  In this paper, we study the (complex) geometry of the set $S$ of the square
roots of $-1$ in a real associative algebra $A$, showing that $S$ carries a
natural complex structure, given by an embedding into the Grassmannian of
$\mathbb{C}\otimes A$. With this complex structure, slice-regular functions on
$A$ can be lifted to holomorphic maps from $\mathbb{C}\times S$ to
$\mathbb{C}\otimes A \times S$ and the values of the original slice-regular
functions are recovered by looking at how the image of such holomorphic map
intersects the leaves of a particular foliation on $\mathbb{C}\otimes A \times
S$, constructed in terms of incidence varieties. In this setting, the quadratic
cone defined by Ghiloni and Perotti is obtained by considering some particular
(compact) subvarieties of $S$, defined in terms of some inner product on $A$.
  Moreover, by defining an analogue of the stereographic projection, we extend
the construction of the twistor transform, introduced by Gentili, Salamon and
Stoppato, to the case of an associative algebra, under the hypothesis of the
existence of sections for a given projective bundle.
  Finally, we introduce some more general classes of "slice-regular" functions
to which the present theory applies in all qualitative aspects.
