  We present Ghostfinger, a technology for highly dynamic up/down fingertip
haptics and control. The overall user experience offered by the technology can
be described as that of tangibly and audibly interacting with a small hologram.
More specifically, Ghostfinger implements automatic visualization of the
dynamic instantiation/parametrization of algorithmic primitives that together
determine the current haptic conditions for fingertip action. Some aspects of
this visualization are visuospatial: A floating see-through cursor provides
real-time, to-scale display of the fingerpad transducer, as it is being moved
by the user. Simultaneously, each haptic primitive instance is represented by a
floating block shape, type-colored, variably transparent, and possibly
overlapping with other such block shapes. Further aspects of visualization are
symbolic: Each instance is also represented by a type symbol, lighting up
within a grid if the instance is providing output to the user. We discuss the
system's user interface, programming interface, and potential applications.
This is done from a general perspective that articulates and emphasizes the
uniquely enabling role of the principle of computation in the implementation of
new forms of instrumental control of musical sound. Beyond the currently
presented technology, this also reflects more broadly on the role of Digital
Musical Instruments (DMIs) in NIME.
