Which Flat Field Offsets Should be Used?

[calvin.harman@…]

Description says "...the function shall use the offsets in the image (in->x0
and in->y0) to determine the appropriate offsets to obtain the correct pixel on
the flat-field."

How should in->image->row0, in->row0, flat->image->row0, and flat->row0 be used
to set the image value and mask value? It isn't clear which reference point
should be used when looping on the indices.