convert coordinate of the cursor to the local coordinate systeme

To convert mouse coordinates into a coordinates of a view, use function V3d_View::Convert :

Convert( me ; Xp,Yp : Integer ; X,Y,Z : out Coordinate)
---Purpose : Converts the projected point into a point
-- in the reference frame of the view corresponding
-- to the intersection with the projection plane
-- of the eye/view point vector.
raises UnMapped from V3d;
-- If the view is not mapped on the window.

Give to it the (x, y) coordinates as the first two actual arguments.