Most of the movement in Playing with Liquid Mercury stems from a single phrase which the dancers learned on the first day we worked with them. Small groups of dancers were then instructed to manipulate this phrase in various ways, for example, breaking it up into smaller parts and reordering them in any way they chose or streamlining it into a series of poses and then spatially confining it to a straight line. Other dancers were given more freedom. One group, for example, was given the beginning of a movement sequence as a starting point and then allowed to complete it themselves (with some direction from the choreographer) keeping in mind a simple metaphor: that they, as a group, were a blob of liquid mercury (hence the inspiration for the title!). We worked with another group on improvisation skills and this was incorporated into the final piece as a section in which they have no set choreography at all. They improvise based on a few basic rules and they may do it however feels appropriate during each performance. Naturally each improvisation is quite different and it adds both uncertainty and excitement to the piece each time it is performed. In these various ways, the dancers were very much involved in creating this piece. Their individual contributions to the choreography make it unique to them thus it is as much their piece as it is ours.

The sound is a combination of two different audio production processes. On the one hand there are sound files (recordings) that the computer simply plays. The recorded sounds include those of running water, ice skating, fireworks, the dancers singing, and some synthesized music that was created by a computer reacting to a video of the dancers' movement. In most cases the original recordings have gone through some sequence of digital processing and mixing so that the resulting audio may differ considerably from the original sounds. The audio elements produced in this way tend to form a background environment in which the dance takes place. On the other hand there is music played live on the computer's built-in synthesizer by the dancers themselves through their movements. The delicate opening and closing piano solos, as well as the rest of the instrumental sounds in the piece--piano and other keyboards, strings, wind instruments, percussion--are produced in this way and are therefore slightly different at each performance.

The relationship between movement and the sound it produces in Playing with Liquid Mercury is relatively simple: the stage divides into two keyboards which the dancers play as they move. The left keyboard is like a piano in that the low notes are on the left (near the wings) and the high notes are on the right (the center of the stage). The right keyboard is the opposite of a piano, with the low notes on the right (again near the wings) and the high notes on the left (again in the center of the stage). As someone walks across the stage the pitch first goes up and then at center stage reverses direction and goes down.

The two stage keyboards, left and right, are very picky. Each one only sounds one note at a time. The note chosen is the one that corresponds to the part of the stage where the most movement is happening. Thus when two dancers are on the same half of the stage, only the one making the largest, fastest gestures is making sound, though sometimes the quieter, slower dancer can get a note or two in edgewise.

The loudness of the sound is determined by the degree of change in the movement. The faster the dancer moves and/or the larger her movements, the louder the sound produced will be. Where she is on stage, in addition to determining pitch, also determines the placement of sound within the stereo audio space, i.e. the panning of the sound between the left and right speakers. Movement on the far left of the stage places sound most in the left speaker. As the dancer moves across the stage to the right the sound travels with her to the right speaker.

Working within this sound environment, the choreography takes most advantage of the horizontal dimension. Movement generally travels across the stage with relatively little emphasis placed on up and down stage. The split keyboard allows the dancers to create dialogues between both sides of the stage, most noticeable when the two keyboards are actually playing two different instruments. The choreography also uses the sharp split in the stage space to highlight sudden entrances and exits at the edges of the stage, movement which wavers across the center line, and stillness in one or both sides of the stage.

The computer's response to the movement comes via a digital camcorder and audiovisual-oriented software. The dance is being filmed by the camcorder at the back of the auditorium, which sends the digital video signal to the computer via firewire. That video signal is then analyzed by the software package Max/MSP/Jitter, an extremely flexible programming environment in which almost any audio responses to video can be built from scratch. In this case, the key process is one in which the computer looks at changes in the visual scene.

The computer compares consecutive frames of digital video by subtracting one from the other, resulting in a ghostly video that works approximately as follows: If there is no movement on stage, there is no change and subtraction of one frame from the next yields a perfectly black image; but if there is movement, the difference between frames is represented as white pixels in the video, resulting in an outline of the moving dancer which gets thicker and brighter the faster and farther she moves. The computer determines which keys on the stage keyboards have the most white in them (i.e. most movement) and plays the pitches corresponding to those keys.

While this is a fairly simple interface for relating sound to movement, it has proved surprisingly flexible and satisfying. The dancers experience the audio result of their movement and thus truly interact with the computer. Sound therefore becomes a partner in their performance of the piece, enriching in subtle ways the experience of dancing for both dancers and audience.