[FieldTrip] reverse source interpolate?

[cornelius abel]

Hello,

does anybody know how to get the uninterpolated source position giving 
the coordinates aquired after interpolation with an anatomy.
In principle i want to get an individual source time course at the 
position i picked from the interpolated grand average source plot. 
Therfore i need the corresponding filter and its position in the source 
structure.

I tried to get the position by applying the inverse transformation 
matrix of the used anatomy like:
pos_before_interpolation=warp_apply(inv(anatomy.transform), 
position_after_interpolation);

but that did not give usefull results :(

Any ideas how it could be done?

Greetings, Cornelius