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Page 316 ï~~A Symbiosis of Animation and Music Robert Pringle Side Effects Software Dept Toronto, ON, Canada M8V 3E7 S rpringle(@gobi.sidefx.com I Abstract An interactive environment for producing musically controlled computer animations is presented. Objects are initially modeled using an interactive toolset. Each object incorporates a script, which is an instance of programming language code and data definitions that permit complex object and animation control. The script language has a number of functions that access MIDI information, which therefore allows music to determine the movement and morphology of animated objects. The tight integration of music and animation in an interactive production environment results in the ability to automatically synchronize complex animation activity to music, and lets musical compositions act as a creative source for graphical sculptoring and animation. 1 Introduction This paper describes an environment that automates animation and music. An interactive toolset has been implemented that permits the modeling of of organic-like objests suitable for subsequent mutation and animation. The graphic objects used are inspired by Todd's and Latham's evolutionary art (Todd & Latham, 1992), and were chosen for their generality and natural, life-like appearance. The system permits the run-time evolution of creatures through the use of scripts that are defined within kernel objects. Scripts contain programming language code that control local and global characteristics of objects. With respect to music, the script language recognizes MIDI (Musical Instrument Digital Interface) files (IMA, 1983). Script functions can access MID! file information. When executed in a temporal fashion, creature characteristics are controllable by the MIDI music composition, in the manner dictated by the script. The net effect of au Brian J. Ross t of Computer Science, Brock University t. Catharines, ON, Canada L2S 3A1 bross~sandbanks.cosc. brocku.ca tomated MIDI control is that the creatures' movement and morphology are automatically synchronized with and derived from music. Consequently, the music data can be considered to be a "sculptor" of graphical objects, and in concert with appropriate script control, can control object and animation events in a variety of complex ways. This work was motived by the desire to implement an animation environment that embraces music as a primary means of animation design and control. We designed an interactive production environment that integrates music and animation into a single environment. In addition, the system is designed to generate graphical information from the music source data, since musical scores are often predefined and cannot be altered to conform to animation requirements. Thus the approach is conducive towards both synchronizing animation events to music and sound effects, as well as using music as a creative source for deriving animation content. Section 2 reviews related work. Section 3 discusses the interactive modeling environment. The musical and animation control functions of the system are described in section 4. Section 5 reviews some example productions. Future extensions conclude the paper in section 6. 2 Background Sound effects and music are an intrinsic requirement to most computer animation productions. The synchronization of sound effects and music to animation traditionally has been a latter-stage post-production task. Digital technology is being successfully applied to this problem, as can be seen by the growing popularity of multimedia authoring systems and non-linear editors. In some cases, however, it is preferable to automate the synchronization of sound and animation - generating an Pringle & Ross 316 ICMC Proceedings 1996
Page 317 ï~~imation behavior appropriate for the audio to be used in the production. For example, the complexity of synchronizing speech to animated character mouth movement is greatly simplified with lip synchronization software. Some commercial animation environments permit external control through system interfaces, which is required for motion capture (eg. Softimage's channels (Softimage, 1995), Prisms' surface operations (Prisms, 1995)). Although these systems can use music data (with the help of additional software support), interfaces permitting interactive inspection and acquisition of music features by the animator are not included. The use of music to control graphical information is not new, although little work has been done in it. "Light organs" were popular in the sixties, in which lights flash in synchronization to musical audio signals. The video More Bells and Whistles is a recent example of an animation derived largely from MIDI source data (Lytle, 1990). Much more work in the automation of music and animation has been directed to the opposite problem of this paper - creating music from visual input information (Evans, 1987; Bidlack, 1992; Nakamura et al., 1993). 