# Cell function

As explained earlier, at least two coordinates systems exist in Bcube : the "reference" coordinates (`ReferenceDomain`

) and the "physical" coordinates (`PhysicalDomain`

). The evaluation of a function on a point in a cell depends on the way this point has been defined. Hence the definition of `CellPoint`

s that embed the coordinate system. Given a `CellPoint`

(or eventually a `FacePoint`

), an `AbstractCellFunction`

will be evaluated and the mapping between the `ReferenceDomain`

to the `PhysicalDomain`

(or reciprocally) will be performed internally if necessary : if an `AbstractCellFunction`

defined in terms of reference coordinates is applied on a `CellPoint`

expressed in the reference coordinates system, no mapping is needed.