Motion Graph for Character Animation: Design Considerations
Inspiring human character has become an active research country in computing machine artworks. It is truly of import for development of practical environment applications such as computing machine games and practical world. One of the popular methods to inspire the character is by utilizing gesture graph.
Since gesture graph is the chief focal point of this research, we investigate the preliminary work of gesture graph and discourse about the chief constituents of gesture graph like distance prosodies and gesture passage. These two constituents will be taken into consideration during the procedure of development of gesture graph. In this paper, we will besides show a general model and future program of this survey.
gesture graph ; distance prosodies ; gesture synthesis ; character life ; gesture passage
For many old ages, character life has become one of the active research countries in computing machine artworks, and it had been used widely in movie and games industries. In this paper the word character refers the human organic structure with the articulated articulations in a hierarchal signifier. Nowadays, gesture gaining control is a popular method to inspire the character. It can bring forth a credible gesture in shorter clip with less calculation clip compared to traditional methods like kinematics.
Although the consequence of life that is produced from gesture gaining control is good, there are some grounds that we need to recycle and redact the gesture gaining control informations. The chief ground is the cost of package and the equipment for gesture gaining control system are really expensive [ 1 ] . In add-on, the gesture gaining control records all the motions harmonizing to the histrion ‘s motions. If we want to utilize a somewhat different action from the original action, we need to make the redaction procedure for gesture informations. We besides need to redact the informations if we want to make the impossible or hard actions.
Sometimes, in some gesture like insistent gestures ; the existent gesture can non be perfect. If we want to utilize exact cyclic gestures we need to change and redact the information. The interaction with the other gesture such as vesture besides can be one of the grounds we need to recycle and change the informations [ 2 ] . The reusability of gesture gaining control informations means that the energizer can make required gestures by recycling gesture informations repeatedly and redacting the gesture informations.
By and large, gesture redaction can be defined as a procedure to bring forth a new gesture by altering or uniting a figure of gestures. Motion passage, gesture blending and gesture insertion are parts of gesture redacting operation that need at least two gestures as inputs. In contrast, joint repositioning, retargeting, smoothing and manner use merely necessitate one gesture as an input.
Gesture blending allows the energizer to alter the simple gesture to complex gesture and creates longer gesture continuance. Gesture blending is used for many grounds such as in making a passage procedure for two gestures that are really widely used in computing machine game development. A passage is normally used merely for altering from one gesture to another gesture and it is used for a certain passage length.
During a passage procedure, two gesture cartridge holders will be combined by extinguishing the discontinuity at the boundary between two gestures. There are many factors that must be considered in gesture passage, but the chief factor is how to do a realistic and natural passage.One of the common ways to recycle and redacting gesture gaining control informations is known as the Motion Graph. In this paper, we focus on several constituents that need to be considered for making the character life utilizing gesture graph. Discussion of this paper is as follows: Section 2 explains the overview for gesture graph. Section 3 discusses several chief constituents in gesture graph such as distance prosodies and gesture passage. Section 4 provides the general model for gesture graph, followed by Section 5 that concludes this paper and province a hereafter research way.
In this subdivision, we will give the overview and look into the preliminary work of gesture graph and discusses several chief constituents in gesture graph.
A. Gesture Graph
The basic thought of gesture graph is to seamlessly link a gesture in a database utilizing a passage [ 1 ] . The graph construction called a gesture graph will be produced. In the gesture graph, edges correspond to gesticulate cartridge holders and nodes represent as points that connect the cartridge holders ( see 1 ) .
The development of gesture graph is inspired by the work of video textures, which is used to keep the original gesture sequences and played back in non insistent watercourses [ 8 ] . Motion graph is a really popular technique to inspire the character for real-time applications and off-line sketch-based gesture synthesis [ 9-12 ] .Based on the annotated bibliography paper that had been published by Gleicher, there are three research work that marked as the first original “motion graph” in computing machine artworks community [ 13 ] .