3 Modeling The objects modeled are inspired by the work of Todd and Latham (Todd & Latham, 1992). They present a graphics system that supports the userinfluenced evolution of graphical "creatures" (eg. Figure 3). Their system is built upon an objectoriented graphics language that permits hierarchical, recursive object definitions similar in spirit to L-systems (Peitgen et al., 1992). A key characteristic of their system is the ascribing of a geneticlike metaphor to the models. By tweaking object "genes"0, a rich variety of models can be derived and evolved. Our system borrows some of the more interesting features of Todd's and Latham's graphical model, incorporating them within a conventional GUI editor. At the lowest level, objects are composed of a number of geometric primitives, such as spheres, tori, and cones. These can then be recursively and hierarchically composed together. The system makes use of transformational inheritance: a graphical transform applied to a parent is inherited by its children. Children objects may have their own specific transforms defined for them as well. The system supplies a number of composite patterns, such as horns and starfish. In concert with standard graphical transforms and userdefined object definitions, a surprisingly rich set of models can be derived. Material and lighting definitions are also assignable to composite and individual objects. 4 Musical control of graphical objects Besides containing graphical transform and material information, each object also may have a script defined for it. A script is a programming language instance executed for that object for each frame of the animation. An object may have a unique script, no script, or share a script with other objects. In the latter case, execution of scripts may use the same inheritance scheme employed with transforms. Figure 1: Object script The scripting language defined is called ACL (Animation Control Language), and is an interpreted language with a syntax and semantics similar to C (Kernighan & Ritchie, 1978). An example script in a script dialogue window is in Figure 1. ACL supports basic data types and control mechanisms (tests, loops, iteration). Because ACL is fairly conventional, we forgo further discussion ICMC Proceedings 1996 317 Pringle & Ross
Page 318 ï~~of its design. Of importance to the animation system, however, are a number of ACL functions that refer to graphical transform parameters of objects, and MIDI events as resident in a globally accessible MIDI data file (IMA, 1983). MIDI (Musical Instrument Device Interface) is a protocol for exhanging information between digital musical instruments and computers. An important feature of MIDI information that is useful in an animation environment is the means for denoting the temporal delays between musical events. The delta field of a MIDI data stream is a packet item that describes the temporal delay in clock ticks between the previously seen and current events in the stream. When MID! data is being transmitted in real-time, the delta parameter is often superfluous, as the real-time transmission of events implicitly denotes their timing in the performance. However, if the MIDI data is being stored in a file, delta values are the means used for reconstructing the timing of events. Given a number of chronological events, their absolute times are computed by adding each event's delta value to a global time value. The system handles the more essential events defined by the MIDI standard. This includes note on, note off, polyphonic aftertouch, channel, and pitch wheel events. Because the current version of the system requires the animator to implement an ACL script that controls an animated object according to some musical criteria, it is important that access to the music data is made as manageable and intelligible as possible. Therefore, a number of MIDI filters are available for removing superflous events from view. For example, most keyboards generate aftertouch signals, and there can be an enormous number of such signals generated during realtime MIDI recording. Filtering these events from view greatly helps the animator. The MIDI display of events in the interactive environment is for information feedback purposes only. When generating the final animation, the MID] data is processed from start to finish, and is correlated in time with the animation. Each script will access the current state of MID! information according to its AOL code. MID! information can obtained through globally-defined variables. For example, in rotate.x = rotate~x + current~note~on; the X-rotation transformation is incremented at a rate determined by the current note being played. Since higher octave notes have higher note numbers, the rotation rate accelerates as higher pitched notes are played. ACL is also able to search ahead in the musical score for particular MIDI events that will occur in the future. This is extremely useful for determining tweening rates, as it permits us to move forward in time from the current frame to see when a desired musical event happens in the future, and thereby determine an appropriate interpolation factor. In combination with the conventional expression and programming language constructs defined in ACL, the musical control possibilities are unlimited. Using flags and conditional processing, an object's script can specify different activities for the object during the course of the animation. In addition, extracted music values can be arithmetically altered and computationally processed. All such computations can then be used to control any animatable characteristic of the object. 