C. New-Word Heuristic
With the method described above, it is possible to write in an
area of about 7 × 5 cm without moving the hand on the writing
surface. This area is sufficiently large to allow for writing sizes
similar to results of writing with a pen on paper. When writing
with a pen, a user occasionally moves their hand forward to be
able to keep writing in a line. Here, moving the hand forward
is not necessary. Instead, the hand can remain at the same spot
while writing multiple words (or word-parts) on top of each
other: When the end of a word is reached, the user lifts the
writing corner, moves it to the left, and then writes the next
word “over” the previous one.
To detect this restart gesture, we apply a straightforward decision
rule based on the measurement rx of the gyroscope of
rotation around the phone’s x-axis, where the combined signals
for “pen up” [see Fig. 3(f)] and “right to left” [see Fig. 3(b)]
leads to a characteristic pattern (see Fig. 5), which allows detection
of the restarts and segmentation of the writing into three
parts: wi the first word, ξi the restart gesture which is discarded,
and wi+1 the second word.
wi contains all sensor measurements from the beginning (or
from the end of ξi−1 ) until rx exceeds a threshold θu . Starting
from there, all observations that exceed a threshold θl are considered
to be part of the restart gesture ξi, and once the rx drops
below threshold θl, the next word wi+1 starts. The thresholds
were chosen to be θu = 0.6 and θl = 0.2 from analyzing multiple
graphs similar to Fig. 5 during the initial implementation of
this method and ahead of any of our user studies.