These three attacks are proposed by Kovar et Al. [ 1 ] , Arikan and Forsyth [ 3 ] , and Lee et Al. [ 4 ] . These three attacks portion the same basic thought that is develop a graph from gesture database and seek the graph to bring forth a gesture that follows the user aim. However, they used a different technique for some topographic points such as on the technique to make passage, search the graph and the distance prosodies.
In old work, we can see that many research workers combine gesture graphs and insertion techniques [ 16-20 ] . These techniques divide gestures into similar behavior and so group similar sections to make insertion and smooth passages. Sang et al [ 21 ] divide gesture into short sections and arranged them into nodes in graph and intermix them to make motive power in real-time.
Taesoo and Sung [ 19 ] concept automated gesture graph for motive power. Treuille et Al. used a simple graph construction to bring forth real-time character life [ 5 ] . They create near-optimal accountants to steer the practical character based on the user end by utilizing a low-dimensional footing representation.For proving and rating procedure, there are several attacks that have been proposed by old research workers to look into the public presentation of their propose technique. As stated by [ 6 ] , the rating procedure of gesture graph can be classified into two classs. The first class is to measure the gesture graph based on the single passages. It besides can be evaluated by analysing the ensuing life.
There is a figure of surveies that depends on human input to mensurate the quality of synthesized gestures [ 7, 8 ] . The statistical theoretical accounts besides are used by [ 9, 10 ] to measure the quality of gestures. Reitsma et Al. introduced a method of measuring a information structured particularly motion graphs [ 11 ] .
III. Distance Prosodies
Normally, to make passage between really different gestures is really hard. On the other manus, if two gestures are similar or about similar, a passage is easy to bring forth utilizing simple insertion method. For that ground, the gestures are need to be compare by utilizing a good gesture similarity technique before the passage can be generated.
This gesture similarity technique can be refer as distance prosodies.Distance prosodies is one of the of import constituents that had been used by many research workers in gesture redacting [ 1, 3, 12, 13 ] . In gesture graph, distance prosodies are used to observe the similar frames for taking the passage points.
From this passage points, the passage can be created between the gesture cartridge holders. Point clouds, prosodies based on joint angles and rule constituents are the chief types of distance prosodies in gesture graph [ 14 ] .
A. Distance Metrics based on Joint-Angles
Since the gesture informations are represented by the joint angle of a skeleton, computation utilizing this method is easier than other distance prosodies method. However, utilizing this distance prosodies method, we need to put a weight as a step. Lee et Al.
and Arikan et Al. are some of the research workers that used this distance prosodies [ 3, 4 ] . The undermentioned expression shows how Lee et Al.
calculates the differences of joint angles [ 4 ] .Where:= distance between frame I and frame J= leaden differences of joint angles= leaden differences of joint speeds= weights speed differences with regard to place differencesThe leaden differences of joint angles can be expressed as follows:Where:= the root joint place of the character at frame I= the root joint place of the character at frame J= the orientation of joint K at frame I= angle that k joint rotates from the orientation of frame I to the orientation of frame J= four that represents the orientation of the K articulation at frame I.
B. Distance Metrics based on Point-Clouds
In point-clouds distance prosodies, two frames of gesture and its neighbouring frames will be converted into point clouds signifier. The distance between these two point clouds can be measured by using the amount of squared Euclidean distance between the corresponding points in the two point clouds. In order to work out the job of happening the point clouds co-ordinate systems, the minimum leaden amount of squared distances will be calculated [ 1 ] . The distance prosodies equation can be defined as:Where:= kth point in the point clouds for frame Mi and M?j,= kth point in the point clouds for frame Mi and M?j,K = articulation index= stiff transmutation composed of a rotary motion by ? grades about the Y ( perpendicular ) axis followed by a interlingual rendition of ( x0, z0 ) in the floor plane.
