path-finding process and collision avoidance with the environment. Concurrent actions are generated based on a simple layering methodology similar to the one proposed in Oshita (2008). Finally, a similar (Lang 2014) full-body inverse kinematics solver was implemented to handle the postures of a character while interacting with objects located within the virtual environment.

Scripting Character Animation

Developing scripting commands for animating a virtual character can be characterized as a complex process since a virtual character should be able to perform variety of actions. In this section, the identifications of the basic scripting commands that are necessary to enable the virtual character to navigate and interact within a virtual environment are presented. Moreover, by introducing additional methods called by the main scripts, the system generates concurrent actions of a character, as well as animates multiple characters simultaneously. Finally, an additional functionality of CHASE for scripting multitask animated sequences for the generation of scenario-related animated characters is presented.

Identifying Scripting Commands

The application that is presented has been developed for users who are inexperienced in programming. Thus, simple, easily memorized scripting commands are necessary. To generate the required scripting commands, one must begin by identifying the possible actions or type of actions that a character should perform. Generally, a character should be able to perform simple actions such as waving its hand, tasks related to locomotion such as moving to a target position, and interaction tasks such as grasping with its hand an object that is located in the three-dimensional environment. It is apparent that these are the three basic types of actions that a virtual character should be able to perform. Based on this general description, three basic scripting commands were developed: the do(parameters), the goTo(parameters), and the interactWith(parameters).

The do(parameters) command provides a character with the ability to perform a single action. The goTo(parameters) forces a character to move within the given virtual environment. The final command is responsible for making the virtual character capable of interacting with a variety of tasks. Hence, the third command, the interactWith(parameters), is responsible for providing the ability to control a variety of the character’s actions.

For these commands, the parameters within the parentheses indicate the possible parameters that each of the scripting commands could receive (see section Command Parameters). Due to the various parameters that each command receives, a user is provided with the means to develop both abstract and specified action of a character. For example, with the goTo(parameters) command, it is possible not only to generate the required locomotion of a character but also to enable a user to gain better control of the synthesized motion of a character, since the user can specify how the locomotion of a character should be generated. The following section presents the basic parameters that each command receives.

Command Parameters

A task assigned to a character can be performed in a variety of different ways. For example, a sequence of locomotion to a target position can be performed by walking, running, etc. motions. Hence, in cases where a user needs a higher level of control of the synthesized motions of a character, parameters that enhance these actual actions generated by the previously mentioned scripting commands should be defined.

The first command that implemented the do(parameters) command enables a user to script simple actions of a character. This command has a single mandatory parameter, which indicates the action that the character should perform. However, optional parameters to specify the body part or the duration of the task can also be used. Specifically, the user can request a single action by calling do(action), as well as specify the target where the action should be performed, the duration of the action, and the body part that should perform the requested action. This command initially permitted a character to perform the requested action without the need to perform a locomotion sequence (i.e., to wave its hand while staying in its position). However, the do(parameters) command can also be used to permit the character to perform locomotion tasks, since one can request that a character performs a walking motion. Based on these parameters that can be inserted into the do(parameters) command, a user has the means not only to generate the requested action but also to generate an action that should fulfill user-specified constraints.

The goTo(parameters) command enables the character to perform locomotion tasks. The user identifies a mandatory parameter, which is the target position that the character should reach. However, the user is also able to use an additional optional parameter that specifies the motion style that will animate the character. Therefore, a character’s locomotion to a target position can be scripted either by (i) inserting the target position such as goTo(target) when a simple walking motion of the character is desired or (ii) inserting goTo(target, motion style) when both target position and motion style are specified.

The final command that is implemented in CHASE, the interactWith(parameters), can be characterized as more complex than the two previously mentioned commands. The reason is that there are numerous possible interactions between a character and an object. If a character is asked to interact with an object, various actions can be generated. Even if it is possible to associate actions with specific body parts of a character in a preprocessing stage, there are also possible variations of the required actions. These variations may be related to the character’s body or to the duration of the display of the action. For example, scripting a character to kick a ball may also require specifying the foot that should perform this action. Moreover, asking a character to knock a door may also require specifying the duration in the knocking. For that reason, four different parameters have been defined. The first two parameters (object name and interaction module) are mandatory. They indicate the object that the character should interact with and the interaction module that should be generated. However, depending on the user’s requirements for generating a specific action, two more optional parameters could also be inserted. The first one (body part) enables the user to choose which of the character’s body parts should perform the requested action. In the current implementation, the user is permitted to choose the hand or foot that will perform the action. The second parameter (duration) enables the user to choose the time (in seconds) required for the requested action.

Based on the possible parameters that each command could receive, the following should be noted. Firstly, while the user did not specify any optional parameter for a scripted command, the system generates the required action taking into account a predefined set of parameters that are associated with each action of the character. For example, if a user requests that a character kick a ball, the system will display only a single kick by the character. The reason is that a ball kicking action is defined as to be performed only once to avoid synthesizing meaningless and repeated motions. Secondly, it should be noted that each optional parameter is independent. This means that the user is not required to specify all of the optional parameters provided by each command. Therefore, the user may control specific components of the requested action.