5 Example Figure 2: Gyroscope Figure 2 shows three screenshots from an animation entitled Gyroscope. Three tori are used. One torus spins at a constant rate. A second tori spins 180 degrees every time a note is played on MIDI channel 1. Should the frame be one in which a "note on" event is occurring on MIDI channel 1, then the velocity is set to a maximum value. Each subsequent frame will have this velocity decremented, until it reaches zero. A third tori also spins 180 degrees, but when notes are played on channel 2. It's initial velocity and deceleration, however, are dependent upon the interval duration between the trigger and the following one in the MIDI stream. Hence it's rotation adapts to the relative rhythm of notes on channel 2. Figure 3 gives an idea of the organic style of creatures possible in the system. This snapshot is based on an animation in which the creature size, shape, and colour is controlled by various aspects of music - such as pitch, velocity, and tempo. Pringle & Ross 318 ICMC Proceedings 1996
Page 319 ï~~Figure 3: Creature 6 Conclusion Our coalescence of music and graphics is more substantial than merely synchronising sound with animation, but rather, uses music as a mechanism for defining the fundamental form of graphical objects and their animated behaviour. Todd and Latham mention the chaotic nature of this style of graphical modeling, as the smallest change in one parameter can profoundly alter a creature's overall form (Todd & Latham, 1992). Likewise, minute changes in the music data can result in wildly different animations. The system is object-oriented in flavour, as the object definitions and scripts are analogous to object instances and methods in object-oriented programming languages. A number of extensions of this work are possible. The system is implemented in C on a Silicon Graphics Elan workstation, using GL and the Forms user interface library. As GL is a Gouraud renderer, utilizing a rendering engine such as a raytracer would be a useful enhancement. The object models themselves can be extended with a more complete subset of the constructions used in Todd's and Latham's systems. The script language is easily extensible, and a library of musically-interesting script functions is possible. A powerful extension would be to permit the mutation of object primitives themselves by the script language, thereby permitting musically-controlled morphing. In addition, it would be interesting to interface the MIDI controls of the system into a commercial animation environment, perhaps replacing ACL with GuI control. A major enhancement worth considering is the real-time control of animation using music data. The most difficult problem arising from real-time control is the prediction of event times, which are required for tweening purposes. For example, a musical source with varying tempo poses great diffi culty to an animation that is trying to synchronize some event to a regular tempo. A possible solution is to perform real-time predictive beat tracking (P.E.Allen & Dannenberg, 1990; Large, 1995). Acknowledgement: This work is supported through NSERC Operating Grant 0138467. References Bidlack, R. 1992. Chaotic Systems as Simple (but Complex) Compositional Algorithms. Computer Music Journal, 16(3), 33-47. Evans, B. 1987. Integration of Music and Graphics through Algorithmic Congruence. Pages 17 -24 of. Proceedings 1987 International Computer Music Conference. IMA. 1983. MIDI 1.0 Specification. International MIDI Association, North Hollywood, CA. Kernighan, B.W., & Ritchie, D.M. 1978. The C Programming Language. Prentice-Hall. Large, E.W. 1995. Beat Tracking with a Nonlinear Oscillator. Pages 24-31 of: Widmer, G. (ed), IJCAI Workshop in Artificial Intelligence and Music. Lytle, W. 1990. More Bells and Whistles. ACM SIGGRAPH Video Review, 62. (video). Nakamura, J., Kaku, T., Noma, T., & Yoshida, S. 1993. Automatic Background Music Generation based on Actors' Emotion and Motions. Pages 147-161 of: Proceedings Pacific Graphics '93, vol. 1. P.E.Allen, & Dannenberg, R.B. 1990. Tracking Musical Beats in Real Time. Pages 140-143 of: Proceedings 1990 International Computer Music Conference. Peitgen, H.-O., Jurgens, H., & Saupe, D. 1992. Chaos and Fractals. Springer-Verlag. Prisms. 1995 (August). Prisms 6.0 Reference Manual. Toronto. Softimage. 1995. Softimage 3D9 User's Guide. Montreal. Todd, S., &z Latham, W. 1992. Evolutionary Art and Computers. Academic Press. ICMC Proceedings 1996 319 Pringle & Ross