Tapper

There are many ways people share the responsibility of making music: they can collaborate in composing and performing music, a composer can leave performing to other musicians, a group of musicians can hire a conductor, a performer can rely on an instrument maker to build a machine capable of turning musical gestures into sound.
A less commonly used way to share musical responsibility is for a machine to be responsible for storing the notes of the score and playing back those notes in the proper order, leaving a human performer with only the responsibilities of the conductor — rhythm, tempo, dynamics, expression, interpretation.
With the advent of computers and computer music, development of such an approach (which, when implemented with computers, is sometimes called the conductor program) has taken off, with several people experimenting with its possibilities.
Tapper application was developed to be a computer-mediated musical performance software. In Tapper, the gestures that control playback are single keystrokes on either a MIDI keyboard or the ASCII (typewriter) keyboard on the computer. Notes are grouped into alternating sets of play groups (which are explicit and indicated in the display) and release groups (which are implicit and not shown).
Play groups are groups of note starts (MIDI NOTE ON events) that happen at the roughly same time, for example, notes in a chord that is played on a certain beat. Release groups are all the notes ends (MIDI NOTE OFF events) that happen between one play group and the following play group. When you press a key on the MIDI keyboard, Tapper plays any note ends in the upcoming release group (there may be none) and all the note starts in the next play group.
When you release a key and there are no more keys pressed, Tapper ends all the notes in the next release group. The dynamic level of the notes being played depends on both the score and the velocity of the MIDI keystroke keypress that triggers them: for each group of notes started by a given keypress, the loudest note in that group will be played at the level dictated by the velocity of the keypress; the other notes in that group will have dynamic levels that maintain their dynamic proportion to the loudest note.
If you use the ASCII (computer) keyboard instead of a MIDI keyboard, only the press actions happen (you have no explicit control of release groups except those between play groups or at the end of the piece), and the dynamic levels are those in the score.