In this paper we introduce the Fourier tag, a synthetic
fiducial marker used to visually encode information and
provide controllable positioning. The Fourier tag is a synthetic
target akin to a bar-code that specifies multi-bit information
which can be efficiently and robustly detected in an
image. Moreover, the Fourier tag has the beneficial property
that the bit string it encodes has variable length as a
function of the distance between the camera and the target.
This follows from the fact that the effective resolution
decreases as an effect of perspective. This paper introduces
the Fourier tag, describes its design, and illustrates its properties
experimentally.