v := struct.node.surface.xdir(sn)
struct.node.surface.xdir(sn) := v

Get/set the orientation of the surface system of node sn such that the \(x\)-direction is aligned with the projection of the surface vector v onto the normal plane, which is defined by the \(z\)-direction.

This operation can only be performed upon nodes that have a valid surface system (to ensure that the \(z\)-direction always corresponds with the average normal direction of at least one of the shell-type SELs using the node).

Returns:v - vector
Accepts:v - vector
Arguments:sn - structure node pointer