= the frame weightBased on the rating procedure that had been done by Basten et Al for these three chief distance prosodies: joint-angle, point clouds and chief constituent, there are several advantages and disadvantages for these distance prosodies [ 14 ] ( See Table I ) . The comparing standards that they used to measure these distance prosodies are path divergence and pes skating. Table II shows the types of distance prosodies and passage methods that had been used by several research workers in their gesture graph method.
Table I. Advantages and disadvantages of distance prosodies [ 14 ]
Distance ProsodiesAdvantages andDisadvantagesBased on joint angles* Good for way planning in extremely constrained countries* Least way divergencePoint-cloud* Least pes skating* Path divergence is highest* Slowest huntPrinciple Component* The graph is faster* No demand to put a weight* Lower way divergence than point-cloud* Slightly slowerTABLE II. Distance prosodies and passage methodAuthor ( s )Comparison Method/Distance MetricsPassage Method[ 1 ]Point CloudBlending[ 4 ]Based on joint angles and speedscuts and adding supplanting maps[ 3 ]Based on joint angles speeds + Joint Accelerationscuts and adding supplanting maps[ 12 ]Point cloud + joint place, joint speeds, joint acclerationsBlending[ 15 ]Joint angles + dynamics-basedBlending
IV. Creating Passages
In gesture graph, after the distance metric is measured to happen airss that are similar, this metric will be applied in the database to happen all the possible passages. If the metric is below some threshold, the passage can be created.
There are many types of passage method in the gesture graph. By doing leap cuts and adding supplanting maps, Arikan et Al. and Lee et Al. are able to make a passage in their gesture graph method [ 3 ] [ 16 ] . Kovar et Al. utilizing a simple additive blending to bring forth a passage between the frames [ 1 ] .As stated by Wang and Bodenheimer, blending is one of the ways for making passage [ 8 ] .
It can be referred to as a impermanent blend that used to alter from one gesture to other gesture. During the procedure of gesture passage from one gesture to another, gesture blend will be started in a certain passage length. The length of gesture blend is determined depending on the blended gestures.
They besides stated that additive blending is a common technique for making passages. Linear interpolating and additive blending are suited to be used for the application that need the efficiency and velocity which has low computational weight.However, there are some drawbacks utilizing this method. It is still difficult and necessitate a important manual labour for bring forthing a good passage utilizing intermixing [ 8 ] . It is still critical to find the blend length and passage points in the cartridge holders. To work out this job, Wang developed the geodetic distance method and the speed method for finding an optimum blend length for gesture passages [ 17 ] .
These methods can be used for many different types of gesture. The geodetic distance method is suited for motive power gestures such as walking and running. On the other manus, speed method is used for unpredictable gestures such as dance and pugilism.
In this subdivision we discuss about a general model for inspiring the character utilizing gesture graph ( see 2 ) . Most of the determinations for planing the model are based on the old plants of legion research workers. For each gesture cartridge holder in the gesture database, we will utilize more than 100 gesture cartridge holders which contained with one walk rhythm. For the procedure of turn uping passage points, most of old plants use a point-cloud method to compare and happen the similar frames in the gesture cartridge holder. With a little alteration of point cloud method, we will accommodate this attack in our gesture graph. The simple additive blending will be used to make a faster passage which is of import for making a governable gesture graph. Since the intermixing passage can bring forth the artefacts such as foot-skate, we will work out this job with suited foot-skate killing method.
VI. Decisions and Future Works
In this paper we investigate the preliminary work of gesture graph and compare the gesture graph constituents like distance prosodies and passage methods. These two constituents are portion of the issues that we need to see during the procedure of development of gesture graph. Distance prosodies will be our future way.
The writer wishes to convey their innermost gratitude and grasp to Malaysian Ministry of Science, Technology and Innovation ( MOSTI ) under ScienceFund grant ( 01-01-06-SF0387 ) for supplying fiscal support of this research.
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