  We study planar noncommutative theories such that the spatial coordinates
${\hat x}_1$, ${\hat x}_2$ verify a commutation relation of the form: $[{\hat
x}_1, {\hat x}_2] = i \theta ({\hat x}_1,{\hat x}_2)$. Starting from the
operatorial representation for dynamical variables in the algebra generated by
${\hat x}_1$ and ${\hat x}_2$, we introduce a noncommutative product of
functions corresponding to a specific operator-ordering prescription. We define
derivatives and traces, and use them to construct scalar-field actions. The
resulting expressions allow one to consider situations where an expansion in
powers of $\theta$ and its derivatives is not necessarily valid. In particular,
we study in detail the case when $\theta$ vanishes along a linear region. We
show that, in that case, a scalar field action generates a boundary term,
localized around the line where $\theta$ vanishes.