A simple example of this capability of the commands illustrates this. While using the do(parameters) command, the user may request that only either the body part or the duration parameter or both of these be filled. In any case, the system’s decision in generating the requested motion is not influenced by other factors since it is capable of recognizing the correct form of the scripted command in all of the aforementioned cases.

The three commands that are examined in this paper in conjunction with the associated parameters that can be used to animate a virtual character are summarized in Table 1. In addition, a small set of possible keywords that the user could employ in order to animate virtual characters is presented. It is assumed that an additional control parameter for the synthesized motion could be quite beneficial, since it enables the user not only to animate a character but also to force the system to synthesize the user’s actual wish. Complete documentation of all possible actions that can be synthesized by the character can be found in the CHASE webpage (URL omitted for review purposes).

Character Animation Scripting Environment, Table 1 Commands and associated parameters that can be used in CHASE to request an action by an animated virtual character

Scripting Concurrent Actions

Virtual characters, such as humans, should be able to perform more than one action simultaneously. This section presents the scripting process for concurrent actions that a character can perform. The concurrent action functionality is based upon the ability to specify the body part that should perform the action in conjunction with the base action that has been requested. The concurrent action lies between the do(parameters) and either the goTo(parameters) or the interactWith(parameters) commands. Specifically, to have a character perform concurrent actions, the do(parameters) command is attached to either the goTo(parameters) or the interactWith (parameters). A simple example follows. To cause a character to perform a motion, such as waving its hand while walking to a target position, the system permits the user to script the desired walking motion of a character and to request the additional motion that the system should generate. Hence, the previous example can be requested simply by scripting goTo(target, walk).do(wave hand, handR). Thus, by permitting the user to generate additional actions of a character, while another action is in progress can, be quite beneficial when more complex animated sequences are required. Therefore, this additional functionality provides a higher level of control over a requested action of a virtual character.

Scripting Multiple Characters

In animated sequences, it is quite common for more than one character to participate in a single scenario. Hence, by extending the three scripting commands, CHASE also enables a user to script more than one character simultaneously. This is achieved by attaching an additional command to one of the three basic commands, called characterName(parameter). This command specifies the character that should perform an action, permitting the user to control multiple characters, in cases where more than one character participates in the animation process. A simple example of forcing a specific character to perform an action follows. Consider a character named Rudy who is required to walk to target. This procedure could be called by simply scripting goTo(target).characterName(Rudy).

Scripting Multiple Tasks

In scenario-related sequences that involve virtual characters, the latter should be able to perform a variety of tasks one after the other. Thus, this paper presents a method to script multiple tasks, such as enabling a user to synthesize long animated sequences. Generally, the tasks that a character can perform are characterized by their linearity. Specifically, a task begins while a previous task is completed, and the procedure continues until there are no other tasks for a character to perform.

Based on the foregoing, a multitask scenario in a general form can be represented as components of an array that has a dimensionality equal to N × 1, where N denotes the total number of tasks that a character should perform. By assigning each of the actions an array called task[index], a user can generate long animated sequences. This is achieved by allowing the user to assign singe tasks at each index value of the task array. A simple example of a multitask scenario appears in Fig. 3, as well as in the accompanying video. Its scripting implementation is represented in Algorithm 1.

Character Animation Scripting Environment, Fig. 3

A multitask scenario generated by using Algorithm 1

Algorithm 1 A simple example for generating a multitask scenario

It is quite common in multitask scenarios to involve multiple characters. Two different approaches can be used in CHASE to script more than one character simultaneously in a multitask scenario. The first approach animates each character one after the other. This means that the action required of a characterB is generated after the action of a characterA has been completed. The reason is that each task of the characters taking part in the multitask scenario have been assigned a different index value of the task array. A simple example of generating the actions of two different characters appears in Algorithm 2. However, a user should be able to animate virtual characters simultaneously in multitask scenarios. This is achieved in CHASE by using a two-dimensional array named tasks[index][index]. In this array the first index value represents the row in which each action in generated, whereas the second index value represents the number of the character. It should be noted that each character should be represented by the same index value while developing a multitask scenario. Hence, the background algorithms that are implemented recognize and generate the requested tasks as separate entries. This enables the user to animate a number of characters simultaneously. A simple example in which there are two characters in a multitask scenario appears in Algorithm 3. It should be noted that a multitask scenario where multiple characters evolve in a general form can be represented as an array that has a dimensionality equal to M × N, where M denotes the total number of characters evolving in the multitask scenario and N denotes the total number of tasks that a character should perform.

Algorithm 2 By placing the actions of two different characters at different index values of the task array, the system generates each character action one after the other

Algorithm 3 A multitask scenario in which there are two characters. In this scenario, characterA moves to its target position while walking, and characterB moves to its target position while running. Finally, characterA punches characterB with his right hand

Conclusions and Future Work

In this paper, a novel scripting environment, called CHASE, for use in animating virtual characters was presented. CHASE enables a user to request a variety of actions that a character can perform by simply using three commands. Each command, which receives a variety of parameters, is associated with specific actions that the character is able to perform. Moreover, the commands communicate with a variety of background algorithms that are responsible for generating the actions requested of the character. In addition to the scripting commands, by introducing three additional functionalities, the user is able to script concurrent actions of a character, multiple characters at the same time, and multitask scenarios in order to generate scenario-related sequences that involve animated characters.

Cross-References

• Teaching Computer Graphics by Application