Survey of Pen-and-Paper Computing - Human-Computer Interaction - page 22

(a) EAN 13 (b) ARToolkit (c) ARTag ID marker

(f) DataGlyph a (d) QR Code (e) reacTIVision marker

Fig. 2.2 Linear and two-dimensional barcodes ( a DataGlyph example by Jeff Breidenbach)

Pattern-based fiducials are used for instance in the widely-used open source
marker tracking library ARToolkit 3 . The fiducial has a black frame which encloses a
black and white pattern (Fig. 2.2 (b)). The library identifies a barcode by comparing
the pattern with a set of pre-registered templates using pattern matching techniques
from computer vision. The advantage of pattern-based fiducials is that the appli-
cation developer has some influence on how the fiducial looks like and can create
meaningful markers. For instance, it can contain a symbol or some text. However,
identification is more error-prone than with other techniques.

Matrix-based fiducials are also known as 2D barcodes. They encode binary data
by a two-dimensional grid of black and white points. Each point encodes one bit
of data, whereby some bits are usually reserved for error correction. Figure 2.2 (c)
depicts an ID marker of ARTag [28]. 2D barcodes allow not only to encode an
identifier but a relatively large amount of data. For instance, one single QR code [46]
(see Fig. 2.2 d) can store more than 4,000 alphanumeric characters. The DataMatrix
[45] code can store more than 2,000 characters. It is for example used for electronic
stamps by the German postal service. The data density of 2D barcodes can be further
increased by varying visual properties of the points, such as color or brightness, but
at the price of a decrease in robustness.
In addition to identifying objects, matrix-based fiducials can also be used for
tracking the location of objects. If the size and shape of the marker is known, it
is possible to calculate the marker's relative position and orientation with full 6

3

http://www.hitl.washington.edu/artoolkit/